Name a substance which can be purified by crystallisation.

Explanation

**Step-by-Step Solution:**

1. **Understanding Crystallization**: Crystallization is a chemical process used to separate a pure substance from an impure solution. It involves the formation of solid crystals from a solution.

2. **Identifying Substances**: There are several substances that can be purified using the crystallization process. Some common examples include:
- Baking soda (sodium bicarbonate)
- Alum (specifically potash alum)
- Tetraamine copper(II) sulfate
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