iodine starch experiment with bag

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Selective Permeability of Dialysis Tubing Lab: Explained

• Selective Permeability of Dialysis Tubing…

This dialysis tube experiment experiment was conducted to investigate the selective permeability of dialysis tubing. The permeability of the tubing to glucose, starch, and iodine (potassium iodide) was tested. The dialysis tubing was clipped to form a bag so that glucose and starch were fed into the bag through the other end, and was also clipped to avoid the seeping of the solution.

Water with several drops of iodine added to it until it was visibly yellow-amber was added to a 400ml beaker. The bag was then placed in the beaker, which was stirred with a magnetic stirrer. It was left there for 30 minutes. It was seen that the color of the solution in the bag changed to blue-black color, which showed that iodine was able to pass through the membrane into the bag.

The solution in the beaker became pale yellow-amber, showing that starch didn’t pass through the membrane into the beaker. To confirm the presence of glucose in the beaker and the bag, a Benedict test was performed on the solutions including tap water (control) too.

The beaker solution turned light brown after Benedict’s solution was added to it and suspended in a water bath for 10 minutes. The dialysis bag solution also changed to brown color, while tap water remained blue. This experiment showed that dialysis tubing is selective in its permeability to molecules. It was permeable to glucose and iodine but not starch.

INTRODUCTION:

PURPOSE: The purpose of the dialysis bag experiment was to test the permeability of dialysis tubing to glucose, starch, and iodine.

Living cells need to obtain nutrients from their environment and get rid of waste materials to their surroundings. This exchange of materials between the cell and its surroundings is crucial to its existence. Cells have membranes composed of a phospholipid bilayer embedded with proteins.

This cell membrane can distinguish between different substances, slowing or hindering the movement of other substances and allowing others to pass through readily. This property of the cell is known as selective permeability (Ramlingam, 2008).

Selective permeability is a property of a cell membrane that allows it to control which molecules can pass (moving into and out of the cell) through the pores of the membrane. Selectively permeable membranes only allow small molecules such as glucose and amino acids to readily pass through and inhibit larger molecules like protein and starch from passing through it.

The dialysis tubing is a semi-permeable membrane tubing used in separation techniques and demonstration of diffusion, osmosis, and movement of molecules across a restrictive membrane (Todd, 2012). It separates dissolved substances of different molecular sizes in a solution, and some of the substances may readily pass through the pores of the membrane while others are excluded. The dialysis tubing is made up of cellulose fibers shaped in a flat tube.

In this dialysis tubing lab experiment, the selective permeability of dialysis tubing to glucose, starch, and iodine (potassium iodide) will be tested. This experiment consists of two tests: the test for starch and the test for reducing sugar. When iodine (potassium iodide) is added to a solution in which starch is present, the solution turns blue-black or purple; otherwise, it remains yellow-amber.

When Benedict’s reagent is added to a solution in which reducing sugar is present and it is heated in a water bath, the solution turns green, yellow, orange, red, and then brick red or brown (with a high concentration of sugar present). Otherwise, the solution remains blue.

Will glucose, starch, and iodine (potassium iodide) readily pass through the pores of the dialysis tubing?

HYPOTHESIS:

Glucose, starch, and iodine (potassium iodide) will readily pass through the membrane of the dialysis tubing.

Dialysis Lab Report Prediction:

The solution in the bag and the beaker will both turn blue-black due to the presence of iodine and starch; the presence of glucose in the bag and beaker will be investigated using Benedict’s test.

• Dialysis Tubing
• Test Tubes rack
• Benedict’s reagent
• Iodine (Potassium Iodide)

EXPERIMENT PROCEDURE:

1) 250 ml of tap water was added to a beaker. Several droppers of Iodine (Potassium Iodide) solution was added to the water until it was visibly yellow-amber in color. The color was then recorded.

2) The dialysis tubing was soaked in water for a few minutes until it began to open. One end of the bag was folded and clipped in order to secure it so that no solution seeped through.

