Which indicator is used to determine whether a solution is acidic or basic by changing color?

• A. pH meter
• B. Thermometer
• C. Barometer
• D. Litmus paper

Answer: (D)

Indicators that change color in response to pH are used to tell whether a solution is acidic or basic. Litmus paper is a classic example of this. When blue litmus is placed in an acidic solution, it changes color to red, signaling acidity. When red litmus is placed in a basic solution, it turns blue, signaling basicity. Neutral solutions generally don’t cause a color change. This simple color shift makes litmus paper a quick and straightforward way to distinguish acids from bases. Other tools like pH meters can give a precise numerical value for how acidic or basic a solution is, but they don’t rely on a color change, while thermometers and barometers measure temperature and pressure, not acidity.