make a hypothesis for what causes the reaction you observed

The Scientific Method Tutorial

The Scientific Method

Steps in the scientific method.

There is a great deal of variation in the specific techniques scientists use explore the natural world. However, the following steps characterize the majority of scientific investigations:

Step 1: Make observations Step 2: Propose a hypothesis to explain observations Step 3: Test the hypothesis with further observations or experiments Step 4: Analyze data Step 5: State conclusions about hypothesis based on data analysis

Each of these steps is explained briefly below, and in more detail later in this section.

Step 1: Make observations

A scientific inquiry typically starts with observations. Often, simple observations will trigger a question in the researcher's mind.

Example: A biologist frequently sees monarch caterpillars feeding on milkweed plants, but rarely sees them feeding on other types of plants. She wonders if it is because the caterpillars prefer milkweed over other food choices.

Step 2: Propose a hypothesis

The researcher develops a hypothesis (singular) or hypotheses (plural) to explain these observations. A hypothesis is a tentative explanation of a phenomenon or observation(s) that can be supported or falsified by further observations or experimentation.

Example: The researcher hypothesizes that monarch caterpillars prefer to feed on milkweed compared to other common plants. (Notice how the hypothesis is a statement, not a question as in step 1.)

Step 3: Test the hypothesis

The researcher makes further observations and/or may design an experiment to test the hypothesis. An experiment is a controlled situation created by a researcher to test the validity of a hypothesis. Whether further observations or an experiment is used to test the hypothesis will depend on the nature of the question and the practicality of manipulating the factors involved.

Example: The researcher sets up an experiment in the lab in which a number of monarch caterpillars are given a choice between milkweed and a number of other common plants to feed on.

Step 4: Analyze data

The researcher summarizes and analyzes the information, or data, generated by these further observations or experiments.

Example: In her experiment, milkweed was chosen by caterpillars 9 times out of 10 over all other plant selections.

Step 5: State conclusions

The researcher interprets the results of experiments or observations and forms conclusions about the meaning of these results. These conclusions are generally expressed as probability statements about their hypothesis.

Example: She concludes that when given a choice, 90 percent of monarch caterpillars prefer to feed on milkweed over other common plants.

Often, the results of one scientific study will raise questions that may be addressed in subsequent research. For example, the above study might lead the researcher to wonder why monarchs seem to prefer to feed on milkweed, and she may plan additional experiments to explore this question. For example, perhaps the milkweed has higher nutritional value than other available plants.

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The Scientific Method Flowchart

The steps in the scientific method are presented visually in the following flow chart. The question raised or the results obtained at each step directly determine how the next step will proceed. Following the flow of the arrows, pass the cursor over each blue box. An explanation and example of each step will appear. As you read the example given at each step, see if you can predict what the next step will be.

Activity: Apply the Scientific Method to Everyday Life Use the steps of the scientific method described above to solve a problem in real life. Suppose you come home one evening and flick the light switch only to find that the light doesn’t turn on. What is your hypothesis? How will you test that hypothesis? Based on the result of this test, what are your conclusions? Follow your instructor's directions for submitting your response.

The above flowchart illustrates the logical sequence of conclusions and decisions in a typical scientific study. There are some important points to note about this process:

1. The steps are clearly linked.

The steps in this process are clearly linked. The hypothesis, formed as a potential explanation for the initial observations, becomes the focus of the study. The hypothesis will determine what further observations are needed or what type of experiment should be done to test its validity. The conclusions of the experiment or further observations will either be in agreement with or will contradict the hypothesis. If the results are in agreement with the hypothesis, this does not prove that the hypothesis is true! In scientific terms, it "lends support" to the hypothesis, which will be tested again and again under a variety of circumstances before researchers accept it as a fairly reliable description of reality.

2. The same steps are not followed in all types of research.

The steps described above present a generalized method followed in a many scientific investigations. These steps are not carved in stone. The question the researcher wishes to answer will influence the steps in the method and how they will be carried out. For example, astronomers do not perform many experiments as defined here. They tend to rely on observations to test theories. Biologists and chemists have the ability to change conditions in a test tube and then observe whether the outcome supports or invalidates their starting hypothesis, while astronomers are not able to change the path of Jupiter around the Sun and observe the outcome!

3. Collected observations may lead to the development of theories.

When a large number of observations and/or experimental results have been compiled, and all are consistent with a generalized description of how some element of nature operates, this description is called a theory. Theories are much broader than hypotheses and are supported by a wide range of evidence. Theories are important scientific tools. They provide a context for interpretation of new observations and also suggest experiments to test their own validity. Theories are discussed in more detail in another section.

The Scientific Method in Detail

In the sections that follow, each step in the scientific method is described in more detail.

Step 1: Observations

Observations in science.

An observation is some thing, event, or phenomenon that is noticed or observed. Observations are listed as the first step in the scientific method because they often provide a starting point, a source of questions a researcher may ask. For example, the observation that leaves change color in the fall may lead a researcher to ask why this is so, and to propose a hypothesis to explain this phenomena. In fact, observations also will provide the key to answering the research question.

In science, observations form the foundation of all hypotheses, experiments, and theories. In an experiment, the researcher carefully plans what observations will be made and how they will be recorded. To be accepted, scientific conclusions and theories must be supported by all available observations. If new observations are made which seem to contradict an established theory, that theory will be re-examined and may be revised to explain the new facts. Observations are the nuts and bolts of science that researchers use to piece together a better understanding of nature.

Observations in science are made in a way that can be precisely communicated to (and verified by) other researchers. In many types of studies (especially in chemistry, physics, and biology), quantitative observations are used. A quantitative observation is one that is expressed and recorded as a quantity, using some standard system of measurement. Quantities such as size, volume, weight, time, distance, or a host of others may be measured in scientific studies.

Some observations that researchers need to make may be difficult or impossible to quantify. Take the example of color. Not all individuals perceive color in exactly the same way. Even apart from limiting conditions such as colorblindness, the way two people see and describe the color of a particular flower, for example, will not be the same. Color, as perceived by the human eye, is an example of a qualitative observation.

Qualitative observations note qualities associated with subjects or samples that are not readily measured. Other examples of qualitative observations might be descriptions of mating behaviors, human facial expressions, or "yes/no" type of data, where some factor is present or absent. Though the qualities of an object may be more difficult to describe or measure than any quantities associated with it, every attempt is made to minimize the effects of the subjective perceptions of the researcher in the process. Some types of studies, such as those in the social and behavioral sciences (which deal with highly variable human subjects), may rely heavily on qualitative observations.

Question: Why are observations important to science?

Limits of Observations

Because all observations rely to some degree on the senses (eyes, ears, or steady hand) of the researcher, complete objectivity is impossible. Our human perceptions are limited by the physical abilities of our sense organs and are interpreted according to our understanding of how the world works, which can be influenced by culture, experience, or education. According to science education specialist, George F. Kneller, "Surprising as it may seem, there is no fact that is not colored by our preconceptions" ("A Method of Enquiry," from Science and Its Ways of Knowing [Upper Saddle River: Prentice-Hall Inc., 1997], 15).

Observations made by a scientist are also limited by the sensitivity of whatever equipment he is using. Research findings will be limited at times by the available technology. For example, Italian physicist and philosopher Galileo Galilei (1564–1642) was reportedly the first person to observe the heavens with a telescope. Imagine how it must have felt to him to see the heavens through this amazing new instrument! It opened a window to the stars and planets and allowed new observations undreamed of before.

In the centuries since Galileo, increasingly more powerful telescopes have been devised that dwarf the power of that first device. In the past decade, we have marveled at images from deep space , courtesy of the Hubble Space Telescope, a large telescope that orbits Earth. Because of its view from outside the distorting effects of the atmosphere, the Hubble can look 50 times farther into space than the best earth-bound telescopes, and resolve details a tenth of the size (Seeds, Michael A., Horizons: Exploring the Universe , 5 th ed. [Belmont: Wadsworth Publishing Company, 1998], 86-87).

Construction is underway on a new radio telescope that scientists say will be able to detect electromagnetic waves from the very edges of the universe! This joint U.S.-Mexican project may allow us to ask questions about the origins of the universe and the beginnings of time that we could never have hoped to answer before. Completion of the new telescope is expected by the end of 2001.

Although the amount of detail observed by Galileo and today's astronomers is vastly different, the stars and their relationships have not changed very much. Yet with each technological advance, the level of detail of observation has been increased, and with it, the power to answer more and more challenging questions with greater precision.