3) The other end of the tubing was opened so that it forms a bag and 4ml of glucose and 3ml of starch was fed into it. The bag was also closed and its content was mixed. The color of the solution was then recorded.

4) The outside of the bag was rinsed in tap water.

5) The magnetic stirrer and then the bag was placed in the beaker. The other end of the bag was made to hang over the edge of the beaker.

6) The bag was left in the beaker for about 30 minutes, as the beaker was being stirred.

7) After 30 minutes, the bag was carefully removed and made to stand in a dry beaker. The final color of the solutions was recorded.

8) Benedict test was performed to test for the presence of reducing sugar in the solution in the bag, beaker and tap water (serves as control).

• a) 3 test tubes were labelled control, bag and beaker.
• b) 2 ml of water was added to the control test tube. 2 ml of the bag solution was added to the bag test tube and 2 ml of the beaker solution was added to the beaker test tube.
• c) 2 ml of Benedict’s reagent was added to each test tube and was suspended in a boiling water bath for 10 minutes. The color change was recorded.

The solution in the bag turned blue-black in color owing to the movement of molecules of iodine from the beaker to the bag which contains starch. The solution in the beaker turned brown after Benedict’s test, indicating the presence of glucose in the beaker. This means that the dialysis tubing was permeable to both glucose and iodine but not starch. It is known that starch didn’t pass because the solution in the beaker, which contains iodine, didn’t turn blue-black in color but remained yellow-amber.

DISCUSSION:

• How can you explain your results?

From the results of the experiment represented in a tabular form above, the hypothesis suggested before carrying out the experiment turned out to be incorrect. The dialysis tubing was not permeable to all three solutions: glucose, starch, and iodine (potassium iodide). Rather, the tubing was permeable to glucose and iodine but not starch.

This could be known from the color change in the solutions in the beaker and the bag. The dialysis tubing was permeable to iodine, so the content of the bag turned blue-black in color, indicating the presence of starch. Glucose also readily passed through the pores of the membrane. After performing Benedict’s test on the solutions, the bag’s solution as well as the beaker’s solution turned brown in color. This shows the presence of reducing sugar in both solutions, meaning that glucose passed into the beaker from the bag.

• From your results, predict the size of iodine (potassium iodide) relative to starch.

From the results of this experiment, it is obvious that glucose and iodine (potassium iodide) have smaller molecular sizes than starch. Because starch had a larger molecular size, the dialysis tubing was not permeable to it (it didn’t allow it to readily pass through the pores of its membrane).

• What colors would you expect if the experiment started with glucose and iodine (potassium iodide) inside the bag and starch in the beaker? Explain.
• The solution in the bag will remain yellow-amber in color at the end of the experiment.
• The solution in the beaker will turn blue-black in color at the end of the experiment.
• After performing Benedict’s test, both solutions will turn brown in color.

The solution in the bag remained yellow-amber in color at the end of the experiment because the dialysis tubing is not permeable to starch, so starch didn’t pass through from the beaker into the bag. The solution in the beaker turned blue-black in color at the end of the experiment because iodine passed from the bag into the beaker through the membrane. After performing Benedict’s test on the bag and beaker solution, both solutions turned brown in color because the dialysis tubing was permeable to glucose, so glucose readily passed from the bag into the beaker through the membrane.

Precautions

• It was ensured that the right quantity of solutions was used in every part of the experiment.
• It was also ensured that the time required for the successful complement of the experiment was adhered to.
• It was ensured that all apparatus used were handled with caution.
• The dialysis tubing was clipped well on both ends to secure it so that no solution seeped through.

It was concluded that the dialysis tubing investigation doesn’t allow all kinds of substances to pass readily through the pores of its membrane. This means that it is selective in its permeability to substances. The dialysis tubing was permeable to glucose and iodine but not to starch. Starch was excluded because it has a larger molecular size than glucose and iodine.

Ramlingam, S. T. (2008). Modern Biology. Onitsha: African First Publishers.

