The correct option for the thermometric substance of an absolute thermometer is **C. helium**. ### Detailed Explanation 1. **Understanding Absolute Thermometers**: - An absolute thermometer is designed to measure temperature based on the absolute scale, which is a temperature scale that starts at absolute zero (0 Kelvin). This type of thermometer is often used in scientific research and applications where precise temperature measurements are crucial. 2. **Thermometric Substances**: - Thermometric substances are materials used in thermometers to measure temperature changes. The effectiveness of a thermometric substance depends on its physical properties, such as thermal expansion, boiling point, freezing point, and the ability to remain in a gaseous state at low temperatures. 3. **Why Helium is the Correct Answer**: - Helium is a noble gas that remains in a gaseous state even at extremely low temperatures. It has a very low boiling point (around -269°C or 4 K), making it suitable for measuring temperatures close to absolute zero. - In absolute thermometers, helium can be used in a gas thermometer setup, where the pressure of the gas is measured to determine the temperature. The relationship between pressure and temperature for an ideal gas is described by the ideal gas law: \[ PV = nRT \] where \( P \) is pressure, \( V \) is volume, \( n \) is the number of moles of gas, \( R \) is the ideal gas constant, and \( T \) is temperature in Kelvin. This relationship allows for accurate temperature readings. 4. **Why the Other Options are Incorrect**: - **A. Alcohol**: While alcohol is commonly used in traditional thermometers (like alcohol thermometers), it has a higher freezing point compared to helium and is not suitable for measuring temperatures near absolute zero. Alcohol expands and contracts with temperature changes, but it cannot function effectively at extremely low temperatures. - **B. Mercury**: Mercury is another common thermometric substance used in thermometers, but it also has limitations. Its freezing point is -38.83°C, which means it cannot measure temperatures below this point. Additionally, mercury is toxic and poses health risks, making it less desirable for certain applications. - **D. Platinum**: Platinum is used in resistance thermometers (like platinum resistance thermometers) due to its stable resistance properties over a range of temperatures. However, it is not a gas and does not function as a thermometric substance in the same way helium does. It is more suitable for precise temperature measurements in a different context, but not for an absolute thermometer. ### Summary of Key Points - **Absolute Thermometers** measure temperature starting from absolute zero (0 K). - **Helium** is the ideal thermometric substance for absolute thermometers due to its low boiling point and gaseous state at low temperatures. - **Alcohol and Mercury** are not suitable for absolute thermometers due to their higher freezing points and limitations in low-temperature measurements. - **Platinum** is used in resistance thermometers but does not serve as a thermometric substance in the context of absolute thermometers. This thorough understanding of the properties of thermometric substances and their applications will help you in your studies and exams related to thermodynamics and temperature measurement.