What do you observe when : Dilute hydrochloric acid reacts with Hg.

Explanation

**Step-by-Step Solution:**

1. **Identify the Reactants**: The reactants in this reaction are dilute hydrochloric acid (HCl) and mercury (Hg).

2. **Understand the Nature of the Reaction**: This is an acid-metal reaction. Hydrochloric acid is an acid, and mercury is a transition metal. When they react, they typically produce a salt and hydrogen gas.

3. **Write the Chemical Equation**: The reaction can be represented by the following chemical equation:
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