Which temperature does pure water freeze at under standard conditions?

Which temperature does pure water freeze at under standard conditions?

• A. -5°C
• B. 0°C
• C. 5°C
• D. -10°C

Answer: (B)

The key idea is the freezing (melting) point of a substance at a given pressure. For pure water at standard atmospheric pressure (1 atm), this point is exactly 0°C, where ice and liquid water are in equilibrium. So under standard conditions, pure water begins to freeze at 0°C. If impurities are present, the freezing point drops a bit (freezing point depression), which is why impure water or salt solutions freeze at temperatures below 0°C.

The key idea is the freezing (melting) point of a substance at a given pressure. For pure water at standard atmospheric pressure (1 atm), this point is exactly 0°C, where ice and liquid water are in equilibrium. So under standard conditions, pure water begins to freeze at 0°C. If impurities are present, the freezing point drops a bit (freezing point depression), which is why impure water or salt solutions freeze at temperatures below 0°C.