Investigating Crystals
Name:
Background: All rocks are composed of minerals. In many rocks these minerals are in the form of
crystals. Initially crystals form when molten material cools. The rate at which the rock cools
determines the size of the crystals, e.g. granite (large crystals) and basalt (small crystals).
Aim:
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Independent variable: What will you change?
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Dependent variable: What will you measure?
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Controlled variables: What will you keep the same?
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Fair test: How will you make sure this test remains fair?
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Prediction: Predict if the crystal size will be smaller or larger when the solution is cooled slowly or
cooled quickly.
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Materials:
For each group
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personal protective equipment (PPE), including safety glasses
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1 x beaker
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2 x test tubes
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Bunsen burner
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matches
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crushed ice in a 250 mL beaker
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thermometer
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prepared saturated solution of aluminium potassium sulphate (alum)
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10–15 g of aluminium potassium sulphate, in solid form
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spatula
Risk assessment: What risks can you identify in this investigation? What measures can be taken to
minimise the risks?
Personal safety
Other
safety glasses
Report all spills and breakages.
gloves
Dispose of chemicals correctly by placing in ice
cream bucket
hair tied back
closed shoes
What are the risks?
How will you manage them?
handling chemical compounds
Wear PPE.
e.g. aluminium potassium sulphate
Collect waste — do not put down drains.
Method:
1. Half fill a beaker with the saturated solution of your chosen chemical compound.
2. Add one spatula of your solid chemical compound to the solution.
3. Using a tripod and gauze mat, heat the solution gently over a Bunsen burner flame,
stirring the solution to dissolve the compound. (Note: Take care as the mixture begins
to boil, it may bubble up.)
4. Repeat steps 2 and 3 until no more solid compound will dissolve in the solution.
5. Divide the contents evenly between two clean test tubes.
6. Place one test tube into a larger beaker of crushed ice to cool the contents
immediately.
7. Leave the other test tube to cool down slowly in an empty beaker.
8. Inspect the contents of both test tubes the next day.
9. Draw and label a diagram of the crystal size from each test tube.
Data collection: How will you record your observations?
Draw a diagram of quickly cooled crystals.
Draw a diagram of slowly cooled crystals.
Discussion:
Data: What does your data tell you? Were the results consistent with your prediction?
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Describe the effect of cooling rate on the size of crystals formed.
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Were the results consistent with your prediction? Explain your response.
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Under what circumstances would a rock cool quickly or slowly?
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Why does the cooling rate affect the size of crystals? (Hint: Think about what is happening at the
particle level.)
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Evaluating the experimental method: What aspects of the method did not work well?
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Conclusion: Write a sentence or two summarising the connection between the results and the aim
of the investigation.
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