Crystals from solutions

Crystals are fascinating objects. They have smooth sides and sharp points. They may have bright colours or sparkle in bright light. Most crystals are made when hot rock cools down but some can be made from substances dissolved in water.

When a solid dissolves in water, its particles separate and spread out between the water particles. This process continues until all the spaces between the particles are full of the solid particles. When a solid dissolves in water it forms a solution. If the dissolving process continues until the solution can take no more of the solid, the solution is called a saturated solution. You can tell when a solution is saturated. If you add any more of the solid to it, the solid just falls to the bottom of the container and rests there. It does not dissolve. Crystals can be made from saturated solutions.

A crystal will not just form anywhere in a saturated solution. It needs a solid object on which to form. The best solid object to use is a small crystal of the same substance. If this crystal has a good shape the crystal which grows on it will have a good shape too. The crystal that encourages crystal growth is called a seed crystal. A thread is put round it and it is suspended in the middle of the saturated solution.

You have to be patient to grow a large crystal. It can take a long time. This means that the saturated solution and its seed crystal have to be put in a place where they can be left safely for weeks. You must not disturb a crystal while it is forming.

All liquids evaporate. This is a process which takes place at the liquid surface. Liquid particles have energy to move about and slide past each other. They pull on each other too. Those particles at the surface have energy to move but are only pulled on three sides. There is nothing pulling on them in the air above them. This difference in the pulls on particles at the surface allows some of them to leave the liquid completely. They use the energy to move to break free of the grasp of the other particles and enter the air. Liquid particles which enter the air are completely separate from each other. They have become particles of a gas.

When the solution and seed crystal are left alone, evaporation occurs at the surface of the saturated solution. When this happens there are fewer water particles left in the solution and fewer spaces to hold the particles of the dissolved solid. This change makes particles of the dissolved solid around the crystal come out of the solution. They settle on the crystal surfaces. The particles form rows like children in a school playground at the end of playtime. The rows cover the surface of the crystal.

At the surface evaporation slowly continues and this makes more rows of particles settle out. The rows stack one above the other and eventually you can see that the crystal has started to grow.

Over the weeks, as evaporation continues, rows and layers of particles build up to form a large crystal, which can then be removed from the saturated solution, cleaned and put on display. People will be keen to see it because crystals are simply fascinating.

Why can't you just put the seed crystal at the bottom of the container?

It might seem a bit fiddly to tie a thread around a small crystal but it is worth it. If the seed crystal were left at the bottom of the container no particles could settle on its underside as the crystal formed. This would give the crystal a flat bottom.

Why do you need a crystal with a good shape?

Because an odd-shaped crystal will grow on an odd-shaped seed crystal. The particles settle out on the crystal surface. If the surface is uneven, the particles will build up in a disorderly way on it and produce an uneven surface on the finished crystal.

Why should you leave the crystal alone while it is forming?

Some people are tempted to take the crystal out of the saturated solution to see how it is growing. Others think it might help to stir up the solution. Both these activities can damage the growing crystal. They can stop the rows and layers of particles settling out evenly. When this happens, the crystal may grow into a less than perfect shape.

Are all crystals the same shape?

No. Some crystals have a cubic shape, some are hexagons, while others are blocks with pointed ends. Each substance produces just one shape of crystal. For example, perfect salt crystals are always cubic.

Does a solution stay saturated when it is evaporating?

Yes, it does. When it loses water particles it must also lose some dissolved solid particles. It only loses particles for which it no longer has space. All the other spaces are still packed with particles of dissolved solid so the solution is still saturated.

Can you grow any crystals without a seed crystal?

Yes, you can. You can make a saturated solution of table salt and pour it into a saucer. Put the saucer on a warm windowsill. After a few days all the water will have evaporated and you will find cubic salt crystals.

What happens if you take a crystal you have made and put it in water?

The water is not saturated. It has spaces for the particles in the crystal to enter. The particles in the crystal separate and enter the spaces and the crystal dissolves.

Why do some crystals, like rock crystal, not dissolve in water?

They were not made by the evaporation of a saturated solution of water. They were made by the cooling down of a rocky mixture. As the rock cooled down substances formed which have crystal shapes. Rocks are made of interlocking crystals and usually they are too small and poorly shaped to be recognised. They are poorly shaped because they did not have enough room to form properly. Sometimes gas bubbles form hollows in rock and crystals then have the space to form properly. Rock crystal is an example of rocky crystal, which forms in hollows in rocks.