Chemical seaweed

Grow a chemical seaweed garden!

Difficulty:
Danger:
Duration:
40 minutes
Experiment's video preview

Safety

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

The “seaweeds” do not grow. What to do?

Correct proportions are very important in this experiment, so make sure you measured precisely 100 mL of water.

Also, make sure that salts do not mix in the beaker, and there is a gap left between them. Otherwise, “seaweeds” will grow poorly.

Be careful: “seaweeds” are very thin and fragile. Do not disturb the beaker for 20–30 minutes.

Why do we need to leave gaps when depositing the salts?

If these compounds happen to mix, it will be hard to distinguish “seaweeds” of different colors. “Seaweeds” growing from the last salt added will dominate over the rest of “seaweeds”, and the result will not be as diverse.

Step-by-step instructions

Prepare a potassium hexacyanoferrate(II) K4[Fe(CN)6] solution.

chemgardens_seaweed_iks_en_iks_01

Put various metal salts into different areas of the bottom of the beaker.

chemgardens_seaweed_iks_en_iks_02

Metal salts gradually dissolve and react with the potassium hexacyanoferrate(II). Insoluble copper, iron and zinc compounds form. These don’t just precipitate out but form “bubbles” because of the osmotic pressure. The fancy chemical seaweed grows from these bubbles.

chemgardens_seaweed_iks_en_iks_03

Disposal

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

Scientific description

Why does the chemical garden form?

Upon entering the solution, the salt crystals start to react with the yellow potassium ferrocyanide (or yellow prussiate). But the reaction is not instant, it begins on the surface of the solid salts. As a result, each of the crystals is covered by a film of non-soluble compounds. However, this film doesn’t cover the crystal completely, it leaves small gaps, it’s through these gaps that more yellow prussiate seeps through. Thus the reaction begins again. It’s at this point in time that slightly-soluble compounds are formed, they are the ones that “break out” through the film and onto the surface. This is pretty much how the fascinating structures come to be at the bottom of the beaker.

Why does the seaweed grow in various manners?

The obvious and simple guess would be that we are using different substances – white zinc sulfate ZnSO4, blue crystals of copper sulfate CuSO4 and a greenish iron(II) sulfate FeSO4. This is actually pretty close to the right answer.

One the one hand, all these compounds are quite similar – they are sulfates of metals. They consist of a positively charged metal ion and a negatively charged sulfate ion, into which they break down (dissociate) in water:

ZnSO4 ↔ Zn2+ + SO42-

CuSO4 ↔ Cu2+ + SO42-

FeSO4 ↔ Fe2+ + SO42-

On the other hand they are different in color and appearance (a fine white powder, greenish small crystals and blueish crystals). But the main thing is the metal, that dictates the difference in properties of these substances. It’s the metal ion which gives each sulfate it’s properties!

This is why the seaweed looks different. Each substance reacts with the solution (K4[Fe(CN)6]) in its own unique manner.

Blue copper sulfate forms small crumbly brown structures, these look a lot more like corals and not seaweed. The white zinc sulfate swiftly grows in thin strands right to the top of the solution and doesn’t change color. Lastly, the greenish iron sulfate gradually turns bright-blue while growing only a couple of millimeters from the bottom of the beaker.

Why does the iron sulfate change color in the solution?

First of all the iron sulfate FeSO4 crystals are covered by a thin film of so called Everitt's salt (K2Fe[Fe(CN)6]).

This salt is formed when potassium ferrocyanide (yellow prussiate) K4[Fe(CN)6] reacts with FeSO4:

K4[Fe(CN)6] + FeSO4 –> K2Fe[Fe(CN)6] + K2SO4

However, this process normally goes unnoticed because this salt oxidizes to Prussian blue KFe[Fe(CN)6], which has a distinct dark-blue color.

As a matter of fact, Prussian blue is a very popular pigment (paint) which is used by many famous painters. For example, in the famous painting «The Starry Night» by the Dutchman Vincent Van Gogh or on the popular woodblock print «The Great Wave off Kanagawa» by the Japanese Katsushika Hokusai the main color tones are set specifically by Prussian blue!

Find out more

The color of copper sulfate CuSO4, which was blue at the beginning, also changes distinctly. The reddish-brown color of the “plants” is a result of potassium-copper ferricyanide (red prussiate) forming:

CuSO4 + K4[Fe(CN)6] → (K2Cu[Fe(CN)6])↓ + K2SO4

How can you make seaweed of a different form?

To create seaweed of different forms we can try to alter the parameters of the experiment. To be more precise these are the parameters of the beaker, the volumes of all the solutions, the amount of crystals used. All of these can be varied. For example, you can use a more concentrated solution of potassium ferrocyanide (just use less water, 50 ml instead of 100 ml). This is a tried and tested method. But on the whole – don’t be scared to experiment yourselves! Try using hot or cold water, pick out the finer or bigger crystals. Make sure to remember the necessary safety precautions when working with chemical compounds though, and do write down the new conditions of your experiments! So that you can compare your results and draw conclusions. This is pretty much the life of modern-day scientist!

Why shouldn’t you shake the beaker during the experiment?

Since the forming structures are just heaps of non-soluble crystals which are formed in rather chaotic ways, they a rather fragile. Even the smallest movement of the solution in the beaker can transform your artificial seaweed into a murky brew of a slushy color. You can check this quite easily, just nudge the seaweed slightly with something like a toothpick or a match.