At which of the following temperatures is the density of pure water at atmospheric pressure greatest?

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

Answer: (B)

Verified step by step guidance

1. Understand that the density of a substance is defined as its mass divided by its volume, \(\rho = \frac{m}{V}\), and for water, density changes with temperature due to thermal expansion or contraction.
2. