Which substance would be the best choice for calibrating a thermometer in an experiment to measure the freezing point of water?

• A. Vinegar (acetic acid solution)
• B. Pure water (H_2O)
• C. Saltwater solution (NaCl in H_2O)
• D. Ethanol (C_2H_5OH)

Answer: (B)

Verified step by step guidance

1. Understand the purpose of calibration: When calibrating a thermometer to measure the freezing point of water, you want to use a substance with a well-known and reproducible freezing point.
2. Recall the freezing points of the given substances: Pure water freezes at 0°C under standard atmospheric pressure, vinegar (acetic acid solution) freezes at a lower temperature due to impurities, saltwater freezes at an even lower temperature because of dissolved salt (freezing point depression), and ethanol freezes at around -114°C.
3. Recognize that the best calibration substance should have a freezing point close to the temperature you want to measure, which is the freezing point of pure water (0°C).
4. Choose pure water because it has a precise and consistent freezing point, making it ideal for calibrating a thermometer intended to measure the freezing point of water.
5. Avoid solutions like vinegar or saltwater because their freezing points vary with concentration and impurities, and ethanol because its freezing point is far from 0°C, making them unsuitable for this calibration.