Answer in one or two word:
A chemical normally used to test the rate of transpiration on either side of a leaf.

Explanation

**Step-by-Step Solution:**

1. **Identify the Chemical**: The question asks for a chemical used to test the rate of transpiration on leaves.

2. **Recall the Function**: The chemical should indicate the presence of water loss through transpiration.

3. **Know the Chemical**: The chemical that is commonly used for this purpose is cobalt chloride.

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