burning candle under a jar experiment

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Rising Water Experiment

Light a fire under middle school science and heat it up! Place a burning candle in the water and watch what happens to the water. Explore how heat affects air pressure for an awesome middle school science experiment. This candle and rising water experiment is a great way to get the kiddos thinking about what is happening. We love  simple science experiment s; this one is super fun and easy!

Candle in Water

This candle experiment is a great way to excite your kiddos about science! Who doesn’t love watching a candle? Remember, adult supervision is required, though! We love  simple science experiment s; this one is super fun and easy!

This science experiment asks a few questions:

• How is the candle flame affected by placing a jar over the candle?
• What happens to the air pressure inside the jar when the candle goes out?

💡 Make sure to check out all our chemistry experiments and physics experiments !

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Candle in a Jar Experiment

You need to change one variable if you want to extend this science experiment or use the  scientific method  for a science fair project .

EXTEND THE LEARNING: You could repeat the experiment with candles or jars of different sizes and observe the changes.

💡Learn more about the scientific method for kids here .

• Middle School Science
• Elementary Grades Science
• Tea light candle
• Bowl of water
• Food coloring (optional)

Instructions:

STEP 1: Put about a half inch of water into a bowl or tray. Add food coloring to your water if you like.

STEP 2: Set a tea candle in the water and light it.

ADULT IS SUPERVISION REQUIRED!

STEP 3: Cover the candle with a glass, setting it in the bowl of water.

Now watch what happens! Do you notice what happens to the level of water under the jar?

Why Does the Water Rise?

Did you notice what happened to the candle and the water level? What’s happening?

The burning candle raises the air temperature under the jar, and it expands. The candle flame uses up all of the oxygen in the glass, and the candle goes out.

The air cools because the candle has gone out. This creates a vacuum that sucks up the water from the outside of the glass.

It then raises the candle up on the water that enters the inside of the glass.

What happens when you remove the jar or glass? Did you hear a pop or popping sound? You most likely listened to this because the air pressure created a vacuum seal, and by lifting the jar, you broke the seal, resulting in the pop!

More Fun Science Experiments

Why not also try one of these easy science experiments below?

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Burning Candle Rising Water Experiment

• December 3, 2020
• 5-6 Year Olds , 7-9 Year Olds , Chemistry , Fire Science , Physics , Rainy Day Science

In our previous experiment , we discussed the candle covered with glass. The basic science behind was the oxygen limitation that made the candle go off.

In an extension of that science activity, I am now going to share another experiment with you. It is also to do with candles and glass, but with a twist.

Apart from the oxygen limitation that puts the candle off, there is also low pressure created in the glass that leads to a vacuum.

This will cause some effects and that looks like magic to kids but the science to all adults. So let us do this magic to our kids and also explain them some science.

Things required

• Ceramic or glass plates
• Glass tumbler
• Matchbox with stick

Steps involved

Fill the plate with water.

Place the candle on the plate and lit it. You can see the candle glowing brightly.

It may float or stand on the water in the plate based on the weight of the candle.

The presence of water does not make any difference to the candle at this stage.

After sometime invert the glass tumbler and place it on the glowing candle.

Imagine the glass will close the candle. In a few minutes, you can witness candle blowing off as the closed glass limits oxygen in the space surrounding the candle.

Another thing you will witness is now the water in the plate enters the glass and you will see the level rising constantly.

Science Behind Candle Rising Water

The basic science here is the lack of oxygen puts off the candle in step 2. At the same time lack of oxygen also lowers the atmospheric pressure and creates a vacuum.

This leads to the water entering the glass from the plate. You can see this like the water level rises in the glass.

Detailed science with chemical equations

The water level rises to 1/10th of the glass before the candles go off is importantly you must note.

There is no air bubble formed. The water level will stay for few minutes once the candle goes off completely.

So both the candle dies out and water rises happens concurrently.

Chemical equation

Oxygen + Candle (wax/paraffin) à Water and Carbon dioxide

O 2 + C n H 2n+2 à H 2 O + CO 2

I have an exercise for older kids here. Yes, ask your older kids to balance the chemical equation taking n as 1. Post the answers for learning.

The oxygen is 2 times more than the carbon dioxide released and hence the air volume reduces.

Let me also explain the physics behind this experiment for physics fans!

Physics facts

The burning candle produces heat which heats the air and thus expansion happens. This will cancel the oxygen depletion slowly and the water level remains down.

When oxygen gets saturated in the glass the candle goes off and the air begins to cool and volume decreases.

