At what temperature does pure water reach its maximum density under normal atmospheric pressure?

• A. 100 °C
• B. 25 °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 normal atmospheric pressure, which is a unique physical property of water.
2. Recall that water behaves anomalously compared to most substances: it expands upon freezing and has a maximum density at a temperature above its freezing point.
3. Recognize that the maximum density of water occurs at approximately 4 °C, which is why ice floats on liquid water and why aquatic life can survive under ice in winter.
4. Note that temperatures like 0 °C correspond to the freezing point of water, and 100 °C corresponds to the boiling point, neither of which is where maximum density occurs.
5. Therefore, the key concept is that water's maximum density is at 4 °C under normal atmospheric pressure, which is a fundamental property important in environmental and physical chemistry.