Diffusion through plastics

Can iodine escape a plastic packet?

30 minutes
Diffusion through plastics



  • 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.
General safety rules
  • 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.
General first aid information
  • 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.
Advice for supervising adults
  • 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.

FAQ and troubleshooting

Can I conduct this experiment without the starch from the set?

Yes! You can conduct this experiment using other sources of starch. One particularly convenient option is water that was used to cook noodles, but you can use any kind of starch – corn starch, potato starch, or rice starch. Try different sources and find the one that works best!

The solution in the beaker turned blue immediately when I immersed the bag in it. What went wrong?

Oops! The plastic bag likely has a small hole in the bottom. Try using another one.

The solution isn’t turning blue!

First, make sure that the reaction in the bag successfully yielded iodine – the mixture should have turned brown. If not, try adding some more СuSO4 crystals and KI solution to the plastic bag. If the mixture is brown, make sure that you used boiling water to make the starch solution. Alternatively, if you used a different source of starch, it is possible that the type of starch in it does not react with iodine. Try using a different one.

Why should I use the blue bottle? What if I use the red bottle?

The substances in the bottle with blue and red labels are identical – both bottles contain copper sulfate CuSO4. We use stickers to help differentiate between them because it’s possible to cross-contaminate the copper sulfate while doing the “Chemical reefs” experiment. It’s no problem if you used the red bottle – just make sure there is enough substance left in the bottle to conduct this experiment. And if not, use another bottle of CuSO4 to reach the needed amount.

I smell something. Is everything okay?

Yes, this isn’t dangerous; you’re smelling the weak scent of iodine. It’s best to conduct this experiment in a well-ventilated area.

I've waited for 10 minutes, but the solution changed colors only slightly. What should I do?

Wait a little bit more. Then carefully swirl the solution in the beaker. If nothing changes, try repeating the experiment.

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 CuSO4 crystals and potassium iodide KI solution in the packet. Neither of these reagents can pass through the packet walls.


The reaction between CuSO4 and KI yields iodine I2 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

Iodine molecules  are a sneaky bunch: they can squeeze in between the polyethylene molecules and slowly penetrate through the seemingly impermeable walls of your packet . While this process isn't particularly fast and only a small amount of I2 molecules can escape, the starch solution is there to detect them, and it turns deep blue.

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 I2 to pass slowly through the packet via a process known as diffusion.

Learn more

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 (C6H10O5)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:

I2 + (C6H10O5)n → I2•(C6H10O5)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 I2 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.