The reduction in air pressure will create a vacuum and hence water level rises.

Also, water initially is in the gas state when there is heat around and later it condenses and rises in level as water.

The same process or science is applied to how storms and hurricanes are formed.

When the sun heats up the air causing its density become low which is the reason for formation of wind and storms.

The high density air moves into the lower density air pockets. When there is enough wind referred to as ‘hurricanes’ causing the water rise and lifted up out of the ocean.

• This experiment is magic for kids aged 3 to 5.
• You can teach some science to kids 5 – 8 as they will know about oxygen etc.
• 8 -12-year-old kids can benefit from the chemical equations and the detailed science of this experiment.

As I always alert you, please make sure to assist or monitor kids when they do this experiment.

It involves fire and you must be around to avoid any accident. Also, dealing with glass dishes needs support which you must provide.

Depending on the age of your kid you decide whether you must take their help or help them or just be around. However, we advise you to be present irrespective of the kid’s age.

Interested in More Air Pressure Experiments? Explore the list below

DIY Drip Drop Water Bottle

Balloon Powered Car

Make a Balloon Rocket

We have tried answering a few usual questions that might arise in the kid’s mind. My little one always ask questions upon which I fumble many times. So here you go with ready-made answers as well.

Twice the time of oxygen is burnt than the available CO2 that decreases the air pressure and hence water level rises. The air cools soon after flames go off and the molecules slow down making the water vapor condense to moisture.

The heat of the flame will start melting the wax near the wick and the liquid wax is drawn up to the wick due to capillary action. The flames heat will vaporize the wax and break them into molecules of oxygen and carbon.

The candle is put off by placing the lid on the candle that is lit. It is another way to extinguish the candle. The lid is placed on the flame which immediately cuts off the oxygen and thus puts off the candle.

The wick gets close to the glass the wax burns off and heats the glass. This might lead to the explosion of the glass. However, when water is kept on the glass this explosion is prevented.

As long as the pressure is low the water rises and when the pressure level equalizes the water level stops rising.

Place the glass flat on the plate to prevent air bubble forming. In case if it is tilted, then the air bubbles will form due to the difference in the pressure level between the inside and outside surface.

When you observe the tall and short burning candles closed with a glass container, surprisingly the tall candle goes out first because the carbon dioxide released travels upwards and suffocates the tall candle making the cold air sink. The short candle utilizes the oxygen in this cold air and stays on for some more time. When all the oxygen is used up, the short candle also dies.

Yes, place a candle in the bowl containing water and lit it. Slowly it goes down melting the wax which forms a protected wax around the wick. This allows the candle to stay on for good amount of time even though the flame has reached the level lower than the water. And of course, after a while water gets into wick and turns the flame off.

Modifications you can try with this experiment

 Here are the few variations to further explore the scientific concepts in this experiment.

• Change the amount of water taken in the plate and observe how does it affects the water level rise.
• Discuss on what happened to the water when the candle is lit.
• How does temperature changes happen when we use different types of colored water?
• You can use colored water made of food coloring, milo, liquid dyes, powdered dye etc.
• Experiment on hot water versus cold water and observe the temperature and air pressure changes.
•  Also try the experiment using two candles versus one candle or more candles etc.
• Use different liquids instead of water and check what are the changes and results.
• Try with different candle weight and height
• Change the glass to narrow and broad
• Make colored water and also increase/lower the water level in the plate
• Try not to lit a candle before and light it only after placing the glass. Yes, you need to lift it a bit and light it. Preheating is avoided to observe for any changes in the results.

Share the results with us that will let all our readers know what happens with all these modifications. In the meantime, I will also try different twists with my kids and post my experience.

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Burning Candle Experiment With Water: Watch The Smokey Glass “Suck” Water In!

Cover a burning candle with glass and the surrounding water goes up inside the glass.

This burning candle experiment with water is pretty neat and one that will amaze many kids.

If you like quick experiments that you can do without buying a science kit for kids , this one is a good one to start with.

Before we move on, just a little reminder. This educational activity uses fire, so adult supervision is recommended.

With that out of the way, vamos a empezar! (Let’s start!)

oxygen, air pressure, candle experiments, water experiments

Doing This Experiment In The Gally Kids Headquarters!

As usual, we started this activity with colored water.

This is optional. But you get to see the water better when it’s a different color.

We chose  red.

Then we followed the instructions which are all straightforward.

The first try wasn’t exactly a success as there was very little water that went  into the glass.

We gave it another try and this time, it was a lot better. It was what we wanted and expected.

