Crystal chemistry

Crystals aren’t just beautiful (and expensive), they’re also a fascinating and a useful part of science. 

Skill development

Learners will develop their working scientifically skills by:

• Using appropriate scientific language and ideas to explain, evaluate and communicate their methods and findings.
• Drawing conclusions and raising further questions that could be investigated, based on their data and observations.
• Asking their own questions about scientific phenomena.

Learning objectives

• Observe that some materials will dissolve in liquid to form a solution.
• Demonstrate that dissolving, mixing and changes of state are reversible changes.

Concepts supported

Children will learn:

• What happens in the process of dissolving, in simple terms.
• That various factors can affect dissolving, and be able to provide examples.

Snowflakes

Equipment

• Pipe cleaner
• String
• Salt
• A clean jar
• Hot water

Procedure

1. Cut the pipe cleaner into three pieces and twist together to make the snowflake shape.
2. Tie one end of the string to the centre of the snowflake and the other end to the pencil to a suitable length.
3. Pour hot water into one of the jars.
4. Add a small amount of salt and stir. Keep adding salt and stirring until it stops dissolving (you will see it on the bottom of the jar).
5. Balance the pencil on the jar, so the snowflake is covered but not touching the jar.
6. Leave overnight

Crystals

Equipment

• Clean jar of hot water x ½
• Clean jar
• Alum powder
• A pencil
• Nylon fishing line (for big crystals) or sewing thread (for smaller crystals)
• Filter paper (eg a coffee filter)

Procedure

1. Add a small amount of alum to the hot water and stir. Keep adding alum and stirring until it stops dissolving (you will see it at the bottom of the jar).
2. Cover the jar with filter paper and leave overnight.
3. The next day, pour the solution into the second clean jar.
4. You should see crystals at the bottom of the first jar - pick the biggest one. Tie one end of the nylon line around the crystal and the other end to a pencil.
5. Balance the pencil on top of the jar, so the crystal is covered by solution but not touching the jar.
6. Cover with filter paper and leave to grow.

What’s the chemistry?

When you add crystals to water, they will keep dissolving until the water becomes saturated. This means that the water cannot ‘hold’ any more of the crystal. When water is hot, it can ‘hold’ more crystals than when it is cold. So as the solution cools, the crystal comes out as a solid. For the ‘crystal snowflakes’ the pipe cleaner gives the crystals a support to ‘grow’ on. For the ‘big crystals’ the first crystal acts as a seed crystal for the rest of the crystals to ‘grow’ on.

Suggested activity use

This activity could be used as a whole-class investigation or as a guided group task to stimulate discussion and questioning. It provides a useful opportunity to demonstrate dissolving and that this is a reversible change. Also, it provides a chance for learners to compare dissolving solids in hot and cold water and observe the differences. Learners could try growing crystals from a range of solids to see which ones grow best.

Practical considerations

The time taken for the crystals to start to develop could vary from a couple of days to a week.

Health and safety considerations need to be taken into account, particularly with regard to learners working with hot water.

Alum powder may be difficult to source for primary schools. Also, other simple equipment, such as several clean jam jars, will need to be sourced prior to the activity.