Question: What are some of the differences between a casual observation and a 'scientific observation'?

Step 2: The Hypothesis

A hypothesis is a statement created by the researcher as a potential explanation for an observation or phenomena. The hypothesis converts the researcher's original question into a statement that can be used to make predictions about what should be observed if the hypothesis is true. For example, given the hypothesis, "exposure to ultraviolet (UV) radiation increases the risk of skin cancer," one would predict higher rates of skin cancer among people with greater UV exposure. These predictions could be tested by comparing skin cancer rates among individuals with varying amounts of UV exposure. Note how the hypothesis itself determines what experiments or further observations should be made to test its validity. Results of tests are then compared to predictions from the hypothesis, and conclusions are stated in terms of whether or not the data supports the hypothesis. So the hypothesis serves a guide to the full process of scientific inquiry.

The Qualities of a Good Hypothesis

• A hypothesis must be testable or provide predictions that are testable. It can potentially be shown to be false by further observations or experimentation.
• A hypothesis should be specific. If it is too general it cannot be tested, or tests will have so many variables that the results will be complicated and difficult to interpret. A well-written hypothesis is so specific it actually determines how the experiment should be set up.
• A hypothesis should not include any untested assumptions if they can be avoided. The hypothesis itself may be an assumption that is being tested, but it should be phrased in a way that does not include assumptions that are not tested in the experiment.
• It is okay (and sometimes a good idea) to develop more than one hypothesis to explain a set of observations. Competing hypotheses can often be tested side-by-side in the same experiment.

Question: Why is the hypothesis important to the scientific method?

Step 3: Testing the Hypothesis

A hypothesis may be tested in one of two ways: by making additional observations of a natural situation, or by setting up an experiment. In either case, the hypothesis is used to make predictions, and the observations or experimental data collected are examined to determine if they are consistent or inconsistent with those predictions. Hypothesis testing, especially through experimentation, is at the core of the scientific process. It is how scientists gain a better understanding of how things work.

Testing a Hypothesis by Observation

Some hypotheses may be tested through simple observation. For example, a researcher may formulate the hypothesis that the sun always rises in the east. What might an alternative hypothesis be? If his hypothesis is correct, he would predict that the sun will rise in the east tomorrow. He can easily test such a prediction by rising before dawn and going out to observe the sunrise. If the sun rises in the west, he will have disproved the hypothesis. He will have shown that it does not hold true in every situation. However, if he observes on that morning that the sun does in fact rise in the east, he has not proven the hypothesis. He has made a single observation that is consistent with, or supports, the hypothesis. As a scientist, to confidently state that the sun will always rise in the east, he will want to make many observations, under a variety of circumstances. Note that in this instance no manipulation of circumstance is required to test the hypothesis (i.e., you aren't altering the sun in any way).

Testing a Hypothesis by Experimentation

An experiment is a controlled series of observations designed to test a specific hypothesis. In an experiment, the researcher manipulates factors related to the hypothesis in such a way that the effect of these factors on the observations (data) can be readily measured and compared. Most experiments are an attempt to define a cause-and-effect relationship between two factors or events—to explain why something happens. For example, with the hypothesis "roses planted in sunny areas bloom earlier than those grown in shady areas," the experiment would be testing a cause-and-effect relationship between sunlight and time of blooming.

A major advantage of setting up an experiment versus making observations of what is already available is that it allows the researcher to control all the factors or events related to the hypothesis, so that the true cause of an event can be more easily isolated. In all cases, the hypothesis itself will determine the way the experiment will be set up. For example, suppose my hypothesis is "the weight of an object is proportional to the amount of time it takes to fall a certain distance." How would you test this hypothesis?

The Qualities of a Good Experiment

• The experiment must be conducted on a group of subjects that are narrowly defined and have certain aspects in common. This is the group to which any conclusions must later be confined. (Examples of possible subjects: female cancer patients over age 40, E. coli bacteria, red giant stars, the nicotine molecule and its derivatives.)
• All subjects of the experiment should be (ideally) completely alike in all ways except for the factor or factors that are being tested. Factors that are compared in scientific experiments are called variables. A variable is some aspect of a subject or event that may differ over time or from one group of subjects to another. For example, if a biologist wanted to test the effect of nitrogen on grass growth, he would apply different amounts of nitrogen fertilizer to several plots of grass. The grass in each of the plots should be as alike as possible so that any difference in growth could be attributed to the effect of the nitrogen. For example, all the grass should be of the same species, planted at the same time and at the same density, receive the same amount of water and sunlight, and so on. The variable in this case would be the amount of nitrogen applied to the plants. The researcher would not compare differing amounts of nitrogen across different grass species to determine the effect of nitrogen on grass growth. What is the problem with using different species of plants to compare the effect of nitrogen on plant growth? There are different kinds of variables in an experiment. A factor that the experimenter controls, and changes intentionally to determine if it has an effect, is called an independent variable . A factor that is recorded as data in the experiment, and which is compared across different groups of subjects, is called a dependent variable . In many cases, the value of the dependent variable will be influenced by the value of an independent variable. The goal of the experiment is to determine a cause-and-effect relationship between independent and dependent variables—in this case, an effect of nitrogen on plant growth. In the nitrogen/grass experiment, (1) which factor was the independent variable? (2) Which factor was the dependent variable?
• Nearly all types of experiments require a control group and an experimental group. The control group generally is not changed in any way, but remains in a "natural state," while the experimental group is modified in some way to examine the effect of the variable which of interest to the researcher. The control group provides a standard of comparison for the experimental groups. For example, in new drug trials, some patients are given a placebo while others are given doses of the drug being tested. The placebo serves as a control by showing the effect of no drug treatment on the patients. In research terminology, the experimental groups are often referred to as treatments , since each group is treated differently. In the experimental test of the effect of nitrogen on grass growth, what is the control group? In the example of the nitrogen experiment, what is the purpose of a control group?
• In research studies a great deal of emphasis is placed on repetition. It is essential that an experiment or study include enough subjects or enough observations for the researcher to make valid conclusions. The two main reasons why repetition is important in scientific studies are (1) variation among subjects or samples and (2) measurement error.

Variation among Subjects

There is a great deal of variation in nature. In a group of experimental subjects, much of this variation may have little to do with the variables being studied, but could still affect the outcome of the experiment in unpredicted ways. For example, in an experiment designed to test the effects of alcohol dose levels on reflex time in 18- to 22-year-old males, there would be significant variation among individual responses to various doses of alcohol. Some of this variation might be due to differences in genetic make-up, to varying levels of previous alcohol use, or any number of factors unknown to the researcher.

Because what the researcher wants to discover is average dose level effects for this group, he must run the test on a number of different subjects. Suppose he performed the test on only 10 individuals. Do you think the average response calculated would be the same as the average response of all 18- to 22-year-old males? What if he tests 100 individuals, or 1,000? Do you think the average he comes up with would be the same in each case? Chances are it would not be. So which average would you predict would be most representative of all 18- to 22-year-old males?

A basic rule of statistics is, the more observations you make, the closer the average of those observations will be to the average for the whole population you are interested in. This is because factors that vary among a population tend to occur most commonly in the middle range, and least commonly at the two extremes. Take human height for example. Although you may find a man who is 7 feet tall, or one who is 4 feet tall, most men will fall somewhere between 5 and 6 feet in height. The more men we measure to determine average male height, the less effect those uncommon extreme (tall or short) individuals will tend to impact the average. Thus, one reason why repetition is so important in experiments is that it helps to assure that the conclusions made will be valid not only for the individuals tested, but also for the greater population those individuals represent.

"The use of a sample (or subset) of a population, an event, or some other aspect of nature for an experimental group that is not large enough to be representative of the whole" is called sampling error (Starr, Cecie, Biology: Concepts and Applications , 4 th ed. [Pacific Cove: Brooks/Cole, 2000], glossary). If too few samples or subjects are used in an experiment, the researcher may draw incorrect conclusions about the population those samples or subjects represent.

Use the jellybean activity below to see a simple demonstration of samping error.

Directions: There are 400 jellybeans in the jar. If you could not see the jar and you initially chose 1 green jellybean from the jar, you might assume the jar only contains green jelly beans. The jar actually contains both green and black jellybeans. Use the "pick 1, 5, or 10" buttons to create your samples. For example, use the "pick" buttons now to create samples of 2, 13, and 27 jellybeans. After you take each sample, try to predict the ratio of green to black jellybeans in the jar. How does your prediction of the ratio of green to black jellybeans change as your sample changes?