Then of course, we couldn’t stop there. We wanted to see if more water would get in if we put in another source of heat. So we put the lighted match in with the candle.

There didn’t seem to be much of a difference in the water level.

The smokey effect is pretty cool.

So we were happy with it.

Burning Candle Experiment With Water Materials

You will need:

• water (colored ones are the best)

Burning Candle Experiment With Water Procedure

• First, pour water into the plate.
• Next, put the candle in the middle of the plate.
• Then, light up the candle. Wait 30-seconds to 1 minute to make sure the candle is properly burning.
• When it’s ready, cover the candle with the glass.
• The candle will continue burning for a few seconds. But then when the candle turns off, this happens.
• The water rises inside the glass!

Why Does The Water Rise In The Candle Experiment?

The water rises in this candle because of air pressure.

But that’s not all there is to it.

What happens is that when you cover the candle with the glass, the air inside it expanded because of the heat. But then when the candle burned off, the air inside it got colder. Cold air contracts so this left a space inside the glass. This also allowed the air inside the glass to have less pressure.

But, the air outside has more pressure. So it pushes the water in until the pressure inside and outside are the same.

Pretty cool and fun activity that educates as well!

Video: Water Rises Up Inside The Glass

Here’s the video of the burning candle experiment with water which we posted on our Gally Kids channel in Youtube . Don’t forget to visit us there! And if you want to get notified every time we post a new video, make sure you subscribe!

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Candle burning investigation

By Tim Jolliff

• No comments

Engage learners in the experimental process as they develop a hypothesis and plan an investigation

This resource accompanies the article Understanding the hypothesis , part of the Teaching science skills series, from Education in Chemistry.

Learning objectives

1 Make predictions using your scientific knowledge and use them to form a hypothesis.

2 Plan an appropriate investigation to test your predictions.

Download this

Download the slides as PowerPoint and pdf , teacher notes as Word and pdf and student worksheet as Word and pdf .

Introduction

Experiments ‘to show’ can be frustrating for learners if they already know what the experiment will show. The approach here is to change the experiment into one where they will not know what happens.

Learners have to use critical thinking to evaluate the alternative suggestions in the concept cartoon. They are then asked to formulate a hypothesis based on their own ideas about what will happen. They will need experimental data in order to test their prediction.

As learners are asked to plan an investigation to test their hypothesis they are prompted to think about what makes a fair test and how to get reliable data.

How to use the resource

This could be used to follow on from a class investigation into the effect of beaker size on the length of time the candle burnt. The slides can be used to guide a class discussion, in combination with or as an alternative to the worksheets. Give learners time to read the concept cartoon and consider their own ideas, then discuss and work towards agreeing on a hypothesis to test.

Alternatively, learners can work through the worksheet in groups, or independently as a homework task.

If the learners plan and carry out their own investigations this will be an activity for a whole lesson or even two. Otherwise all or part can be used as an activity at the start or end of a lesson.

There is an opportunity to evaluate some real experimental data (this might motivate learners to carry out the experiment to obtain better evidence). Learners are then asked to briefly think about the difficulties of showing only slight effects in results, as in medical research.

The follow-up task asks learners to use creative skills to produce their own concept cartoons. You can show learners more examples of concept cartoons from  our collection .

Differentiation

This activity was created to be challenging, requiring learners to use critical thinking. By structuring as a class discussion you can use the discussion guidance in the teacher notes to offer prompts or ask questions to help guide learners needing more support. Alternatively, asking learners to work in small groups for some parts of the activity will give them a chance to support one another. More confident learners could complete the worksheet independently.

Answers and discussion guidance can be found in the teacher notes.

More resources

• Use  research-based tips  to help learners form scientific questions. 
• Discover ideas for open investigations where learners can practise developing hypotheses with activities from  In search of solutions . 
• Explore the reactivity series of metals and displacement reactions with these  experiments and videos for 14–16 students , use the pause-and-think questions to discuss what is being tested and why. 
• Find out how  senior principal scientist, Misbah  uses computers to help make and test predictions about catalysts.

Candle investigation: hypothesis and planning - presentation

Candle investigation: hypothesis and planning - teacher notes, candle investigation: hypothesis and planning - student worksheet, additional information.

This resource was originally created by the author as part of our Chemistry for the gifted and talented collection. It has been updated and made more accessible.