Todd, I. S. (2012). Dialysis: History, Development and Promise. World Scientific Publishing Co Pte Ltd.

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18 Comments

Could oxygen pass through the dialysis tubing

so what was the chemical formula for this experiment?

After 24 hours of leaving the bag in the iodine solution; -The dialysis bag turned dark blue/purple, Explain. -The fructose test strip turned positive when dipped in the solution, Explain.  

if the dialysis represent the membrane of a root air cell, and the sugar solution inside represent the cells cytoplasm, which is hypotonic, hypertonic or isotonic. is there any movement of iodine molecules?

What is the purpose of the Iodine Solution?

you added starch and glucose to dialysis tubing, a semipermeable membrane that mimics the plasma membrane of cells. The filled tubing which was placed in a beaker of water containing iodine. What is the purpose of the iodine?

Is the iodine entering the dialysis tube an example of diffusion or osmosis? or can osmosis only occur with water?

what was the purpose of placing the dialysis tubing containing starch solution into the beaker of distilled water

What were the limitations of your experiment ?

What about the NaCl? I did this lab but we had a question if NaCl moved out of the tube.

what did not diffuse through the membrane

Starch and Benedict’s solution.

Maybe the starch and its size.

How can you explain the change in weight of the cells?

osmosis of water

Include an analysis maybe? all around good job though!

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• Experiments

Diffusion through plastics

Can iodine escape a plastic packet?

• Copper(II) sulfate
• Potassium iodide
• Starch (soluble)
• Put on protective gloves and eyewear.
• Conduct the experiment on the plastic tray.
• Observe safety precautions when working with boiling water.
• Avoid inhaling iodine vapors from the packet.
• Do not allow chemicals to come into contact with the eyes or mouth.
• Keep young children, animals and those not wearing eye protection away from the experimental area.
• Store this experimental set out of reach of children under 12 years of age.
• Clean all equipment after use.
• Make sure that all containers are fully closed and properly stored after use.
• Ensure that all empty containers are disposed of properly.
• Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
• Do not replace foodstuffs in original container. Dispose of immediately.
• In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
• If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
• In case of inhalation: Remove person to fresh air.
• In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
• In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
• In case of injury always seek medical advice.
• The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
• This experimental set is for use only by children over 12 years.
• Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
• The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
• The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
• Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

Step-by-step instructions

First, prepare a starch solution. Starch hardly dissolves in cold water, but will grudgingly dissolve in hot water. And while starch may not be overly enthusiastic about dissolving, it is really keen to react with iodine. When it does, it forms a dark blue compound. For this reason, starch can be used to detect iodine.

Now take a polyethylene packet. Polyethylene doesn't allow any water through, so your packet should be completely watertight.

Mix some copper sulfate CuSO 4 crystals and potassium iodide KI solution in the packet. Neither of these reagents can pass through the packet walls.

The reaction between CuSO 4 and KI yields iodine I 2 inside the packet.

Iodine forms inside the packet, but the starch solution in the beaker turns blue. Can you guess what happened?

Dispose of solid waste together with household garbage. Pour solutions down the sink and wash with an excess of water.

Scientific description

How does the iodine get out of the packet and into the beaker.

To the human eye, the packet appears seamless, with no holes or cracks. The idea that some substances can pass through its walls seems strange, but it’s true! In fact, the package is riddled with micropores, which allow small molecules such as iodine I 2 to pass slowly through the packet via a process known as diffusion.

Diffusion is the process of movement of molecules from areas with higher concentrations to areas with lower concentrations. It occurs mostly due to molecules’ constant motion and the presence of gaps between them. This process stops if the concentration of the substance is equal in both areas. This is exactly what happens here! Iodine molecules slowly diffuse through the packet into the area where there are no or few iodine molecules. Liquid diffusion can take anywhere from several minutes to several hours. Diffusion in solids occurs over the course of several years. These processes can be accelerated by increasing temperature or other external influences.

Why does the solution in the beaker turn blue.