Measurement Error

The second reason why repetition is necessary in research studies has to do with measurement error. Measurement error may be the fault of the researcher, a slight difference in measuring techniques among one or more technicians, or the result of limitations or glitches in measuring equipment. Even the most careful researcher or the best state-of-the-art equipment will make some mistakes in measuring or recording data. Another way of looking at this is to say that, in any study, some measurements will be more accurate than others will. If the researcher is conscientious and the equipment is good, the majority of measurements will be highly accurate, some will be somewhat inaccurate, and a few may be considerably inaccurate. In this case, the same reasoning used above also applies here: the more measurements taken, the less effect a few inaccurate measurements will have on the overall average.

Step 4: Data Analysis

In any experiment, observations are made, and often, measurements are taken. Measurements and observations recorded in an experiment are referred to as data . The data collected must relate to the hypothesis being tested. Any differences between experimental and control groups must be expressed in some way (often quantitatively) so that the groups may be compared. Graphs and charts are often used to visualize the data and to identify patterns and relationships among the variables.

Statistics is the branch of mathematics that deals with interpretation of data. Data analysis refers to statistical methods of determining whether any differences between the control group and experimental groups are too great to be attributed to chance alone. Although a discussion of statistical methods is beyond the scope of this tutorial, the data analysis step is crucial because it provides a somewhat standardized means for interpreting data. The statistical methods of data analysis used, and the results of those analyses, are always included in the publication of scientific research. This convention limits the subjective aspects of data interpretation and allows scientists to scrutinize the working methods of their peers.

Why is data analysis an important step in the scientific method?

Step 5: Stating Conclusions

The conclusions made in a scientific experiment are particularly important. Often, the conclusion is the only part of a study that gets communicated to the general public. As such, it must be a statement of reality, based upon the results of the experiment. To assure that this is the case, the conclusions made in an experiment must (1) relate back to the hypothesis being tested, (2) be limited to the population under study, and (3) be stated as probabilities.

The hypothesis that is being tested will be compared to the data collected in the experiment. If the experimental results contradict the hypothesis, it is rejected and further testing of that hypothesis under those conditions is not necessary. However, if the hypothesis is not shown to be wrong, that does not conclusively prove that it is right! In scientific terms, the hypothesis is said to be "supported by the data." Further testing will be done to see if the hypothesis is supported under a number of trials and under different conditions.

If the hypothesis holds up to extensive testing then the temptation is to claim that it is correct. However, keep in mind that the number of experiments and observations made will only represent a subset of all the situations in which the hypothesis may potentially be tested. In other words, experimental data will only show part of the picture. There is always the possibility that a further experiment may show the hypothesis to be wrong in some situations. Also, note that the limits of current knowledge and available technologies may prevent a researcher from devising an experiment that would disprove a particular hypothesis.

The researcher must be sure to limit his or her conclusions to apply only to the subjects tested in the study. If a particular species of fish is shown to consume their young 90 percent of the time when raised in captivity, that doesn't necessarily mean that all fish will do so, or that this fish's behavior would be the same in its native habitat.

Finally, the conclusions of the experiment are generally stated as probabilities. A careful scientist would never say, "drug x kills cancer cells;" she would more likely say, "drug x was shown to destroy 85 percent of cancerous skin cells in rats in lab trials." Notice how very different these two statements are. There is a tendency in the media and in the general public to gravitate toward the first statement. This makes a terrific headline and is also easy to interpret; it is absolute. Remember though, in science conclusions must be confined to the population under study; broad generalizations should be avoided. The second statement is sound science. There is data to back it up. Later studies may reveal a more universal effect of the drug on cancerous cells, or they may not. Most researchers would be unwilling to stake their reputations on the first statement.

As a student, you should read and interpret popular press articles about research studies very carefully. From the text, can you determine how the experiment was set up and what variables were measured? Are the observations and data collected appropriate to the hypothesis being tested? Are the conclusions supported by the data? Are the conclusions worded in a scientific context (as probability statements) or are they generalized for dramatic effect? In any researched-based assignment, it is a good idea to refer to the original publication of a study (usually found in professional journals) and to interpret the facts for yourself.

Qualities of a Good Experiment

• narrowly defined subjects
• all subjects treated alike except for the factor or variable being studied
• a control group is used for comparison
• measurements related to the factors being studied are carefully recorded
• enough samples or subjects are used so that conclusions are valid for the population of interest
• conclusions made relate back to the hypothesis, are limited to the population being studied, and are stated in terms of probabilities

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• How to Write a Strong Hypothesis | Steps & Examples

How to Write a Strong Hypothesis | Steps & Examples

Published on May 6, 2022 by Shona McCombes . Revised on November 20, 2023.

A hypothesis is a statement that can be tested by scientific research. If you want to test a relationship between two or more variables, you need to write hypotheses before you start your experiment or data collection .

Example: Hypothesis

Daily apple consumption leads to fewer doctor’s visits.

Table of contents

What is a hypothesis, developing a hypothesis (with example), hypothesis examples, other interesting articles, frequently asked questions about writing hypotheses.

A hypothesis states your predictions about what your research will find. It is a tentative answer to your research question that has not yet been tested. For some research projects, you might have to write several hypotheses that address different aspects of your research question.

A hypothesis is not just a guess – it should be based on existing theories and knowledge. It also has to be testable, which means you can support or refute it through scientific research methods (such as experiments, observations and statistical analysis of data).

Variables in hypotheses

Hypotheses propose a relationship between two or more types of variables .

• An independent variable is something the researcher changes or controls.
• A dependent variable is something the researcher observes and measures.

If there are any control variables , extraneous variables , or confounding variables , be sure to jot those down as you go to minimize the chances that research bias  will affect your results.

In this example, the independent variable is exposure to the sun – the assumed cause . The dependent variable is the level of happiness – the assumed effect .

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Step 1. Ask a question

Writing a hypothesis begins with a research question that you want to answer. The question should be focused, specific, and researchable within the constraints of your project.

Step 2. Do some preliminary research

Your initial answer to the question should be based on what is already known about the topic. Look for theories and previous studies to help you form educated assumptions about what your research will find.

At this stage, you might construct a conceptual framework to ensure that you’re embarking on a relevant topic . This can also help you identify which variables you will study and what you think the relationships are between them. Sometimes, you’ll have to operationalize more complex constructs.

Step 3. Formulate your hypothesis

Now you should have some idea of what you expect to find. Write your initial answer to the question in a clear, concise sentence.

4. Refine your hypothesis

You need to make sure your hypothesis is specific and testable. There are various ways of phrasing a hypothesis, but all the terms you use should have clear definitions, and the hypothesis should contain:

• The relevant variables
• The specific group being studied
• The predicted outcome of the experiment or analysis

5. Phrase your hypothesis in three ways

To identify the variables, you can write a simple prediction in  if…then form. The first part of the sentence states the independent variable and the second part states the dependent variable.

In academic research, hypotheses are more commonly phrased in terms of correlations or effects, where you directly state the predicted relationship between variables.

If you are comparing two groups, the hypothesis can state what difference you expect to find between them.

6. Write a null hypothesis

If your research involves statistical hypothesis testing , you will also have to write a null hypothesis . The null hypothesis is the default position that there is no association between the variables. The null hypothesis is written as H 0 , while the alternative hypothesis is H 1 or H a .

• H 0 : The number of lectures attended by first-year students has no effect on their final exam scores.
• H 1 : The number of lectures attended by first-year students has a positive effect on their final exam scores.

If you want to know more about the research process , methodology , research bias , or statistics , make sure to check out some of our other articles with explanations and examples.

• Sampling methods
• Simple random sampling
• Stratified sampling
• Cluster sampling
• Likert scales
• Reproducibility

 Statistics

• Null hypothesis
• Statistical power
• Probability distribution
• Effect size
• Poisson distribution

Research bias

• Optimism bias
• Cognitive bias
• Implicit bias
• Hawthorne effect
• Anchoring bias
• Explicit bias

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A hypothesis is not just a guess — it should be based on existing theories and knowledge. It also has to be testable, which means you can support or refute it through scientific research methods (such as experiments, observations and statistical analysis of data).

Null and alternative hypotheses are used in statistical hypothesis testing . The null hypothesis of a test always predicts no effect or no relationship between variables, while the alternative hypothesis states your research prediction of an effect or relationship.