• 11-14 years
• 14-16 years
• Presentation
• Reactions and synthesis
• Communication skills
• Able and talented

Specification

• WS.2.1 Use scientific theories and explanations to develop hypotheses.
• WS.2.2 Plan experiments or devise procedures to make observations, produce or characterise a substance, test hypotheses, check data or explore phenomena.
• WS2.1 Use scientific theories and explanations to develop hypotheses.
• WS2.2 Plan experiments or devise procedures to make observations, produce or characterise a substance, test hypotheses, check data or explore phenomena.
• 2a Use scientific theories and explanations to develop hypotheses
• 2b Plan experiments or devise procedures to make observations, produce or characterise a substance, test hypotheses, check data or explore phenomena
• IaS1.1 in given contexts use scientific theories and tentative explanations to develop and justify hypotheses and predictions
• IaS1.6 plan experiments or devise procedures by constructing clear and logically sequenced strategies to: make observations, produce or characterise a substance, test hypotheses, collect and check data, explore phenomena
• WS.1.2a Use scientific theories and explanations to develop hypotheses
• WS.1.2b Plan experiments or devise procedures to make observations, produce or characterise a substance, test hypotheses, check data or explore phenomena
• applying knowledge of chemistry to new situations, interpreting information and solving problems
• planning or designing experiments to test given hypotheses or to illustrate particular effects, including safety measures
• use scientific theories and explanations to develop hypotheses
• plan experiments or devise procedures to make observations, produce or characterise a substance, test hypotheses, check data or explore phenomena
• use scientific theories and explanations to develop hypotheses;
• plan experiments or devise procedures to make observations, produce or characterise a substance, test hypotheses, check data or explore phenomena;
• 2. Recognise questions that are appropriate for scientific investigation, pose testable hypotheses, and evaluate and compare strategies for investigating hypotheses.
• 3. Design, plan and conduct investigations; explain how reliability, accuracy, precision, fairness, safety, ethics, and the selection of suitable equipment have been considered.
• 4. Produce and select data (qualitatively/ quantitatively), critically analyse data to identify patterns and relationships, identify anomalous observations, draw and justify conclusions.
• 5. Review and reflect on the skills and thinking used in carrying out investigations, and apply their learning and skills to solving problems in unfamiliar contexts.

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Mocomi Kids

Under Water - Science Experiments for Kids | Mocomi

• Experiments

https://mocomi.com/embed/content.php?c=4597|Candle Under Water|https://mocomi.com/candle-under-water/

Candle Under Water from Mocomi

Candle Under Water

Requirements:-.

• A glass cup
• Coloured ink
• Kitchen Lighter

WHAT TO DO:-

Pour some water into the saucer and then add a few drops of colour to it and mix well. Now, light up the candle and place it in the middle of the saucer and then cover it up with the cup.

WHAT HAPPENS:-

With the flame still burning inside the cup, some of the water starts collecting inside the glass, coming in from outside, and when the flame goes off there is a much bigger gush of water into the glass.

WHY THIS HAPPENS:-

The flame uses up the oxygen in the cup, creating low pressure. Atmospheric pressure then tries to force its way in. Becuase the water at the base of the cup has create an airtight seal, the pressure difference between the inside and outside of the cup causes the water to rise till the pressure (inside the cup) is equal to the pressure outside. TRY THIS:- Try this experiment with different sizes of candles to see if more water is absorbed into the glass.

For more such science experiments and articles, visit  https://mocomi.com/learn/science/ .

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Cancel reply.

What are we proving through this

Hi Rere, The hypothesis could be pressure differences inside and outside a receptacle, will equalise. Thanks for writing in.

what is the hypothesis for this under water candle project?

As the water level rises where the candle is kept Mostafa, so part of the candle is under water.

Why the name of Exp. Candle under water????

Thanks, Mostafa.

what a wonderful project!

Thanks Koree!

this is useful thanks koree aged 12

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Lit Candle Covered With a Glass

Author: ChemistryViews.org

What happens if you cover a lit candle in a water basin with a glass? The glass becomes foggy on the inside and the flame fades and eventually goes out. Then the water level in the glass rises.

The burning candle produces carbon dioxide and water in the form of water vapor. The glass becomes foggy due to this water. The flame goes out, of course, from a lack of enough oxygen in the glass.

And why does the water level rise? The air is heated while the glass moves downwards. It expands and escapes before the glass touches the water. If the air in the glass cools down and its volume decreases, a negative pressure is created inside the glass. The water is pressed into the glass by the greater external air pressure.

This video is part of:

• Chemistry Advent Calendar 2016 , ChemViews Mag. 2016 . DOI: 10.1002/chemv.201600101

Very helpful explanation thank u very much.