The beaker contains a starch solution. Starch is a tasteless, white powder resembling flour in consistency. It is a polysaccharide consisting of amylose and amylopectin, with the formula (C 6 H 10 O 5 ) n . Moreover, it acts as an iodine indicator. An indicator is a substance that gives a visible signal (such as a change in color) regarding the presence or absence of a certain chemical compound. Iodine reacts with starch to form a complex, deep blue compound.

The equation of the reaction of iodine and starch is:

I 2 + (C 6 H 10 O 5 ) n → I 2 •(C 6 H 10 O 5 ) n

That’s interesting!

More about iodine's properties.

Solid iodine exists as purple-black crystals. But if you leave it in the open air, it quickly turns into a purple vapor. Such a transition from a solid state directly to a gas phase, without passing through a liquid state, is called sublimation.

Surprisingly, though, if you skin your knee, you’ll most likely treat it with a brownish-orange iodine solution! How is this possible? Iodine I 2 dissolves well in ethanol, so you can find iodine-alcohol solutions as disinfectants in some first aid kits.

Furthermore, iodine can be found in some thyroid hormones responsible for the human body’s growth and metabolic processes. If you have too much or too little iodine in your diet, your body may have trouble functioning well.

Dozens of experiments you can do at home

One of the most exciting and ambitious home-chemistry educational projects The Royal Society of Chemistry

Chemistry experiments

Diffusion and Osmosis | Iodine starch experiment with bag | Science Experiments | elearnin

This experiment shows the movement of particles through a membrane For this experiment you will need: • Water • Starch solution • Iodine • Dropper • Zipper plastic bag PROCEDURE: • Mix the starch solution in the water in a beaker. • With the help of the dropper put some iodine solution in the zipper bag. • Zip the plastic bag. • Now turn up side down to check whether there is any leak. • Submerge the plastic bag into the beaker with starch solution. • Leave the arrangement for half an hour. • A layer of deep purple-black color layer is formed on the membrane of the plastic bag and the color slowly diffuses into the starch water. EXPLANATION: Iodine is used to test for the presence of starch. When Iodine reacts with starch, it turns deep purple-black. The iodine molecules are small enough to pass through the membrane of the plastic bag, however starch and water molecules are too big to pass through the membrane. The movement of the iodine through the plastic membrane is functionally the same as movement of molecules through biological membranes, that is, any cell membrane. The molecules will move from higher concentration to lower concentration. Osmosis is the net movement of solvent molecules (in this case, iodine) through a partially permeable membrane (like a plastic bag) into a region of higher solute (water) concentration, in order to equalize the solute concentrations on the two sides. Here, by partially permeable membrane or semi-permeable membrane, we mean a permeable to the solvent, but not the solute. Diffusion is that physical process in which any solvent moves, without input of energy, across a semi permeable membrane separating two solutions of different concentrations. source

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28 Comments

so the iodine diffuses into the semipermeable membrane (the plastic bag) from high concentration to low concentration meaning it doesn’t use any form of energy. when it enters the semipermeable membrane it reacts with the starch and turns a dark purple/blue/black colour. i am currently studying this and although the definition of osmosis was slightly muddled up, it was still a pretty good vid. as osmosis specifically refers to the net movement of water across a semipermeable membrane and on the rare occasion, ethanol too, the movement of the molecules actually refers to diffusion.

The explanation needs to be seriously fixed. How can a wrong statement like this get thousands of views? Its too dangerous for anyone who trusts on everything they see on the internet.

help me I hate this

exiting videos i luv them

What is the ratio of starch to water?

horrible explanation you really need to do your own work and not go on to google or other search engines . common man step up your game.

Thank you alot thiz help me for expo….. again tysm

so what's the starch diffusion or osmosis?

good vid i love it

hi im ms lavender

is it dengrous for our helthly

Osmosis is definitely NOT the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration. Osmosis refers ONLY to the movement of WATER across a membrane from an area of higher WATER concentration to an area of lower WATER concentration.