Hypothesis testing is a formal procedure for investigating our ideas about the world using statistics. It is used by scientists to test specific predictions, called hypotheses , by calculating how likely it is that a pattern or relationship between variables could have arisen by chance.

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Evidence of Chemical Change

Test tubes with colourful liquids (simonkr, iStockphoto)

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Learn how to tell if a chemical reaction has taken place.

Every day,  matter  undergoes changes. These changes can be classified as either physical changes or chemical changes.

Physical changes  can almost always be reversed. For example, when liquid water freezes and forms ice, you still have water. It is just in a different  state . If you dissolve sugar in a glass of water, you can evaporate the water to get back to the solid sugar. This is a special kind of physical change called a  phase change . A phase change is a change in state or form in which no new substance forms.

Our world is also full of  chemical changes  that take place every day. They help us stay warm, feed ourselves, drive cars and play games on our smartphones. Everything you see around you is either undergoing or has undergone chemical change. This includes inside of you!

A chemical change, or  chemical reaction , is a process by which one or more substances are changed into others through chemical means. This may involve a substance joining together with another substance. It may also involve a substance breaking apart into different substances. A chemical change is a change in which at least one new substance forms.

Conservation of Mass

When a chemical or physical change happens, you can observe different types of things. One thing that will always stay constant is mass. This means that the total mass of what you begin with will equal what you end up with. This is easiest to measure in a  closed system  in which no matter can escape.

For instance, let’s take a common combustion reaction such as burning a wood log. To prove the conservation of mass, you could measure the mass of the log before you burned it. After it has been burned, you could measure the mass of the ashes. But the mass of the ashes would be much lower than the mass of the log. So where did the rest of the mass go? It went into the air! Carbon dioxide gas and water vapour are products of combustion reactions. If you collected and measured the gases produced as the log burned, then you would find the missing mass.  Antoine Lavoisier  did exactly this to prove his point! We now call this idea the  Law of the Conservation of Mass .

" Nothing is lost, nothing is created, everything is transformed ".

Antoine Lavoisier (1789)

Other scientists had experimented and proven this law before Lavoisier, but Lavoisier usually gets the credit for it because he published it in what is considered the first  chemistry book . This book encouraged a shift from  alchemy  to modern chemistry. What made his work different was that he used math to describe what happened when mixing elements. His work spotlighted the need to measure and count. We can see this math in action when we balance chemical equations .

How to Tell When a Chemical Reaction May Have Occurred

We can observe many different things when a chemical reaction takes place. We might observe a change in temperature, the emission of light, a change in colour, a release of gas, or a change in the amount of  reactants  or  products . All of these observations are useful when determining whether a chemical reaction has happened.

Heat or Light

Many reactions involve a transfer of energy. In some cases the energy may be felt as heat energy or seen as light energy.  Often, both light and heat can be observed at the same time!

When a reaction gives off heat, we call this an exothermic reaction . This means that heat is transferred from the reactants to the external environment. When the opposite occurs and a chemical reaction absorbs heat from its environment, it feels cold. We call this an endothermic reaction . The emission of light from a very hot substance is called incandescence.

Many chemical reactions give off light and/or heat. A familiar example of a chemical reaction that gives off both heat and light is the combustion of propane in a gas stove or BBQs.

Sometimes light is given off without the temperature being high. We call this  luminescence . A good example of this happens when you bend a  glowstick . The bending allows two chemicals inside to react and give off light. When the light is as a result of an inorganic chemical reaction, we call it  chemiluminescence . When a living organism uses chemicals in its body to emit light, we call it  bioluminescence .

Colour Change

A  change in colour  is another sign that a chemical reaction has occurred. A good example of this is when something  rusts . Pure iron is naturally silver in colour. We don’t usually observe it this way on Earth because it reacts easily with the oxygen in the air.

The balanced chemical equation of this reaction is:

4Fe + 3O 2  + 6H 2 O → 4Fe(OH) 3

As you may already know, rust is a reddish colour. It is responsible for the beautiful colours we see in the Grand Canyon, and the colour of the soil on Mars.

Another impressive colour change reaction is that of potassium permanganate and sugar (glucose). In this reaction, the colour changes as more reactants are turned into products! But no need for lab chemicals to see this for yourself. Try this activity to see this at home!

Release of Gas

Sometimes a chemical reaction produces a gas as one of its products. When this happens, we can observe bubbles or fumes.

An example of a chemical reaction that produces a gas is the reaction between baking soda and lemon juice (citric acid). If you mix baking soda with lemon juice (or vinegar), both substances quickly react to form bubbles. The bubbles are made of carbon dioxide gas (CO 2(g) ). Carbon dioxide bubbles are also responsible for the fizz you see in soft drinks. The burning of paper or wood (a type of combustion reaction) also produces carbon dioxide gas as well as water vapour.

Misconception Alert! Be careful to not confuse a phase change with a gas formation. Water vapor formed when heating water is not evidence of a chemical change!

In some chemical reactions, the gas produced can be highly toxic. This can happen at home if certain cleaning products are mixed together. Breathing the gas created from mixing some  cleaning products can even result in death . This is why you should never use two different cleaning products together - ever!

Precipitate Formation

Sometimes when two solutions undergo a chemical reaction, one of the new products formed from the reaction is not  soluble . This means that it is not able to dissolve in the new solution. Instead, we can observe a solid that is distinct and separate from the solution. We call this solid a  precipitate . You have probably heard the word “precipitation”, but maybe not when talking about chemistry. We also use the word precipitation to refer to the types of water that fall from the sky. The solid in a precipitation reaction also often falls, or sinks, to the bottom of the solution. Precipitation reactions are all double replacement reactions .

The video below shows many examples of a formation of a precipitate, including ones in which colourless solutions form colourful solid products. Who knew that chemistry could be so beautiful! 

Living Space

Explore the optimal environmental conditions for human life. How do you think your classroom conditions compare to those on the International Space Station? Free project for grades 6-9 students.

Law of Conservation of Mass experiment  (2014) This video (6:21 min.) by Zoe Friedland shows a young person demonstrating the conservation of mass using the vinegar and baking soda experiment.

Chemical Changes Versus Physical Changes The Chem4Kids website has a section which explains the difference between physical changes and chemical changes.

Physical and Chemical Changes in the Kitchen This article from Let’s Talk Science looks at the chemical and physical changes that happen when we cook and bake.

How do we know a chemical reaction has taken place? This page offers exercises for students who want to validate their understanding of chemical and physical changes.

Can Mixing Cleaning Chemicals Kill You?  (2019) This video (5:19 min.) by Reactions explains how mixing cleaning products can emit toxic gases.

How Does Bioluminescence Work?  (2017) This video (5:15 min.) by It's Okay To Be Smart explains how living organisms can produce light.

Chemical Precipitation Reactions are Beautiful Chemistry! This video from the Science Pirate presents different precipitation reactions.

Evidence of a Chemical Reaction. (2021, February 9). Retrieved from https://chem.libretexts.org/@go/page/47498

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What Is a Chemical Reaction? Definition and Examples

Chemical reactions are the backbone of chemistry and, arguably, life itself. Understanding what a chemical reaction is, how to represent it, how to categorize it, and how to distinguish it from a physical change is vital.

What Is a Chemical Reaction?

A chemical reaction is a process in which the chemical structure of a substance changes, leading to the formation of a new substance with different properties. In other words, the reactants convert into products through the breaking and formation of chemical bonds .

Describing Chemical Reactions Using Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction. Reactants are written on the left-hand side, and products on the right, separated by an arrow indicating the direction of the reaction. Combinations of coefficients, element symbols, subscripts, and superscripts indicate the chemical formulas of the reactants and products and their quantities. For each chemical formula, the cation (positive-charged part) of a compound gets listed before the anion (negative-charged part). For example, you write NaCl for sodium chloride rather than ClNa.

A balanced chemical equation follows conservation of mass and charge. There are exactly the same number of atoms of each element on both the reactant and product sides of the equation. The net electrical charge is also the same for both sides of the equation.

Examples of Chemical Reactions

For example, here are some chemical reactions represented as chemical equations:

• The formation of water from hydrogen and oxygen: 2H 2 + O 2 → 2H 2 O
• The combustion of methane: CH 4 + 2O 2 → CO 2 + 2H 2 O
• The decomposition of calcium carbonate: CaCO 3 → CaO + CO 2

How to Recognize a Chemical Reaction

Not all changes involving matter are chemical reactions. A chemical reaction is a chemical change , which means the starting materials are chemically different from the ending materials. In contrast, matter also changes form via physical changes. But, in a physical change , the chemical identity of matter does not change.

