Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Explanation

### Step-by-Step Solution

1. **Identify the Gas Liberated**: When an acid reacts with a metal, the gas that is usually liberated is hydrogen gas (H₂).

2. **Example of the Reaction**:
- Consider the reaction between zinc (Zn), a metal, and hydrochloric acid (HCl).
- The chemical reaction can be represented as:
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