This is so easy as one person Unlike me with a group and I'm little

i really understood this lesson and it was practically the same stuff my teacher taught me thanks

What acts as the cytoplasm and the cell wall?

What mil plastic bags are you using? I tried this and it didn't work, but I was using pretty thick/sturdy bags.

@ Ethiopian boy, why don't appreciate and focus the concept instated finding wrong or correct . Anyway good job bro.

THIS REALLY HELPS! THANK YOU!

Love the visuals but- Your definition of osmosis needs to be corrected- since it is the movement of WATER (not just anything) across a membrane from high to low concentration!

this experiment is too wrong!

This experiment will give a better understanding about osmosis and diffusion

what would be the effect of higher temperature?

Isn't osmosis exclusively the diffusion of water through a membrane and nothing else?

Thank you so much for making this video. It helped me alot with my biology homework. I saw this lab sheet I hadn't done (late/missing work) but this video helped me get the data I needed. 🙂

I'm not quite sure if the explanation is the fact that water molecules are not small enough to make it through the membrane, because they should not be so much bigger than iodine (I would expect them to be smaller, as a matter of fact). A more plausible explanation would be that plastic, being an organic polymer, is mainly non-polar, so is iodine, which allows the latter to pass through the plastic membrane. Starch molecules are indeed too big, but water molecules do not pass through the membrane due to its polarity.

no, starch was placed inside the bag, the iodine reacted with the starch in the bag

I actually had that same question.

So if it turned purple inside the bag, isn't that an indication that starch can pass the membrane too?

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How Can Diffusion Be Observed? - Bio 1

Introduction: In this lab, you will observe the diffusion of a substance across a semipermeable membrane. Iodine is a known indicator for starch. An indicator is a substance that changes color in the presence of the substance it indicates. Watch as your teacher demonstrates how iodine changes in the presence of starch.

Prelab Observations: Describe what happened when iodine came into contact with starch.

• Fill a plastic baggie with a teaspoon of cornstarch and a half a cup of water tie bag. (This may already have been done for you)
• Fill a beaker halfway with water and add ten drops of iodine.
• Place the baggie in the cup so that the cornstarch mixture is submerged in the iodine water mixture.
• Wait fifteen minutes and record your observations in the data table
• While you are waiting, answer the questions.

1. Define diffusion. ______________________________________________

2. Define osmosis. ____________________________________

3. Why is iodine called an indicator? _____________________________________________

4. Molecules tend to move from areas of _______ concentration to areas of ______ concentration.

What's in the Bag?

We're going to think about concentrations now, which substances are more or less concentrated depends on which one has the most stuff in it.

1. Which is more concentrated in starch? [ beaker / baggie ]

2. Which is more concentrated in iodine? [ beaker / baggie ]

3. With regard to iodine, which is hypertonic? [ beaker / baggie ]

4. With regard to starch, which is hypertonic? [ beaker / baggie ]

Make Predictions:

1. If the bag is permeable to starch, which way would the starch move? [ into bag / out of bag ]

2. If the bag is permeable to iodine, which way would the iodine move? [ into bag / out of bag ]

3. If the bag is permeable to iodine, what color would you expect it to change? [ orange / purple / no change ]

What about the solution in the beaker? [ orange / purple / no change ]

4. If the bag is permeable to starch, what color would you expect it to change? [ orange / purple / no change ]

Observations:

Write your observations in the table below:

Post Lab Analysis

1. Based on your observations, which substance moved, the iodine or the starch?

2. How did you determine this?

3. The plastic baggie was permeable to which substance?

4. Explain how the bag is a model for the cell.

5. Sketch the cup and baggie in the space below. Use arrows to illustrate how diffusion occurred in this lab.

6. What would happen if you did an experiment in which the iodine solution was placed in the baggie, and the starch solution was in the beaker? Be detailed in your description.