For example, when you melt an ice cube into liquid water, the chemical identity of the ice and the water is the same (H 2 O). Melting (and any other phase transition) is an example of a physical change. No chemical reaction occurs. However, when you combine baking soda (NaHCO 3 ) and vinegar (CH 3 COOH), the two chemical undergo a chemical reaction that produces sodium acetate (NaC 2 H 3 O 2 ), water (H 2 O), and carbon dioxide (CO 2 ).

You can’t see the atoms and molecules in action and in the examples of melting ice and reacting baking soda and vinegar, you start with a transparent substance and end with one. So, how do you know which is a physical change and which is a chemical reaction? There are several indicators of a chemical change:

• Color change
• Forming a gas or bubbles
• Forming a precipitate
• Temperature change
• Releasing or absorbing light or sound
• Irreversibility (Most chemical changes are irreversible, while most physical changes are reversible.)
• Changing chemical properties

Melting ice is reversible and does not really meet the other criteria for a chemical change, so it is a physical change. Mixing baking soda and vinegar results in bubbles, a temperature change, and new chemical properties.

Types of Chemical Reactions

There are many different types of chemical reactions , but there are four main classes:

Synthesis (Combination) Reactions

• Description : Two or more substances combine to form a single product.
• General Reaction: A + B → AB
• Example : N 2 + 3H 2 → 2NH 3

Decomposition Reactions

• Description : A single compound breaks down into two or more simpler substances.
• General Reaction: AB → A + B
• Example : 2H 2 O → 2H 2 + O 2

Single-Replacement Reactions

• Description : One element replaces another element in a compound.
• General Reaction: A + BC → AC + B
• Example : Zn + 2HCl → ZnCl 2 + H 2

Double-Replacement Reactions

• Description : The cations and anions of two different molecules switch places.
• General Reaction: AB + CD → AD + CB
• Example : AgNO 3 + NaCl → AgCl + NaNO 3

Other Types of Reactions

There are many other types of reactions, such as:

• Redox Reactions : Involves electron transfer.
• Acid-Base Reactions : Involves the transfer of a proton.
• Complexation Reactions : Formation of complex ions.
• Polymerization : Formation of polymers from monomers.

Importance of Chemical Reactions

Chemical reactions are at the heart of chemistry. Understanding their mechanisms, types, and representations helps us grasp more complex concepts and applications. From the combustion that powers our cars to the metabolic reactions that keep us alive, chemical reactions are indispensable to our daily lives. Applications include:

• Medication formulation
• Making cleaners
• Making disinfectants
• Waste treatment
• Food processing
• Energy production
• Material design
• Atkins, Peter W.; Julio de Paula (2006). Physical Chemistry (4th ed.). Weinheim: Wiley-VCH. ISBN 978-3-527-31546-8.
• IUPAC (1997). Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi: 10.1351/goldbook
• Wintterlin, J. (1997). “Atomic and Macroscopic Reaction Rates of a Surface-Catalyzed Reaction”. Science . 278 (5345): 1931–4. doi: 10.1126/science.278.5345.1931
• Zumdahl, Steven S.; Zumdahl, Susan A. (2000).  Chemistry  (5th ed.). Houghton Mifflin. ISBN 0-395-98583-8.

Related Posts

Module 1: Introduction to Biology

Experiments and hypotheses, learning outcomes.

• Form a hypothesis and use it to design a scientific experiment

Now we’ll focus on the methods of scientific inquiry. Science often involves making observations and developing hypotheses. Experiments and further observations are often used to test the hypotheses.

A scientific experiment is a carefully organized procedure in which the scientist intervenes in a system to change something, then observes the result of the change. Scientific inquiry often involves doing experiments, though not always. For example, a scientist studying the mating behaviors of ladybugs might begin with detailed observations of ladybugs mating in their natural habitats. While this research may not be experimental, it is scientific: it involves careful and verifiable observation of the natural world. The same scientist might then treat some of the ladybugs with a hormone hypothesized to trigger mating and observe whether these ladybugs mated sooner or more often than untreated ones. This would qualify as an experiment because the scientist is now making a change in the system and observing the effects.

Forming a Hypothesis

When conducting scientific experiments, researchers develop hypotheses to guide experimental design. A hypothesis is a suggested explanation that is both testable and falsifiable. You must be able to test your hypothesis, and it must be possible to prove your hypothesis true or false.

For example, Michael observes that maple trees lose their leaves in the fall. He might then propose a possible explanation for this observation: “cold weather causes maple trees to lose their leaves in the fall.” This statement is testable. He could grow maple trees in a warm enclosed environment such as a greenhouse and see if their leaves still dropped in the fall. The hypothesis is also falsifiable. If the leaves still dropped in the warm environment, then clearly temperature was not the main factor in causing maple leaves to drop in autumn.

In the Try It below, you can practice recognizing scientific hypotheses. As you consider each statement, try to think as a scientist would: can I test this hypothesis with observations or experiments? Is the statement falsifiable? If the answer to either of these questions is “no,” the statement is not a valid scientific hypothesis.

Practice Questions

Determine whether each following statement is a scientific hypothesis.

Air pollution from automobile exhaust can trigger symptoms in people with asthma.

• No. This statement is not testable or falsifiable.
• No. This statement is not testable.
• No. This statement is not falsifiable.
• Yes. This statement is testable and falsifiable.

Natural disasters, such as tornadoes, are punishments for bad thoughts and behaviors.

a: No. This statement is not testable or falsifiable. “Bad thoughts and behaviors” are excessively vague and subjective variables that would be impossible to measure or agree upon in a reliable way. The statement might be “falsifiable” if you came up with a counterexample: a “wicked” place that was not punished by a natural disaster. But some would question whether the people in that place were really wicked, and others would continue to predict that a natural disaster was bound to strike that place at some point. There is no reason to suspect that people’s immoral behavior affects the weather unless you bring up the intervention of a supernatural being, making this idea even harder to test.

Testing a Vaccine

Let’s examine the scientific process by discussing an actual scientific experiment conducted by researchers at the University of Washington. These researchers investigated whether a vaccine may reduce the incidence of the human papillomavirus (HPV). The experimental process and results were published in an article titled, “ A controlled trial of a human papillomavirus type 16 vaccine .”

Preliminary observations made by the researchers who conducted the HPV experiment are listed below:

• Human papillomavirus (HPV) is the most common sexually transmitted virus in the United States.
• There are about 40 different types of HPV. A significant number of people that have HPV are unaware of it because many of these viruses cause no symptoms.
• Some types of HPV can cause cervical cancer.
• About 4,000 women a year die of cervical cancer in the United States.

Practice Question

Researchers have developed a potential vaccine against HPV and want to test it. What is the first testable hypothesis that the researchers should study?

• HPV causes cervical cancer.
• People should not have unprotected sex with many partners.
• People who get the vaccine will not get HPV.
• The HPV vaccine will protect people against cancer.

Experimental Design

You’ve successfully identified a hypothesis for the University of Washington’s study on HPV: People who get the HPV vaccine will not get HPV.

The next step is to design an experiment that will test this hypothesis. There are several important factors to consider when designing a scientific experiment. First, scientific experiments must have an experimental group. This is the group that receives the experimental treatment necessary to address the hypothesis.

The experimental group receives the vaccine, but how can we know if the vaccine made a difference? Many things may change HPV infection rates in a group of people over time. To clearly show that the vaccine was effective in helping the experimental group, we need to include in our study an otherwise similar control group that does not get the treatment. We can then compare the two groups and determine if the vaccine made a difference. The control group shows us what happens in the absence of the factor under study.

However, the control group cannot get “nothing.” Instead, the control group often receives a placebo. A placebo is a procedure that has no expected therapeutic effect—such as giving a person a sugar pill or a shot containing only plain saline solution with no drug. Scientific studies have shown that the “placebo effect” can alter experimental results because when individuals are told that they are or are not being treated, this knowledge can alter their actions or their emotions, which can then alter the results of the experiment.

Moreover, if the doctor knows which group a patient is in, this can also influence the results of the experiment. Without saying so directly, the doctor may show—through body language or other subtle cues—their views about whether the patient is likely to get well. These errors can then alter the patient’s experience and change the results of the experiment. Therefore, many clinical studies are “double blind.” In these studies, neither the doctor nor the patient knows which group the patient is in until all experimental results have been collected.

