Give two requirements for vapour phase refining.

Explanation

### Step-by-Step Text Solution:

1. **Definition of Vapour Phase Refining**: Vapour phase refining is a method used to purify metals by converting them into volatile compounds. These compounds can then be collected and decomposed to yield pure metal.

2. **Requirement 1: Formation of a Volatile Compound**:
- The first requirement for vapour phase refining is that the metal must be able to form a volatile compound. This is crucial because the ability to form a volatile compound allows for the collection and storage of the compound for further processing.

3. **Requirement 2: Decomposability of the Volatile Compound**:
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