7. Why is it not a good idea to store iodine in a plastic bag? Related Documents:

Other Resources on Cell Transport

Observe Diffusion in a Bag - Intro biology, same lab with simplified directions and questions Transport Across the Cell Membrane - simple diagram shows how molecules enter the cell through diffusion, facilitated diffusion, and osmosis Investigation: Why Are Cells So Small? - use bromothymol blue and agar to model how diffusion occurs in cells Observing Osmosis - use an egg, vinegar, corn syrup, will take a few days

Optional Lab Activities

Osmosis and diffusion, lab objectives.

At the conclusion of the lab, the student should be able to:

• define the following terms: diffusion, osmosis, equilibrium, tonicity, turgor pressure, plasmolysis
• describe what drives simple diffusion (why do the molecules move?)
• list the factors that may affect the speed of simple diffusion
• list which molecules, in general, can freely diffuse across the plasma membrane of a cell
• describe what drives osmosis (why do water molecules move?)
• explain why water moves out of a cell when the cell is placed in a hypertonic solution
• explain why water moves into a cell when the cell is placed in a hypotonic solution
• describe what physically happens to a cell if water leaves the cell
• describe what physically happens to a cell if water enters the cell

Introduction

Understanding the concepts of diffusion and osmosis is critical for conceptualizing how substances move across cell membranes. Diffusion can occur across a semipermeable membrane; however diffusion also occurs where no barrier (or membrane) is present. A number of factors can affect the rate of diffusion, including temperature, molecular weight, concentration gradient, electrical charge, and distance. Water can also move by the same mechanism. This diffusion of water is called osmosis .

In this lab you will explore the processes of diffusion and osmosis. We will examine the effects of movement across membranes in dialysis tubing, by definition, a semi-permeable membrane made of cellulose. We will also examine these principles in living plant cells.

Part 1. Diffusion Across a Semi-Permeable Membrane: Dialysis

• Cut a piece of dialysis tubing, approximately 10 cm.
• Soak the dialysis tubing for about 5 minutes prior to using.
• Tie off one end of the tubing with dental floss.
• Use a pipette and fill the bag with a 1% starch solution leaving enough room to tie the other end of the tubing.
• Tie the other end of the tubing closed with dental floss.
• Fill a 250 mL beaker with distilled water.
• Add Lugol’s iodine to the distilled water in the beaker until the water is a uniform pale yellow color.
• Place the dialysis tubing bag in the beaker.
• The movement of starch
• The movement of iodine
• The color of the solution in the bag after 30 minutes
• The color of the solution in the beaker after 30 minutes
• Add the dialysis bag to the beaker and allow the experiment to run for 30 minutes. Record the colors of both the dialysis bag and the beaker.

Lab Questions

• Is there evidence of the diffusion of starch molecules? If so, in which direction did starch molecules diffuse?
• Is there evidence of the diffusion of iodine molecules? If so, in which direction did iodine molecules diffuse.
• What can you say about the permeability of the dialysis membrane? (What particles could move through and what particles could not?)
• What is the difference between a semi-permeable and a selectively permeable membrane

Part 2. Plasmolysis—Observing Osmosis in a Living System, Elodea

If a plant cell is immersed in a solution that has a higher solute concentration than that of the cell, water will leave/enter (circle one) the cell. The loss of water from the cell will cause the cell to lose the pressure exerted by the fluid in the plant cell’s vacuole, which is called turgor pressure. Macroscopically, you can see the effects of loss of turgor in wilted houseplants or limp lettuce. Microscopically, increased loss of water and loss of turgor become visible as a withdrawal of the protoplast from the cell wall (plasmolysis) and as a decrease in the size of the vacuole (Figure 1).

• Obtain a leaf from the tip of an Elodea Place it in a drop of water on a slide, cover it with a coverslip, and examine the material first at scanning, then low power objective and then at high power objective.
• Locate a region of health. Note the location of the chloroplasts.  Sketch a few cells. For the next step, DO NOT move the slide .
• While touching one corner of the coverslip with a piece of Kimwipe to draw off the water, add a drop of 40% salt solution to the opposite corner of the coverslip. Do this simultaneously.  Be sure that the salt solution moves under the coverslip. Wait about 5 minutes, then examine as before. Sketch these cells next to your sketch of cells in step two, note the location of the chloroplasts. Label it 40% salt solution .
• What happened to the cells in the salt solution?
• Assuming that the cells have not been killed, what should happen if the salt solution were to be replaced by water?
• Are plant cells normally hypertonic, hypotonic, or isotonic to their environment? Why?
• Can plant cells burst? Explain.

