**Correct Option: C. Mercury thermometer** ### Explanation of the Correct Answer A mercury thermometer operates on the principle of thermal expansion of a liquid—in this case, mercury. When the temperature increases, the mercury expands and rises in the glass tube of the thermometer. Conversely, when the temperature decreases, the mercury contracts and falls back down the tube. This expansion and contraction are directly related to temperature changes, allowing the thermometer to provide a precise reading of the temperature. **How a Mercury Thermometer Works:** 1. **Construction**: A mercury thermometer consists of a glass tube that is sealed at one end and contains mercury. The other end is open to the atmosphere and has a scale marked on it. 2. **Thermal Expansion**: As the temperature rises, the kinetic energy of the mercury molecules increases, causing them to move apart and occupy a larger volume. This results in the mercury rising in the tube. 3. **Reading the Temperature**: The height of the mercury column corresponds to a specific temperature on the scale, allowing for an accurate reading. ### Why Other Options Are Incorrect **A. Bimetallic thermometer** - **Explanation**: A bimetallic thermometer uses two different metals that expand at different rates when heated. These metals are bonded together, and as they expand, they bend, moving a pointer on a dial to indicate temperature. While this method also measures temperature, it does not rely on the expansion of a liquid, making it different from a mercury thermometer. **B. Infrared thermometer** - **Explanation**: An infrared thermometer measures temperature by detecting the infrared radiation emitted by an object. It does not involve any liquid or physical expansion; instead, it uses sensors to convert infrared radiation into a temperature reading. Therefore, it does not operate on the principle of liquid expansion. **D. Thermocouple** - **Explanation**: A thermocouple consists of two different metals joined at one end. When the junction is heated, it produces a voltage that can be correlated to temperature. Like the infrared thermometer, it does not involve the expansion of a liquid, making it fundamentally different from a mercury thermometer. ### Summary of Key Concepts - **Mercury Thermometer**: Operates on the principle of liquid expansion; mercury expands and contracts with temperature changes. - **Bimetallic Thermometer**: Uses two metals that bend with temperature changes; does not involve liquid. - **Infrared Thermometer**: Measures temperature based on emitted infrared radiation; no liquid involved. - **Thermocouple**: Generates voltage from temperature differences; does not use liquid expansion. ### Revision Summary - Mercury thermometers measure temperature through the expansion of mercury in a glass tube. - Bimetallic thermometers use metal expansion, not liquid. - Infrared thermometers detect radiation, not temperature through liquid. - Thermocouples rely on voltage generation from temperature differences, not liquid expansion.