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Candle chemistry

Experiment with the chemistry of candles and make a flame jump through the air.

Make  a blown out candle relight as if by magic.

ExpeRiment  to find out how long a candle will burn in different amounts of air.

Learn  about the chemistry of how a candle burns.

About this activity

Lisa and Josh make a candle relight as if by magic. They investigate how long it takes for a covered candle to go out, and find out why a candle can keep burning for a longer time in a larger jar than in a small one.

In this fun, free science experiment to do at home with young children, Lisa shows Josh how to relight a candle without touching the wick. When a candle is blown out, the wick stays hot, and wax continues to be drawn up through it before evaporating. This wax gas above the candle can be relit, meaning that a flame will appear to jump from Lisa’s lighter to the candle wick.

Josh times how long it takes for candles to go out when covered by different sized jars. A candle flame is the result of a chemical reaction between wax gas and oxygen in the air. When you trap the candle in a jar, it only has a limited amount of oxygen. Josh finds out that in larger jars, there’s more oxygen so the candle can keep burning for longer, but that the flame will eventually go out.

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Why does the water rise?

It's a very popular experiment ( eg ), from elementary school : put a burning candle on a dish filled with water, cover the candle with an inverted glass: after a little while, the candle flame goes out and the water level inside the glass rises.

The standard explanation (as I recall it) was that combustion "burns" oxygen, and the consummed volume accounts for the extra water that goes inside the glass. Is this correct? I remember feeling (years later) uncomfortable with the explanation, because "to burn" is certainly not "to dissapear": I thought that oxygen combustion produces (mainly) $CO_2$ and hence one oxygen molecule would produce another $CO_2$ molecule, and the volume would remain basically the same. Perhaps $CO_2$ dissolves into the water? I would doubt that.

To add to my confusion, others state that the main cause is not the oxygen combustion but the changes of air temperature, that decreases when the flame goes out and makes the air inside the glass contract... which would rather invalidate the experiment as it was (and is) traditionally taught to students.

What is the right explanation?

(image from here )

Update : As from webpage linked in accepted answer, there are several effects here, but it's fair to say that the "traditional" explanation (consumption of oxygen) is wrong. Oxygen (plus paraffin) turn into $CO_2$ (plus water) (a representative reaction: $C_{25}H_{52}+38O_2 \to 25CO_2+26H_2O$ ). This would account for a small reduction in volume ( $25/38 \approx 2/3$ ), even assuming that this is the complete chemical picture (it's not) and that water condenses ( $CO_2$ dissolves in water poorly and very slowly). The main cause here is thermal expansion-contraction of air.

• home-experiment
• physical-chemistry

• $\begingroup$ Is there a stackexchange for chemistry? Maybe they could provide better help. $\endgroup$ –  Lemon Commented Jan 4, 2012 at 1:58
• $\begingroup$ @jak Not yet. $\endgroup$ –  Manishearth Commented Mar 15, 2012 at 7:21
• $\begingroup$ @Manishearth Yes there is - chemistry.stackexchange.com It is in beta, though. $\endgroup$ –  Dave Coffman Commented Jul 28, 2014 at 22:06
• $\begingroup$ @DaveCoffman look at the date on that comment. I moderate Chem.SE, I know about it :P $\endgroup$ –  Manishearth Commented Jul 28, 2014 at 22:28
• $\begingroup$ Geez - Sorry about that. $\endgroup$ –  Dave Coffman Commented Aug 2, 2014 at 18:19

3 Answers 3

I found two web pages that explain the phenomenon quite well, and even looks into the misconceptions people have.

The candle flame heats the air in the vase, and this hot air expands. Some of the expanding air escapes out from under the vase — you might see some bubbles. When the flame goes out, the air in the vase cools down and the cooler air contracts. The cooling air inside of the vase creates a vacuum. This imperfect vacuum is created due to the low pressure inside the vase and the high pressure outside of the vase. We know what you're thinking, the vacuum is sucking the water into the vase right? You have the right idea, but scientists try to avoid using the term "suck" when describing a vacuum. Instead, they explain it as gases exerting pressure from an area of high pressure to an area of low pressure. A common misconception regarding this experiment is that the consumption of the oxygen inside of the bottle is also a factor in the water rising. Truth is, there is a possibility that there would be a small rise in the water from the flame burning up oxygen, but it is extremely minor compared to the expansion and contraction of the gases within the bottle. Simply put, the water would rise at a steady rate if the oxygen being consumed were the main contributing factor (rather than experiencing the rapid rise when the flame is extinguished). (1)

The page from Harvard goes into more detail on the argument versus the error for the incorrect statement.

Argument : Oxygen is replaced by Carbon dioxide. So, there is the same amount of gas added than taken away. Therefore, heat alone most be responsible for the water level change. Source of the Error : A simplified and wrong chemical equation is used, which does not take into account the quantitative changes. The chemical equation has to be balanced correctly. It is not true that each oxygen molecule is replaced by one carbon dioxide molecule during the burning process; two oxygen molecules result in one carbon dioxide molecule and two water molecules (which condense). Remember oxygen is present in the air as a diatomic molecule. [A reader clarifies the water condensation in an email to me as follows: If the experiment were done with the sealing fluid able to support a temperature greater than 212 F and the whole system held above this temperature then the water product of combustion would remain gaseous and the pressure within the vessel would increase as a result of three gaseous molecules for every two prior to combustion and the sealing fluid would be pushed out.] Argument : Carbon dioxide is absorbed by the water. Thats why the oxygen depletion has an effect. Source of the Error : This idea is triggered from the fact that water can be carbonized or that the oceans absorb much of the carbon dioxide in the air. But carbon dioxide is not absorbed so fast by water. The air would have to go through the water and pressure would need to be applied so that the carbon dioxide is absorbed during the short time span of the experiment. Argument : The experiment can be explained by physics alone. During the heating stage, air escapes. Afterwards, the air volume decreases and pulls the water up. Source of the Error : the argument could work, if indeed the heating of the air would produce enough pressure that some air could leave. In that case, some air would be lost through the water. But one can observe that the water level stays up even if everything has gone back to normal temperature (say 10 minutes). No bubbles can be seen. Argument : It can not be that the oxygen depletion is responsible for the water raising, because the water does not rise immediately. The water rises only after the candle dims. If gas would be going away, this would lead to a steady rise of the water level, not the rapid rise at the end, when the candle goes out. Source of the Error : It is not "only" the oxygen depletion which matters. There are two effects which matter: the chemical process of the burning as well as a physical process from the temperature change. These effects cancel each other initially. Since these effect hide each other partially, they are more difficult to detect. (2)

It clearly has more to do with the temperature differences than any conversion of gases. Especially considering that a volume of oxygen and carbon dioxide will be nearly identical to human eye observation.

• 4 $\begingroup$ I'd trust Harvard (second footnote I am guessing). $\endgroup$ –  Skava Commented Jan 4, 2012 at 3:11
• 2 $\begingroup$ Yes "Skava", now go to bed! $\endgroup$ –  Larian LeQuella Commented Jan 4, 2012 at 3:12
• 3 $\begingroup$ This answer is useful in pointing the best explanation I've seen (the second link), but the text is plainly copied other pages (should be formatted as quotes) and does not make clear the general summary/conclusion. $\endgroup$ –  leonbloy Commented Jan 4, 2012 at 13:49
• $\begingroup$ I'd question one thing from that answer, though: Nowhere is a vacuum created. There's always air in the glass, and it always fills the whole space not occupied by water. When the air cools down, it doesn't contract by itself, only its pressure goes down (intuitively: Since the molecules get slower, they hammer less onto the water surface). As result the water is pressed more in by the air outside than out by the air inside, and thus flows inside. This rising water compresses the air inside, which causes air density and thus pressure inside to rise again until equilibrium is reached. $\endgroup$ –  celtschk Commented Jan 18, 2012 at 5:47
• 1 $\begingroup$ The second quotation seems to contradict the first one: first says "you might see some bubbles", the second one: "No bubbles can be seen". $\endgroup$ –  Ruslan Commented Jul 4, 2018 at 9:25

I have not actually tried this experiment, but I will make at least a few observations:

Hypothesis 1: The burning of oxygen is responsible for the reduced air pressure.

Prediction - if the burning of oxygen is the sole cause of the change in pressure, we should expect to see the water in the glass rise at a more or less constant rate from the moment the environment is sealed until the burning stops. After the candle extinguishes, there should be no more change in water level.

Hypothesis 2: The reduction in temperature after the candle extinguishes is responsible for the reduced air pressure.

Prediction - if the temperature change is the sole cause of the change in pressure, we should expect to see no change in water level while the candle is burning (in the limit that the glass was lowered very slowly). After the burning stops, the water should rise at a rate related to the temperature drop and eventually stop as the experimental setup comes to room temperature.

In order to test which explaination is correct, you should be able to merely perform the experiment and match the observation with the prediction. Of course, in real life it may be a combination of these two factors or perhaps include other reasons not listed here.

