Red crystals

Growing red crystals out of potassium ferricyanide

Difficulty:
Danger:
Duration:
2 hours
Experiment's video preview

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
  • Observe safety precautions when working with boiling water.
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

How to preserve the crystals?

With time, these crystals will “wear off” because the water they initially contained constantly evaporates. In order to slow down this process, coat the crystals with a colorless nail polish and seal them in a transparent container. Crystals are quite fragile, but with such a “protection” you’ll be able to enjoy its beauty for at least half a year (unless you decide to subject them to a structural test).

Step-by-step instructions

  1. Pour 100 mL of freshly boiled water in the beaker.
  2. Collect 10 mL of water with a syringe.
  3. Pour water taken with the syringe into a plastic cup. Add one vial оf potassium hexacyanoferrate K3[Fe(CN)6] (3.8 g).
  4. Place the cup into the beaker with hot water.
  5. Stir the contents in the cup for 10 min.
  6. Carefully pour out the obtained solution into a Petri dish.
  7. Within an hour, crystals will start growing in the Petri dish. And in a couple of days, the crystals will become even larger!
Graphical step-by-step instruction

Expected result

As the solution of potassium hexacyanoferrate (III) K3[Fe(CN)6] cools down and the water evaporates, beautiful red crystals form

Disposal

Dispose of solid waste together with household garbage.

Scientific description

How are crystals formed?

The formation of a crystal is a marvelous triumph of order over chaos. Just think of the amazing regularity and the uniform precision of the many small components that make up a crystal. Then think of how these many small components are all precisely incorporated in a perfect structure that we call a crystal.

Crystals may be composed of various components such as molecules (for example, sugar), ions (for example: table salt NaCl) or atoms (for example, diamonds C). A very fascinating and interesting crystal is one made of metals. Yes, metallic crystals! In a metallic crystal, the atoms of a metal are bound together via an electron cloud shared throughout the entire crystal. Some of these crystals consist of an endless chain of metal atoms (for example, crystals of cesium or bismuth). There are also some complex crystals that have both ions and molecules.

The world around us is moving all the time. Yet, in this reality of constant change, crystals show fundamental stability and permanency. How is this possible?

Crystal growth can be compared to the construction of a brick house. If the bricks are simply dumped into a pile, they’ll remain a pile of bricks. However, if each brick is carefully and patiently put into place, stable and solid walls can be built. The same applies to crystals. Under the right conditions, the principal components - the “bricks” - form a strong, precise, carefully assembled structure. It’s important to remember that unlike real bricks, the “bricks” that form crystals can bond to each other without any additional “mortar” or other binding material.

For example, oppositely charged ions attract one another. Thanks to the unique interlacing arrangement of these ions in a crystal, the attraction of the oppositely charged ions outweighs the repulsion of the similarly charged ions.

That’s exactly what happens in a crystal of red prussiate of potash, or potassium hexacyanoferrate (III) K3[Fe(CN)6].

In the first half of the 20th century, scientists discovered new technologies which allowed them to closely examine crystalline structures. This, in turn, helped them to determine the orientation of the atoms and molecules in a crystal and helped them to precisely measure the distance between them. For example, the study of red prussiate of potash crystals (which we obtained in this experiment) demonstrates that each negatively charged ion [Fe(CN)6]3– is surrounded by six positively charged ions K+, while K+ ions are also surrounded by three or four negatively charged ions. These ions are bound by the electrostatic attraction which hinders the crystal from crumbling into a powder. Although a crystal may be crushed into fine dust, it would require a lot of work.

Cyanate

Why does red prussiate of potash precipitate out of the solution and form a crystalline structure?

In this experiment, red prussiate of potash is dissolved in a given amount of water at an elevated temperature of almost 100 oC (212 oF). As the solution slowly cools down and the water evaporates, the ions start to feel very “crowded”. Some of the ions pushed away and leave the solution to form a crystalline precipitate.

Is it possible to dissolve the crystals and repeat the experiment?

Yes, it is possible to dissolve the crystals and repeat the experiment. The red prussiate of potash crystals are soluble in water.