What is a proper method to calibrate a thermometer?

Using ice water to calibrate a thermometer is based on the fact that the freezing point of water is a consistent and reliable reference point. When a thermometer is immersed in ice water, it should ideally read 32°F (0°C), which represents the freezing temperature of water under standard atmospheric conditions.

This method is particularly effective because it provides a stable temperature for calibration. Ice water is relatively easy to prepare and maintain at the correct temperature as long as the ice is present and the water is at equilibrium. Holding the thermometer in ice water for 30 seconds gives it sufficient time to adjust to the water temperature, ensuring an accurate reading.

The other options may not provide reliable or consistent calibration points. For instance, placing a thermometer in hot water does not ensure that the thermometer accurately reflects the temperature, as the reading can vary based on water conditions or the type of thermometer used. Leaving it at room temperature would not provide a definitive reference point since room temperatures can vary significantly. Placing it in boiling water may not be suitable for all thermometers, particularly those not rated for high temperatures, leading to potentially erroneous results or even damage to the instrument. Thus, immersing a thermometer in ice water is the most standardized and effective method for calibration.