Name one indicator which turns red on adding an acid.

Answer

**Step-by-Step Solution:**

1. **Understand the Question**: The question asks for an indicator that turns red when an acid is added.

2. **Identify Common Indicators**: One of the most common indicators used in chemistry to test for acids and bases is litmus paper.

3. **Know the Properties of Litmus Paper**: Litmus paper comes in two colors: blue and red. The blue litmus paper is used to test for acidic solutions, while red litmus paper is used to test for basic solutions.

4. **Determine the Reaction with Acid**: When blue litmus paper is exposed to an acidic solution, it changes color and turns red. This indicates the presence of an acid.

5. **Conclude the Answer**: Therefore, the indicator that turns red when an acid is added is **blue litmus paper**.

**Final Answer**: Blue litmus paper.

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