Which of the following is an acid-base indicator?

Question

Accepted Answer

Explanation:

1. **Why Option 3 (Turmeric) is Correct:**
   Turmeric is a natural acid-base indicator. It contains a compound called curcumin, which changes color in response to pH changes. In acidic or neutral solutions, turmeric maintains its yellow color. However, in basic or alkaline solutions, it turns a reddish-brown color. This color change property makes turmeric an effective and widely used natural indicator for detecting the presence of a base.

2. **Why the Other Options are Incorrect:**

- **Option 1 (Vinegar):**
     Vinegar is not an acid-base indicator; it is an acidic substance primarily composed of acetic acid. It does not change color in response to pH changes. Instead, vinegar itself can be used to test for the presence of a base by neutralizing it, but it does not serve as an indicator that visually signals pH changes.

- **Option 2 (Lime Water):**
     Lime water is a solution of calcium hydroxide and is basic in nature. It is not an acid-base indicator. Lime water is often used to test for the presence of carbon dioxide gas, as it turns milky when carbon dioxide is bubbled through it due to the formation of calcium carbonate. However, it does not change color in response to pH changes and thus does not function as an indicator.

- **Option 4 (Baking Soda):**
     Baking soda, or sodium bicarbonate, is a basic compound. It is not an acid-base indicator. While it can react with acids to produce carbon dioxide gas, it does not exhibit a color change that would indicate a change in pH. Therefore, it cannot be used as an indicator for acid-base reactions.

In conclusion, turmeric is the only option among the given choices that acts as an acid-base indicator due to its ability to change color in response to pH changes, specifically turning reddish-brown in basic conditions.