Grow crystals out of thin air!

40 minutes



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the safety underlay and in a well-ventilated area.
  • Avoid inhaling iodine vapors from the beaker.
  • Do not touch iodine crystals even with gloves.
  • Remove protective gloves before lighting the splint and put them back on after blowing out the candle.
  • Keep a bowl of water nearby when working with fire.
  • Keep flammable materials and hair away from flame.
  • Do not touch the stove after the experiment. Wait until it cools down.
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

What did we use CaCl2 for?

The air always contains a certain amount of water vapor. Like iodine, it bumps into the surface of the cold cup and transitions into another state. But unlike iodine, it becomes liquid, not solid. We don’t want water to interfere with the growth of the iodine crystals, so we need to remove it somehow. This is where calcium chloride CaCl2 comes in handy! CaCl2 is a hygroscopic substance – this means that it readily absorbs moisture from its surroundings. This curious property of CaCl2 allows us to remove condensation as it appears and observe our shiny iodine crystals.

Step-by-step instructions

Prepare some ice in advance.


When mixed, copper sulfate CuSO4 and potassium iodide KI react with each other: I- ions share their electrons with Cu2+ and pair up, turning into I2 molecules.


Heat will make the reaction more intense. Stir the mixture and prepare the stove.


Now, add sodium thiosulfate Na2S2O3 to some water—it will come in handy at the very end of the experiment.


When heated, the mixture reacts intensely, yielding purple iodine I2 vapor. But as the vapor hits the cold cup, it turns into shiny, needlelike crystals.


Do you remember the sodium thiosulfate Na2S2O3 solution you made? This is where it comes into play! Iodine I2 is very volatile and should not be inhaled. But it can easily be turned back into I- by exposing it to Na2S2O3 solution.



Please refer to local regulations when disposing of chemicals. Dispose of other solid waste with household garbage. Pour leftover solutions down the sink. Wash with an excess of water. Make sure to dissolve all the iodine crystals in the sodium thiosulfate solution.

Scientific description

These purple vapors  and shiny crystals  are two states of one substance—iodine . The transition directly from a gaseous  to a solid  state, bypassing the liquid state, is called deposition. In their gaseous state , molecules of iodine I2  move quickly and freely, but as soon as they bump into a cold surface , they lose energy, instantly slowing down and packing closely together to form a solid . This resembles the process by which snowflakes or frost patterns form from water vapor in the air. However, we usually don't see water vapor in the humid winter air: it is colorless, in contrast to the bright iodine vapor .

According to one version of events, a cat was involved in the discovery of iodine! In 1811, Parisian chemist Bernard Courtois was studying seaweed ash with his beloved cat sitting on his shoulder. Suddenly, it jumped down and knocked a jar of sulfuric acid right onto a heated brazier of seaweed ash. Incidentally, seaweed ash contains iodine compounds, somewhat similar to the KI you used in the experiment, and sulfuric acid H2SO4 is even more keen than CuSO4 to take electrons from such compounds. Clouds of bright purple smoke rose from the brazier, which turned out to be the vapors of a new chemical element!

What chemical reaction occurs between KI and CuSO4?

Potassium iodide KI contains iodide ions I , which are basically iodine atoms  with an extra electron , and copper sulfate CuSO4 contains a Cu2+ ion , a copper atom  with two electrons missing. Cu2+ can take an electron from I, turning it into I{0} . The latter  likes to bond in pairs, forming stable iodine molecules I2 . This electron exchange and I2 formation can be written as the following equations:

I + Cu2+ → Cu+ + I0I0 + I0 → I2

If we combine these equations, we get this one:

2I + 2Cu2+ → 2Cu+ + I2

Don’t forget that we also have potassium K+ and sulfate SO42\- ions! They don't change during the reaction, so we can just add them to both sides of the equation in equal quantities. This introduces the formulas KI and CuSO4 on the left. Now the whole equation of the chemical reaction looks like this:

4KI + 2CuSO4 → I2 + 2CuI + 2K2SO4

That’s interesting!