Both placebo treatments and double-blind procedures are designed to prevent bias. Bias is any systematic error that makes a particular experimental outcome more or less likely. Errors can happen in any experiment: people make mistakes in measurement, instruments fail, computer glitches can alter data. But most such errors are random and don’t favor one outcome over another. Patients’ belief in a treatment can make it more likely to appear to “work.” Placebos and double-blind procedures are used to level the playing field so that both groups of study subjects are treated equally and share similar beliefs about their treatment.

The scientists who are researching the effectiveness of the HPV vaccine will test their hypothesis by separating 2,392 young women into two groups: the control group and the experimental group. Answer the following questions about these two groups.

• This group is given a placebo.
• This group is deliberately infected with HPV.
• This group is given nothing.
• This group is given the HPV vaccine.
• a: This group is given a placebo. A placebo will be a shot, just like the HPV vaccine, but it will have no active ingredient. It may change peoples’ thinking or behavior to have such a shot given to them, but it will not stimulate the immune systems of the subjects in the same way as predicted for the vaccine itself.
• d: This group is given the HPV vaccine. The experimental group will receive the HPV vaccine and researchers will then be able to see if it works, when compared to the control group.

Experimental Variables

A variable is a characteristic of a subject (in this case, of a person in the study) that can vary over time or among individuals. Sometimes a variable takes the form of a category, such as male or female; often a variable can be measured precisely, such as body height. Ideally, only one variable is different between the control group and the experimental group in a scientific experiment. Otherwise, the researchers will not be able to determine which variable caused any differences seen in the results. For example, imagine that the people in the control group were, on average, much more sexually active than the people in the experimental group. If, at the end of the experiment, the control group had a higher rate of HPV infection, could you confidently determine why? Maybe the experimental subjects were protected by the vaccine, but maybe they were protected by their low level of sexual contact.

To avoid this situation, experimenters make sure that their subject groups are as similar as possible in all variables except for the variable that is being tested in the experiment. This variable, or factor, will be deliberately changed in the experimental group. The one variable that is different between the two groups is called the independent variable. An independent variable is known or hypothesized to cause some outcome. Imagine an educational researcher investigating the effectiveness of a new teaching strategy in a classroom. The experimental group receives the new teaching strategy, while the control group receives the traditional strategy. It is the teaching strategy that is the independent variable in this scenario. In an experiment, the independent variable is the variable that the scientist deliberately changes or imposes on the subjects.

Dependent variables are known or hypothesized consequences; they are the effects that result from changes or differences in an independent variable. In an experiment, the dependent variables are those that the scientist measures before, during, and particularly at the end of the experiment to see if they have changed as expected. The dependent variable must be stated so that it is clear how it will be observed or measured. Rather than comparing “learning” among students (which is a vague and difficult to measure concept), an educational researcher might choose to compare test scores, which are very specific and easy to measure.

In any real-world example, many, many variables MIGHT affect the outcome of an experiment, yet only one or a few independent variables can be tested. Other variables must be kept as similar as possible between the study groups and are called control variables . For our educational research example, if the control group consisted only of people between the ages of 18 and 20 and the experimental group contained people between the ages of 30 and 35, we would not know if it was the teaching strategy or the students’ ages that played a larger role in the results. To avoid this problem, a good study will be set up so that each group contains students with a similar age profile. In a well-designed educational research study, student age will be a controlled variable, along with other possibly important factors like gender, past educational achievement, and pre-existing knowledge of the subject area.

What is the independent variable in this experiment?

• Sex (all of the subjects will be female)
• Presence or absence of the HPV vaccine
• Presence or absence of HPV (the virus)

List three control variables other than age.

What is the dependent variable in this experiment?

• Sex (male or female)
• Rates of HPV infection
• Age (years)
• Revision and adaptation. Authored by : Shelli Carter and Lumen Learning. Provided by : Lumen Learning. License : CC BY-NC-SA: Attribution-NonCommercial-ShareAlike
• Scientific Inquiry. Provided by : Open Learning Initiative. Located at : https://oli.cmu.edu/jcourse/workbook/activity/page?context=434a5c2680020ca6017c03488572e0f8 . Project : Introduction to Biology (Open + Free). License : CC BY-NC-SA: Attribution-NonCommercial-ShareAlike

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Observing chemical changes

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Try this microscale practical to explore the chemical changes in displacement, redox and precipitation reactions

In this visually engaging series of experiments, students observe and identify the chemical changes that take place in a range of reactions on a microscale. The practical features displacement, redox and precipitation reactions between various salts, including potassium manganate(VII), barium nitrate and ammonium vanadate(V).

The experiments should take approximately 20 minutes.

• Eye protection
• Student worksheet
• Clear plastic sheet (eg ohp sheet)
• Magnifying glass

Solutions should be contained in plastic pipettes. See the accompanying  guidance on apparatus and techniques for microscale chemistry , which includes instructions for preparing a variety of solutions.

• Barium nitrate, 0.2 mol dm –3
• Sodium sulfate, 0.5 mol dm –3
• Lead nitrate, 0.5 mol dm –3
• Ammonia solution, 3 mol dm –3
• Ammonium vanadate(V), 0.2 mol dm –3 (acidified with sulfuric acid)
• Hydrochloric acid, 1 mol dm –3
• Sodium hydroxide, 1 mol dm –3
• Potassium manganate(VII), 0.01 mol dm –3
• Silver nitrate, 0.1 mol dm –3
• Copper(II) sulfate, 0.2 mol dm –3
• Iron(II) sulfate, 0.2 mol dm –3
• Iron(III) nitrate, 0.2 mol dm –3
• Potassium thiocyanate, 0.2 mol dm –3
• Zinc metal granules

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout (splash-resistant goggles to BS EN166 3).
• Silver nitrate, AgNO 3 (aq), 0.1 mol dm –3  is an eye irritant. Keep separate from organic waste containers. See CLEAPSS Hazcard HC087 and CLEAPSS Recipe Book RB077.
• Lead nitrate, Pb(NO 3 ) 2 (aq), 0.5 mol dm –3  is a reproductive toxin, causes eye damage, causes damage to organs (especially the CNS) and is harmful to the aquatic environment. Avoid inhalation and skin contact. See CLEAPSS Hazcard HC057a and CLEAPSS Recipe Book RB053.
• Ammonia solution, NH 3 (aq), 3 mol dm –3  is CORROSIVE. See CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006.
• Ammonium vanadate(V), NH 4 VO 3 , 0.2 mol dm –3  (acidified with sulfuric acid) is a mutagen and extremely toxic if inhaled – but not by any other route. See CLEAPSS Hazcard HC009B.
• Sodium hydroxide solution, NaOH(aq), 1 mol dm –3  is corrosive. See CLEAPSS Hazcard HC091a and CLEAPSS Recipe Book RB085.
• Copper(II) sulfate solution, CuSO 4 (aq), 0.2 mol dm –3  causes eye damage and is HAZARDOUS to the aquatic environment. See CLEAPSS Hazcard HC027c and CLEAPSS Recipe Book RB031.
• Barium nitrate, Ba(NO 3 ) 2 (aq), 0.2 mol dm –3 – see CLEAPSS Hazcard HC011 and CLEAPSS Recipe Book RB010.
• Sodium sulfate, Na 2 SO 3 (aq), 0.5 mol dm –3 – see CLEAPSS Hazcard HC098B and CLEAPSS Recipe Book RB107.
• Hydrochloric acid, HCl(aq), 1 mol dm –3 – see CLEAPSS Hazcard HC047a and CLEAPSS Recipe Book RB043.
• Iron(II) sulfate, FeSO 4 .7H 2 O(aq), 0.2 mol dm –3 – see CLEAPSS Hazcard HC055B and CLEAPSS Recipe Book RB051.
• Iron(III) nitrate, Fe(NO 3 ) 3 .9H 2 O(aq), 0.2 mol dm –3 – see CLEAPSS Hazcard HC055C and CLEAPSS Recipe Book RB052.
• Potassium manganate(VII), 0.01 mol dm –3 – see CLEAPSS Hazcard HC081 and CLEAPSS Recipe Book RB073.
• Potassium thiocyanate, KSCN(aq), 0.2 mol dm –3 – see CLEAPSS Hazcard HC095A and CLEAPSS Recipe Book RB122.
• Zinc metal granules – see CLEAPSS Hazcard HC107.

