At what temperature is pure water at its maximum density under standard atmospheric pressure?

• A. 25 °C
• B. 100 °C
• C. 4 °C
• D. 0 °C

Answer: (C)

Verified step by step guidance

1. Understand that the problem asks for the temperature at which pure water reaches its maximum density under standard atmospheric pressure.
2. Recall that water exhibits an unusual property compared to most substances: it reaches its maximum density at a temperature above its freezing point.
3. Recognize that the maximum density of water occurs at approximately 4 °C, which is a well-known physical property of water.
4. Note that at temperatures below 4 °C, water expands as it approaches the freezing point (0 °C), which is why ice floats on liquid water.
5. Therefore, the temperature at which pure water has its maximum density under standard atmospheric pressure is 4 °C.