Which substance is commonly used as an acid-base indicator, which turns red in acidic solutions and blue in basic solutions?

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**Solution:**

1. **Litmus paper** is a widely used acid-base indicator in laboratories.

2. It is derived from lichens, which are symbiotic associations between fungi and algae.

3. The key property of litmus paper is that it turns **red in acidic solutions** (pH < 7) and **blue in basic solutions** (pH > 7).

4. Other indicators like methyl orange and bromothymol blue also indicate pH changes but have different color changes and pH ranges.

5. Phenol is not an acid-base indicator.

**Answer:** Litmus paper

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