Chemical reefs

Create a reef in your Petri dish!

10 minutes



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
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

Why should I use a small Petri dish?

In the beginning, when using only two reagents, a small Petri dish will yield faster and clearer results. It’s better to use a large Petri dish when making reefs with three or four different compounds.

I want to do this experiment again. What combinations should I try?

To learn more about the different compounds and their combinations, please read the scientific description of this experiment. But we recommend trying ammonium iron(III) sulfate NH4Fe(SO4)2 (iron alum) and potassium hexacyanoferrate(II) K4[Fe(CN)6]. Their blue reefs are quite nice.

I decided to repeat the experiment with other compounds, but I didn't get any results.

It's great that you decided to try the experiment using other substances! Yes, this is possible for some pairs of compounds. Take, for example, CuSO4 and NH4Fe(SO4)2 or K4[Fe(CN)6] and Na2CO3. Combining these substances will only yield indistinct results.

When I repeated the experiment with the same compounds, I didn't get the result I was expecting. Why not?

This can happen if you reuse a Petri dish from a previous experiment without washing out the leftover salt residues. This can make the results more ambiguous and less beautiful.

I stirred the salts and didn't get a beautiful reef.

That’s OK. Stirring the salts will interfere with the experiment. Try the experiment again, but this time just portion the compounds onto the bottom of the Petri dish and leave them for a few minutes. The salts will dissolve in the water and form a reef.

The reagents aren’t sticking to the cotton swabs. What should I do?

First, try to wet the cotton swabs more thoroughly. Perhaps they weren't damp enough to hold the reagents. You can also try pouring small quantities of the reagents directly into the Petri dish without using cotton swabs.

I want to repeat the experiment. Can I use the same Petri dish?

Yes, you can, but be sure to wash the Petri dish with plenty of water. Otherwise, the experiment may yield unpredictable results.

Step-by-step instructions

This reaction will take place in an aqueous solution.


You will only need small amounts of the dry compounds.


Let the crystals dissolve in the water.


Where the dissolved compounds meet, a reaction takes place and a nebulous brown “reef” appears.


You can use different combinations of CuSO4, K4[Fe(CN)6, Na2CO3, and NH4Fe(SO4)2 to create different kinds of reefs. You can even create four different kinds of reefs at once, but be sure to use a large Petri dish and wash it thoroughly between the experiments.


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

Scientific description

When you see a chemical formula such as CO2, you can guess that one molecule of the compound  contains one C atom  (carbon) and two O atoms  (oxygen). Does a bottle of CuSO4 contain lots of CuSO4 molecules then? Not really. There's no such thing as a CuSO4 molecule; rather, this compound consists of two separate parts known as ions: Cu2+  and SO42- . Compounds like this are called ionic compounds. When solid, their ions are packed together, but once in water, the ions can swim their separate ways.

Some combinations of ions, like CuSO4, break up easily in water (i. e. they dissolve easily), while other combinations are not so easily separated. When we put CuSO4  and K4[Fe(CN)6 crystals in water, both of these compounds dissolve easily, but when Cu2+  ions meet [Fe(CN)6]4-  ions in the middle of the dish, they stick together to form brown Cu2[Fe(CN)6 particles, which are really reluctant to dissolve. That's what our glorious "reef" is made of.

Try combining different compounds and see which of them contain ions that can fit together to form "reefs". Blue Fe4[Fe(CN)6]3  reefs are especially beautiful!

How do these reefs form?

The reefs materialize as a result of the formation of an insoluble compound Cu2[Fe(CN)6] during the reaction between copper(II) sulfate CuSO4 and potassium hexacyanoferrate(II) K4[Fe(CN)6]:

K4[Fe(CN)6] + 2CuSO4 → Cu2[Fe(CN)6]↓ + 2K2SO4

But first, the substances dissolve in water. When dissolving, they split into ions:

CuSO4 → Cu2+ + SO42–

K4[Fe(CN)6] → [Fe(CN)6]4– +4K+

A salt metathesis reaction takes place. When the counterions meet, they form the brown precipitate Cu2[Fe(CN)6] that you see in the middle of the Petri dish.

Follow up

Try various combinations of the solid reagents: copper sulfate CuSO4, potassium hexacyanoferrate(II) K4[Fe(CN)6], sodium carbonate Na2CO3, and ammonium iron(III) sulfate NH4Fe(SO4)2 (iron alum). You can also conduct this experiment in the large Petri dish and create four different kinds of reefs at once!

That’s interesting!

Diffusion in the living world

You can observe diffusion every day! When something is burning on the stove, you can smell it throughout the house, right? This phenomenon also applies to the human body! Our organism needs to obtain oxygen and dispose of carbon dioxide to survive. These compounds enter and exit the lungs by diffusion!

Diffusion is also what causes the carbon dioxide we breathe out to disperse in space and mix with oxygen instead of lingering in a cloud around us. That's why we can breathe quietly for a long time in a closed room. However, from time to time, you have to ventilate the room and let in fresh air saturated with oxygen, which (again due to diffusion) spreads quickly through the area.

This phenomenon is very important for plants too. Plants get the nutrients and water they need from the soil via diffusion. This is how plants can "eat" and grow!