Additional measures such as putting an oxygen indicator in the glass (say a fresh slice of apple) or a thermometer would provide further insight.

• 1 $\begingroup$ As oxygen is burned - how many moles of CO2 do you get for each mole of O2 used? $\endgroup$ –  Martin Beckett Commented Jan 3, 2012 at 23:15
• 1 $\begingroup$ @MartinBeckett: Not to mention it's mostly carbon monoxide because it's imperfect burning. $\endgroup$ –  Mike Dunlavey Commented Jan 4, 2012 at 3:15
• 1 $\begingroup$ @MartinBeckett: The pertinent equation seems to be something like $C_{25} H_{52} + 38 O_2 => 25 C O_2 + 26 H_2 O$. So for 1 mole of oxygen we have 0.65 moles of $C O_2$ - a moderate reduction, and this assuming water condenses. $\endgroup$ –  leonbloy Commented Jan 4, 2012 at 14:40
• 1 $\begingroup$ @leonbloy - although with a smoky candle you do get a lot of CO. Plus since O2 is only 20% of air it would at most be a (1-0.65)*0.21 = 7% change in volume even with full combustion $\endgroup$ –  Martin Beckett Commented Jan 4, 2012 at 16:26
• $\begingroup$ @MartinBeckett: you are right, of course. See the Harvard link in the other answer for the complete picture. $\endgroup$ –  leonbloy Commented Jan 4, 2012 at 16:36

I will make this into an answer because the idea behind this question is used in an ancient medical metho d which was still used by practical nurses and even prescribed by old fashioned doctors when I was a child more than half a century ago in Greece. It is now used in alternative medicine practices

The air inside the cup is heated and the rim is then applied to the skin, forming an airtight seal. As the air inside the cup cools, it contracts, forming a partial vacuum and enabling the cup to suck the skin, pulling in soft tissue, and drawing blood to that area.

I think it was the invention of antibiotics which diminished rapidly its use, which was mainly for bronchitis pneumonia and similar afflictions, at least in Greece.

As far as the question goes, no liquids to confuse the issue of its being a strongly temperature dependent effect.

• $\begingroup$ Indeed, the practice is known as "cupping" and is often offered at spas and other health resorts. $\endgroup$ –  AdamRedwine Commented Jan 4, 2012 at 13:15
• $\begingroup$ +1 In spanish: "ventosa". I've seen it applied by my grandmother many years ago. $\endgroup$ –  leonbloy Commented Jan 4, 2012 at 13:37

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The burning candle - rising water experiment

This is a classic experiment. On a plate with a little water place a candle and light it. Place a drinking glass over it. The light goes out while sucking up the water.

At least two things happen:

• The oxygen O₂ is changed to CO₂
• The air is heated up a few degrees
• Water is sucked up

But then I tried to change the experiment with a vase (big glass) and opening upwards and place the candle at the bottom. Then lit the candle and place plastic over the opening. The result was that the light went out but it didn't suck down the plastic. So it there is no vacuum in the glass? A strange thing is that the plastic was pulsing like it was breathing. It can be seen here in my video https://youtu.be/aolz8Y27Lsk

Why is the experiment so different from the other?

• experimental-chemistry

• $\begingroup$ Obviously the plastic didn't completely seal the vase. $\endgroup$ –  MaxW Commented Mar 19, 2020 at 19:10
• $\begingroup$ Your chemical description is incomplete. $\endgroup$ –  Karl Commented Mar 19, 2020 at 19:41
• $\begingroup$ There must be a leak somewhere, because the gas volume does not change during the combustion. And the volume must change during the combustion. It must increase or decrease, as explained by Karsten Theis $\endgroup$ –  Maurice Commented Mar 19, 2020 at 19:54
• $\begingroup$ The seal on your vessel has to be perfect for this to work. An inverted glass vessel achieves this; it is very, very unlikely that a plastic seal will do so. $\endgroup$ –  matt_black Commented Mar 20, 2020 at 10:49

2 Answers 2

Changes in volume

The chemical reaction for burning a candle is something like this:

$$\ce{C25H52 + 38 O2(g) -> 25 CO2(g) + 26 H2O(g)}$$

For every 38 dioxygen molecules used, you are making 25 carbon dioxide molecules and 26 water molecules (which start out as a gas, but will condense once they reach an area of lower temperature such as the glass surface). So once the flame is out and the water has condensed, the volume should be less than that of the fresh air (20 % of the volume initially are oxygen. Its volume will be replaced by that of carbon dioxide, so the volume will go from 100% to 25/38 * 20% + 80% = 93%). While the candle is burning, however, the temperature is higher, and some of the water will be in the gas phase. Also, while the air was already hotter than room temperature before the container was covered, the temperature will rise as soon as the influx of cooler (fresh) air is cut off.

