**Correct Option: C. Mercury thermometer** ### Explanation of Why the Answer is Correct: A mercury thermometer operates based on the principle of thermal expansion of a liquid, specifically mercury. When the temperature of the mercury increases, it expands and rises in a narrow glass tube. 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. **Step-by-Step Explanation:** 1. **Principle of Thermal Expansion**: - All liquids expand when heated and contract when cooled. This is known as thermal expansion. The degree to which a liquid expands or contracts is dependent on its properties and the temperature change. 2. **Construction of a Mercury Thermometer**: - A typical mercury thermometer consists of a glass tube that is sealed at one end and contains mercury. The other end is open to the atmosphere, allowing the mercury to move freely. - The tube is marked with a scale (usually in degrees Celsius or Fahrenheit) that indicates the temperature based on the height of the mercury column. 3. **How It Works**: - When the thermometer is placed in a hot environment, the heat causes the mercury to absorb energy, which increases its kinetic energy. As a result, the mercury expands and moves up the tube. - The height of the mercury column corresponds to a specific temperature on the scale, allowing for an accurate reading. 4. **Calibration**: - Mercury thermometers are calibrated at known temperature points (like the freezing point of water at 0°C and the boiling point at 100°C) to ensure accuracy. ### Why the Other Options Are Wrong or Weaker: **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. While it also measures temperature through expansion, it does not use a liquid, making it different from the mercury thermometer. **B. Digital thermometer**: - Digital thermometers use electronic sensors (like thermocouples or thermistors) to measure temperature. They convert the temperature reading into a digital format for display. This method does not involve thermal expansion of a liquid, so it is not applicable to the question. **D. Infrared thermometer**: - Infrared thermometers measure temperature by detecting the infrared radiation emitted by an object. They do not rely on any physical change in a substance (like thermal expansion of a liquid) to measure temperature. Instead, they provide a non-contact method of measuring temperature, which is fundamentally different from the principle used in a mercury thermometer. ### Common Pitfalls: - Confusing the principle of thermal expansion with other temperature measurement principles, such as electrical resistance or radiation detection. - Misunderstanding the construction and operation of different types of thermometers, leading to incorrect identification of their working principles. ### Revision Summary: - A mercury thermometer measures temperature through the thermal expansion of mercury, which rises or falls in a glass tube. - It is calibrated against known temperature points for accuracy. - Other types of thermometers (bimetallic, digital, infrared) use different principles and do not rely on liquid expansion. - Understanding the specific working principle of each thermometer type is crucial for accurate identification and application.