Cover the table on your worksheet with a clear plastic sheet, then follow steps 1–10 below.

• Put two drops of barium nitrate solution into box 1 (at the top of the middle column). Add two drops of sodium sulfate solution to the drops of barium nitrate solution.
• Put two drops of lead nitrate solution into box 2. Add two drops of potassium iodide solution to the drops of lead nitrate solution.
• Put two drops of iron(III) nitrate solution into box 3. Add one drop of potassium thiocyanate solution to the iron(III) nitrate solution.
• Put two drops of copper(II) sulfate solution into box 4. Add two drops of ammonia solution to the copper(II) sulfate solution.
• Put two drops of ammonium vanadate(V) solution into box 5. Add one drop of hydrochloric acid, then a small piece of zinc metal to the ammonium vanadate(V) solution.
• Put two drops of iron(II) sulfate solution into box 6. Add two drops of sodium hydroxide solution to the iron(II) sulfate solution.
• Put two drops of potassium manganate(VII) solution into box 7. Add two drops of iron(II) sulfate solution to the potassium manganate(VII) solution.
• Put two drops of barium nitrate solution into box 8. Add two drops of sodium hydroxide to the barium nitrate solution. Observe, and record any changes over the next ten minutes.
• Put one drop of silver nitrate solution into box 9. Add one drop of iron(II) sulfate to the silver nitrate solution. Observe closely using a magnifying glass.
• Put two drops of copper(II) sulfate solution into box 10. Add a small piece of zinc metal to the copper sulfate solution.

Teaching notes and expected observations

The following notes explain the expected observations for each step/reaction:

• A dense white precipitate of barium sulfate forms. Barium sulfate is used as a barium meal in medicine since it is opaque to X-rays. Because it is very insoluble it is non-toxic, unlike other, soluble, barium compounds.
• A bright yellow precipitate of lead nitrate forms. Lead nitrate is a very effective pigment but it is toxic.
• A deep red colour is produced due to iron(III) thiocyanate ions. This reaction is used to test for the presence of iron.
• A deep blue colour of tetra-amminocopper(II) forms. There may also be some light blue precipitate of copper(II) hydroxide.
• Bubbles (of hydrogen) are seen. The yellow colour of the ammonium vanadate gradually changes (as the vanadium is reduced) to blue owing to the formation of the vanadium(IV) ion (VO 2+ ). The colour then changes to green due to the vanadium(III) ion (V 3+ ) and finally to lilac due to the vanadium(II) ion (V 2+ ). The changes in oxidation states of vanadium salts have been investigated for applications in battery technology.
• A greenish precipitate of iron(II) hydroxide forms. This gradually changes to the brown iron(III) hydroxide as the iron is oxidised.
• The deep purple colour of the potassium manganate(VII) gradually fades first to the brown manganese(IV) dioxide and then to the pale pink manganese(II) ion (Mn 2+ ). Manganese(II) compounds in solution usually appear virtually colourless. However, a solid manganese(II) salt is pink.
• Barium hydroxide forms. This is soluble so nothing is seen at first. Barium hydroxide is alkaline and gradually absorbs carbon dioxide from the air to form the insoluble barium carbonate. The drop takes on a hazy appearance as a skin of barium carbonate forms on the surface.
• A glittering of metallic silver forms as the iron(III) reduces the silver nitrate. This is seen clearly using a magnifying glass.
• The surfaces of the pieces of zinc turn red-brown as copper metal deposits via a displacement reaction. The blue colour of the copper(II) sulfate solution fades.

Steps 9 and 10 in the procedure both involve the displacement of a valuable, but less reactive, metal using a less valuable, but more reactive, metal. This could be used as a topic for discussion.

Observing chemical changes - student sheet

Observing chemical changes - teacher notes, additional information.

This resource is part of our  Microscale chemistry  collection, which brings together smaller-scale experiments to engage your students and explore key chemical ideas. The resources originally appeared in the book  Microscale chemistry: experiments in miniature , published by the Royal Society of Chemistry in 1998.

© Royal Society of Chemistry

Health and safety checked, 2018

• 11-14 years
• 14-16 years
• 16-18 years
• Practical experiments
• Reactions and synthesis

Specification

• 5.5.12 use, as qualitative detection tests, the formation of precipitates of the hydroxides of Cr³⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺ and Cu²⁺ with NaOH(aq) and NH₃(aq) and,where appropriate, their subsequent dissolution;
• 5.5.13 recall the reduction of VO²⁺ (acidified ammonium metavanadate) by zinc to form VO²⁺, V³⁺ and V²⁺;
• 1.9.14 describe the test for Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺, Zn²⁺ and Mg²⁺ ions in solution using sodium hydroxide solution and ammonia solution;
• 1.9.15 describe the tests for the following: chloride, bromide and iodide (using silver nitrate solution);
• The electrochemical series as a series of metals arranged in order of their ability to be oxidised (reactions, other than displacement reactions, not required).
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• Preparatory

Lesson Explainer: Effects of Temperature and Concentration on Rates of Reactions Science • Third Year of Preparatory School

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In this explainer, we will learn how to describe and explain the effect temperature and concentration have on the rate of chemical reactions.

The speed at which a chemical reaction takes place is known as the rate of reaction. Usually, the rate of reaction describes how some variable changes over a certain period of time. A common way to measure the rate of a chemical reaction is to measure how the concentrations of the reactants and products change over a certain period of time.

Definition: Rate of Reaction

• The rate of reaction measures how reactant or product concentrations change per unit time.

The rate of a chemical reaction can be affected by many factors. By changing some of these factors, the rate of reaction can be increased or decreased.

The factors that affect the rate of reaction include surface area, temperature, concentration, and the addition of catalysts. We will focus on temperature and concentration.

In order for two particles to react, they must first collide. In addition, the particles must have a certain amount of energy when they collide.

Any factor that can increase the frequency of collisions, or the energy of the particles, will likely increase the rate of reaction.

Example 1: Identifying in Which Box of Particles the Number of Collisions Will Be Greatest

The boxes below represent a chemical reaction between the red and the blue particles. In which box will the number of collisions be greatest?

A chemical reaction occurs when reactants collide with each other. The greater the number of collisions that occur, the more likely the reaction to happen and the faster the rate of reaction.

There are several factors that can affect the rate of reaction. However, from the question and diagram, we can see that we are given four boxes each containing different numbers of particles. The size of the box is also the same in each case.

If the particles are moving randomly, then the more particles there are, the more collisions there are likely to be.

We can see from the diagram that box A contains the greatest number of particles. Therefore, the number of collisions is likely to be greatest in box A.

The answer is box A.

One way to increase the number of collisions is by increasing the temperature. As the temperature increases, the particles gain energy and move faster. The faster the particles move, the more likely they are to collide with each other.

In the diagram below, the larger the arrow, the faster the particle is moving. At higher temperatures, the particles have more energy and so a larger arrow.

The effect of temperature on the rate of reaction can easily be demonstrated in a laboratory experiment. In this experiment, one effervescent tablet is put into a flask that contains hot water and a second tablet is put into a different flask that contains cold water.

The tablet reacts with the water to produce carbon dioxide gas. The experimental setup is shown below.

By measuring the volume of gas produced in each experiment, the rates of reaction can be determined and compared.

The results of this experiment are shown in the graph below:

At the higher temperature, the particles have more energy and move around faster. This increases the number of collisions between particles and increases the rate of reaction.

A faster rate of reaction increases the volume of gas produced at the start of the reaction, resulting in a steeper line on the graph. However, as the mass of the tablet and volume of water remain constant, the final amount of gas produced is the same.

Example 2: Relating Temperature to the Frequency of Collisions between Molecules

The boxes below each contain an equal number of reactant molecules. The boxes are heated to different temperatures. Which box will have the greatest frequency of collisions between molecules?

In order for two reactant molecules to react, they have to collide. There are several factors that can increase the number of collisions between reactant molecules. One of these is temperature.

We are told that each box contains the same number of reactant molecules, so the frequency of collisions is not going to be affected by a different number of molecules. However, the temperature of each box is different, and so, the main effect on the frequency of collisions will be the change in temperature.

As the temperature increases, the reactant molecules gain energy and move faster. The faster the molecules are moving, the more likely they are to collide and the greater the frequency of collisions will be.

The higher the temperature, the greater the frequency of collisions between molecules. Looking at the diagram, we can see that the box with the highest temperature is box D. Therefore, the answer is box D.