How hot will it get?

A tea light produces heat at about 30 J/s, and (with a molar heat of combustion of about 15,000 kJ / mol) used up $\pu{80 \mu mol}$ oxygen per second. This means you need about 10 mL of fresh air each second the candle burns. The container has a volume of about 400 mL, so the candle should burn <40 seconds. In the video, it burned about 2 minutes (not bad for an estimate). Using up all the oxygen in the container would give off about 1.2 kJ, which is sufficient to raise the temperature by 1000 degrees Celsius if it were isolated. As it is, most of the heat flows into the container.

What is different when using water?

Water has a higher heat capacity, so it is more efficient in cooling down the gas (and the container). As a consequence, the water level will rise appreciably. As MaxW mentions in the comments, the plastic probably did not seal perfectly. The pulsing could be intermittent flow of gas, or temperature fluctuations because of mixing.

• 1 $\begingroup$ So in the next experiment I will 1) put water at the bottom to increase coolant 2) light the candle and cover immediately 3) use more heat resistant plastic 4) use better and more rubber bands. Goal: To see the plastic cover be sucked in the container (will that happen?) (Sorry for making such a lousy experiment) $\endgroup$ –  hschou Commented Mar 19, 2020 at 22:06
• 1 $\begingroup$ @hschou Instead of a candle, you could use some steel wool soaked in vinegar. It will use up the oxygen without making carbon dioxide (the oxygen ends up being rust). This experiment removes two complications - the carbon dioxide and the large changes in temperature. But I encourage you to try your 2nd experimental design as well - the first try is always lousy, and you only learn by repeating experiments. $\endgroup$ –  Karsten ♦ Commented Mar 20, 2020 at 0:17

Why do most people want to believe that on burning fuel little or no carbon monoxide is created?

Yes, it could be true, mainly CO2 is formed, but read these qualifying comments from a source :

A properly designed, adjusted, and maintained gas flame produces only small amounts of carbon monoxide, with 400 parts per million (ppm) being the maximum allowed in flue products.

And, per the same source:

Incomplete combustion occurs because of: Insufficient mixing of air and fuel. Insufficient air supply to the flame. Insufficient time to burn. Cooling of the flame temperature before combustion is complete.

Also, per Wikipedia , to quote:

In the presence of oxygen, including atmospheric concentrations, carbon monoxide burns with a blue flame, producing carbon dioxide.[10]

So, many combustion reactions perhaps should be expressed in steps including:

$\ce{2 CO + O2 -> 2 CO2}$

So, perhaps more likely, the presence of CO is more of a reality than many would want to believe.

In the case of your experiment, leakage (from possible thermal exposure damage and/or a poor seal) along with incomplete combustion may be some route causes (as carbon monoxide is not very soluble in water, 27.6 mg/L as compared to CO2 with 1,450 mg/L at 25 °C).

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Candle Under Glass Experiment

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Keep a candle burning underwater.

Magic tricks can't hold a candle to this scientific triumph! Watch your child create a bizarre candle souvenir by getting a candle to burn below water. With just the open flame above the water's surface, this process will transform the candle into a hollow tube of wax. The water surrounding the candle is the secret to this "trick." It keeps the exterior of the candle at a cool temperature, so the area touching the water never melts. 

What You Need:

• Taper candle
• Cigarette lighter or candle

What You Do:

• Have your child cut off four 4” lengths of duct tape.
• Invite him to fasten the candle to the bottom of the bowl with duct tape.
• Help him fill the bowl with water just to the top of the candle. Be careful not to submerge the wick!
• Light the candle for your child. Instruct him to be careful of the open flame!
• Encourage your child to observe what happens. The candle should burn all the way to the bottom of the bowl, leaving a thin tube of wax.

What Happened?

If you had lit and burned the candle in a waterless bowl, it would have burned normally and eventually consumed the whole candle. This is caused by the action of both heat flow and a high temperature. Get your child thinking: Why did the candle burn differently when he added water to the bowl? It only seems like magic that the candle burned while it was underwater!

Water has the amazing quality of absorbing heat energy, and because you used cold water, even more heat energy was absorbed. Once the candle's heat energy dissipated into the water, the heat had much less impact on the outer surface of the candle, resulting in the candle's strange transformation.

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