Overall Conclusions

• Review your hypothesis for each experiment. Was your original hypothesis supported or rejected for each experiment. Explain why or why not. This should be based on the best information collected from the experiment. Explain how you arrived at this conclusion.
• If it was incorrect, give the correct answer, again based on the best information collected from the experiment.

Sources of Error

• Identify and explain two things that people may have done incorrectly that would have caused them to get different answers from the rest of the class. Be  specific .
• Biology 101 Labs. Authored by : Lynette Hauser. Provided by : Tidewater Community College. Located at : http://www.tcc.edu/ . License : CC BY: Attribution
• BIOL 211 - Majors Cellular [or Animal or Plant]. Authored by : Carey Schroyer and Diane Forson. Provided by : Open Course Library. Located at : http://opencourselibrary.org/biol-211-majors-cellular-or-animal-or-plant/ . License : CC BY: Attribution

Middle School Science Blog

Free lesson plans and resources for grades 5-8 by liz belasic (liz larosa), diffusion lab – iodine & cornstarch.

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Materials and Set Up – this was so easy and inexpensive to do and had the same effect as using dialysis tubing. Great demo/lab as part of our unit on osmosis and diffusion!

For every two students:

• handout from Biology Corner
• large beaker
• inexpensive sandwich bag – non sealing (I used Wegmans 150 ct)
• 1 tbsp corn starch
• 50 mL water
• rubber band
• clothes pin
• graduated cylinder
• 100 mL Iodine dilution

Iodine Preparation

• 20 ml Iodine added to 500 mL of water
• measure out 100 mL of diluted iodine for each group

Prelab Prep:

• Place one bag over each beaker
• Add 1 tbsp of cornstarch to each bag
• Add 50 mL of water to each bag
• Check for leaks
• Use a rubber band on each one to keep closed
• Clip bag to beaker

Observations

• Iodine is able to pass through the plastic bag, the starch is not
• Have students lift the bag out to see the changes that are taking place

Update – I let the set up sit over the weekend, and when I came in today, the water was almost completely clear – looks like just about all of the iodine moved into the bag:

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5 thoughts on “ Diffusion Lab – Iodine & Cornstarch ”

Is there an alternative to iodine or do you know where to find cheap iodine? I don’t have access to a chemistry lab.

I think I bought some from the supermarket or drugstore, sorry I do not recall.

Hi, I’m just finishing up investigations with my 7th graders on osmosis and diffusion. Oddly, I tried this very set up hoping that inexpensive baggies would work. We had tried with a z-loc brand bags and had no luck ( just letters to the company letting them know how non permeable their baggies were) so I used the store brand baggie and we were still disappointed. I guess we all felt better about our sandwiches wrapped in these baggies but not to see the diffusion. We did a great investigation with carrots and salt and then moved onto “naked eggs”. I had been very hopeful the baggie would work. Glad it did for your crew.

Like Liked by 1 person

We used either Wegman’s brand fold over sandwich bags or ShopRite – I threw out the box without writing it down 😦 -Sorry it didn’t work for your class 😦 The carrots sounds like a great idea, I saw one using potato slices that I thought about using as well. I have never tried the egg one, but heard it works well 🙂

Update – it was Wegmans brand, box was still in my room 🙂

Late to the party but I am using ziplock brand locking and it is working but I had to increase the amount of iodine to about 1/2 and 1/2 for it to work in a reasonable timeframe

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Why Does Iodine Turn Starch Blue?

