Does temperature change crystal size?
The question
Do alum crystals grow bigger when the solution cools slowly or quickly?
Slow cooling gives large crystals. That is why iron meteorites, which cooled over millions of years inside asteroids, have metal crystals centimetres long.
Equipment
- Potash alum (potassium aluminium sulfate) from a school or chemistry supplier
- Warm water from a kettle, prepared by an adult
- Three identical jars, a spoon and filter paper
- A box of towels or a cool bag for slow cooling, and a fridge for fast cooling
- A hand lens and a ruler
Method
- With an adult, dissolve as much alum as will dissolve in warm water, then filter it into three jars.
- Jar 1: wrap in towels in a box so it cools slowly. Jar 2: leave at room temperature. Jar 3: put in the fridge once it is cool enough to handle.
- After a day, look at the crystals with a hand lens. Leave the jars for a week, keeping them still and covered.
- What to measure
- The number of crystals and the length of the five largest in each jar, after one day and after a week.
- How to present it
- Photos with a scale, a table and a bar chart, and an explanation using the particle model linked to igneous rocks and iron meteorites.
- Take it further
- Find out how the Widmanstätten pattern in iron meteorites formed, and why it cannot be made in a factory.
Safety
Alum is low hazard but must not be eaten; wash hands after handling. An adult handles the hot water. Label the jars clearly and keep them away from food.