Temperature is a very important factor for controlling the rate of reactions in food. Placing food in a cool place, such as a refrigerator or freezer, slows down the chemical reactions that spoil food. As a result, food can be preserved and last longer.

High temperatures are often used when cooking food. The higher temperature increases the rate of reaction and helps cook food quicker and more thoroughly.

The effect of concentration on the rate of reaction can be explained by looking at the frequency of collisions.

Consider the reaction between the purple particles A and the green particles B shown in the diagram below.

If the concentration of B is increased, then the number of particles of B present increases. This is shown in the diagram below.

An increase in the number of particles will result in an increase in the number of collisions. A greater number of collisions causes an increase in the rate of reaction.

The effect of concentration on the rate of reaction can be demonstrated using the reaction of iron wool and oxygen.

Iron wool, also known as steel wool, can be burned in the presence of oxygen. However, the speed and intensity of this reaction changes when the concentration of oxygen changes.

When burned over a Bunsen burner, the iron wool is being burned in air. Air contains 2 1 % of oxygen, a medium to low concentration. The rate of reaction is quite low, and the iron wool burns relatively slowly.

However, when burned in pure oxygen the reaction is much more rapid and intense. The concentration of pure oxygen is ∼ 1 0 0 % , much greater than air. The increase in oxygen concentration increases the rate of reaction and results in a more vigorous and fast reaction.

These two experiments are shown in the image below.

Example 3: Explaining the Different Rates of Combustion in Air Compared with Pure Oxygen

Why is the combustion of aluminum in air slower than in pure oxygen?

• The temperature of oxygen in air is greater than in pure oxygen.
• The temperature of pure oxygen is greater than air.
• The concentration of oxygen in air is less than in pure oxygen.
• The concentration of oxygen in air is greater than in pure oxygen.

The process of combustion usually refers to the reaction of a substance with oxygen. Here, aluminum is reacted with oxygen under two different conditions.

The combustion of aluminum in air is most likely performed using a Bunsen burner. Air usually contains around 2 1 % oxygen, a relatively low amount of oxygen.

The combustion of aluminum with pure oxygen most likely involves conditions where there is ∼ 1 0 0 % oxygen. We can see that the difference between burning in air and in pure oxygen is the amount, or concentration, of oxygen present.

From this, we can conclude that the difference in the rate of combustion is because of the different concentrations of oxygen. Our answer is therefore likely to be either C or D.

Concentration can affect the rate of reaction by changing the number of reactant molecules present. The more reactant molecules there are, the greater the number of collisions that will occur between them and the faster the rate of reaction is.

As concentration increases, the rate of reaction increases.

The combustion of aluminum in air is slower because the concentration of oxygen is lower than in pure oxygen. This statement matches with choice C, and so our answer is C.

Another experiment that shows the effect of concentration on the rate of reaction is the reaction of magnesium with hydrochloric acid.

In this experiment, one conical flask contains dilute hydrochloric acid and a different flask contains concentrated hydrochloric acid. Into each conical flask is placed an identical piece of magnesium of the same size and mass.

The chemical equation for the reaction between magnesium and hydrochloric acid is M g ( ) + 2 H C l ( ) M g C l ( ) + H ( ) s a q a q g 2 2

Therefore, by measuring the volume of hydrogen gas produced over time, any change in the rate of reaction can be determined.

The setup of this experiment is shown in the image below:

By plotting a graph of the volume of hydrogen gas produced against time, the rates of reaction for each experiment can be determined. A graph showing the rate of reaction for dilute and concentrated hydrochloric acid is shown below:

The graph shows that a greater volume of hydrogen gas is produced over a short period of time when concentrated hydrochloric acid is used. This shows that the rate of reaction increases as the concentration increases.

As the concentration of hydrochloric acid increases, the number of acid particles present increases. As a result, there is a greater number of collisions between the acid and the magnesium particles, and so, there is an increase in the rate of reaction.

Example 4: Ordering Experiments with Differing Concentration by Their Rate of Reaction

A chemist performs a series of experiments to determine the effect of concentration on the rate of a reaction. They pour an equal amount of hydrochloric acid of different concentrations into four test tubes, then they place an identical piece of magnesium ribbon into each of the test tubes. The experiment setup is shown below.

From slowest to quickest, what is the likely ordering of the rate of reaction for the four experiments?

There are several factors that can affect the rate of reaction. These include concentration and surface area. In the experiment, the volume of hydrochloric acid used is kept the same. An identical piece of magnesium is also used, and so, the surface area and mass are kept the same.

The only factor that is changing is the concentration of hydrochloric acid. The concentration is greatest for experiment D and lowest in experiment B.

For a reaction to occur, the reactant molecules must collide with each other. Increasing the number of collisions increases the rate of reaction.

When the concentration is increased, the number of acid particles present in the solution increases. The increased number of acid particles will result in a greater number of collisions and therefore a faster rate of reaction.

If the rate of reaction increases as the concentration increases, then the order of the rate reaction from slowest to quickest will correspond to the order from the lowest to the greatest concentration.

From slowest to quickest, the likely ordering is B, C, A, D, which corresponds to answer choice D. The correct answer is therefore D.

Example 5: Identifying Which Set of Conditions Gives the Greatest Rate of Reaction

In a series of experiments, a student changes both the concentration and the temperature. The conditions for each experiment are shown below. In which conical flask is the rate of reaction likely to be highest?

The rate of a reaction is affected by both temperature and concentration. For a reaction to occur, reactant particles must collide with each other. Any factor that increases the number of collisions is likely to increase the rate of reaction.

As the temperature increases, the particles are given more energy and can move faster. As a result, there is likely to be a greater number of collisions and a faster rate of reaction. Therefore, the rate of reaction increases as the temperature increases.

As the concentration increases, the number of reactant particles increases. With a greater number of particles present, there is likely to be a greater number of collisions and a faster rate of reaction. Therefore, the rate of reaction increases as the concentration increases.

From the two statements above, we can conclude that the rate of reaction is likely to be highest when both the temperature and the concentration are greatest.

In the diagram above, we can see that the highest temperature is 5 0 ∘ C and the highest concentration is 2 mol/L , which occurs in experiment C.

The rate of reaction is therefore likely to be highest for experiment C.

• For a chemical reaction to occur, reactant particles must collide with each other.
• Generally, as the number of collisions between reactant particles increases, the rate of reaction increases.
• When the temperature increases, the particles gain more energy and the number of collisions increases, causing the rate of reaction to increase.
• The effect of temperature on the rate of reaction can be seen experimentally by reacting effervescent tablets with water and measuring the volume of gas produced.
• Increasing the concentration increases the number of particles present. There is a greater number of collisions, and so, the rate of reaction increases.
• The combustion of substances such as iron wool in pure oxygen is faster than in air because the concentration of oxygen is lower in air.
• The effect of concentration on the rate of reaction can be seen experimentally by reacting magnesium with different concentrations of hydrochloric acid and measuring the volume of gas produced.

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Some chemical reactions change temperature, change color, produce a gas, or make a solid from two liquids. Try this reaction and see what it does! 

Here's what to do:

• Pour 1 tablespoon of hydrogen peroxide into a cup. Place the thermometer into the cup.
• Hold the thermometer and the cup so they do not fall over. Read the temperature and write it down as your “Starting Temperature”.
• Measure 1 teaspoon of yeast. While the thermometer is still in the cup, dump all the yeast into the cup. Gently swirl the cup while you look at the temperature.

What did you observe?

What to expect

The yeast and hydrogen peroxide will produce bubbles and the temperature will increase.

What's happening in there?

When yeast was added to hydrogen peroxide, a chemical in the yeast causes a reaction in which the hydrogen peroxide breaks apart to form oxygen gas and water. The oxygen was in the bubbles you saw. This reaction causes the temperature to go up.

What else could you try?

Try Another Temperature Changer!

Be sure to review the safety instructions on page 1 before proceeding.

• Place 1 tablespoon of vinegar in a cup. Put the thermometer in the cup. Hold the thermometer and cup so they do not fall over. Read the temperature and write it down as the “Starting Temperature”.
• Measure 1 teaspoon of baking soda. With the thermometer still in the cup, have your adult partner dump all the baking soda in the cup.

• Gently swirl the cup while looking sat the thermometer. What did you observe?

The vinegar and baking soda will produce bubbles and the temperature will decrease.

When the baking soda was added to vinegar, a chemical reaction takes place that produces carbon dioxide gas. This reaction causes the temperature to go down.

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