Author: Catharina Goedecke

Iodine Test

Using iodine to test for the presence of starch is a common experiment. A solution of iodine (I 2 ) and potassium iodide (KI) in water has a light orange-brown color. If it is added to a sample that contains starch, such as the bread pictured above, the color changes to a deep blue. But how does this color change work?

Starch is a carbohydrate found in plants. It consists of two different types of polysaccharides that are made up of glucose units which are connected in two different ways. One is the linear amylose and the other is the branched amylopectin (pictured below).

Amylose is the compound that is responsible for the blue color. Its chain forms a helix shape, and iodine can be bound inside this helix (pictured below).

Charge-Transfer Complexes

The colors are caused by so-called charge transfer (CT) complexes. Molecular iodine (I 2 ) is not easily soluble in water, which is why potassium iodide is added. Together, they form polyiodide ions of the type I n – , for example,  I 3 – , I 5 – , or I 7 – . The negatively charged iodide in these compounds acts as charge donor, the neutral iodine as a charge acceptor. Electrons in such charge-transfer complexes are easy to excite to a higher energy level by light. The light is absorbed in the process and its complementary color is observed by the human eye.

In the case of the aqueous solution of polyiodides, the absorptions of the different species lead to an overall brownish color. Once amylose is added, it forms another CT complex, Here, the amylose acts as a charge donor and the polyiodide as an acceptor. This complex absorbs light of a different wavelength than polyiodide, and the color turns dark blue.

Polyiodide Chains

The exact structure of the polyiodides inside the amyloid helix is not clear. The amylose-iodine complex is amorphous (i.e., it does not form ordered crystals), which has made it difficult to determine its structure. It has been proposed that the species inside the helix are repeated  I 3 – or I 5 – units.

However, Ram Seshadri, Fred Wudl, and colleagues, University of California, Santa Barbara, USA, have found evidence that infinite polyiodide chains I n x– are contained in the amylose-iodine complex [1]. The team investigated a related system, a pyrroloperylene–iodine complex, to study its properties as an organic electronic conductor. The material is crystalline, and therefore, the team was able to determine its structure using X-ray crystallography. They found nearly linear polyiodide chains in-between stacks of pyrroloperylene. It turned out that the material containing these chains absorbs light at very similar wavelengths to the amylose-iodine complex, which supports the hypothesis that similar polymeric chains form in the iodine test for starch.

[1] Sheri Madhu, Hayden A. Evans, Vicky V. T. Doan-Nguyen, John G. Labram, Guang Wu, Michael L. Chabinyc, Ram Seshadri, Fred Wudl, Infinite Polyiodide Chains in the Pyrroloperylene-Iodine Complex: Insights into the Starch-Iodine and Perylene-Iodine Complexes , Angew. Chem. Int. Ed. 2016 , 55 , 8032–8035. DOI: 10.1002/anie.201601585

• Der Iod-Stärke-Komplex (in German), www.chemieunterricht.de 2006 . (accessed November 24, 2016)
• The structure of the blue starch-iodine complex , Wolfram Saenger, Naturwissenschaften 1984 , 71 , 31–36. DOI: 10.1007/bf00365977

this article is quite helpful .. thank you ✨

Thank you guys

where the h*** is the volume and page number to cite this

Dear Reader, We do not have volumes or page numbers. You can cite the article as: C. Goedecke, Why Does Iodine Turn Starch Blue?, ChemistryViews 2016 . https://doi.org/10.1002/chemv.201600103 Best regards, Your ChemistryViews Team

Interesting light absorbing properties

Nice informative post

Very informative thank you very much

Who is the publisher?

Dear Reader, You can cite the article as: C. Goedecke, Why Does Iodine Turn Starch Blue?, ChemistryViews 2016. https://doi.org/10.1002/chemv.201600103 Best regards, Your ChemistryViews Team

Very interesting and helpful

Very helpful imformation, thank you

Wow that’s awesome

this is awesome

really cool but no words

would iodiDE alone form the same reaction? or is iodiNE specifically needed to react with the glycogen

Thanks for this.. very helpful

What does it mean, when the water become crystal clear after you added iodine into the water?

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