The correct option is **C. Liquid-in-glass thermometer**. ### Detailed Explanation 1. **Understanding the Liquid-in-Glass Thermometer**: - A liquid-in-glass thermometer consists of a glass tube that contains a liquid, usually mercury or colored alcohol. - As the temperature increases, the liquid expands and rises in the tube. Conversely, when the temperature decreases, the liquid contracts and falls back down the tube. - The height of the liquid column corresponds to the temperature, which can be read against a calibrated scale marked on the glass. 2. **Why Option C is Correct**: - The defining feature of a liquid-in-glass thermometer is its reliance on the thermal expansion of a liquid. This principle is based on the fact that most liquids expand when heated and contract when cooled. - Mercury is often used because it has a high thermal conductivity, a wide temperature range, and does not wet the glass, allowing for a clear reading. Colored alcohol is used in many household thermometers because it is safer and can be easily seen. 3. **Why the Other Options are Incorrect**: - **A. Bimetallic thermometer**: - 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. This method does not involve liquid expansion, making it different from the liquid-in-glass thermometer. - **B. Digital thermometer**: - Digital thermometers use electronic sensors to measure temperature. They convert the temperature reading into a digital format, often using thermocouples or thermistors. There is no liquid involved in this type of thermometer, so it does not fit the description provided in the question. - **D. Infrared thermometer**: - Infrared thermometers measure temperature from a distance by detecting the infrared radiation emitted by an object. They do not use any liquid and are based on the principles of thermography, making them fundamentally different from liquid-in-glass thermometers. ### Common Pitfalls - Students may confuse the liquid-in-glass thermometer with other types of thermometers that also measure temperature but do so through different mechanisms (like bimetallic or digital thermometers). - It's important to remember that the key characteristic of the liquid-in-glass thermometer is the use of liquid expansion, which is not a feature of the other options. ### Summary for Revision - **Liquid-in-glass thermometers** measure temperature through the expansion of liquids (mercury or colored alcohol). - **Bimetallic thermometers** use the bending of two metals to indicate temperature. - **Digital thermometers** rely on electronic sensors and do not use liquid. - **Infrared thermometers** measure temperature from emitted infrared radiation, not through liquid expansion. By understanding these distinctions, you can confidently identify the correct type of thermometer based on its operational principles.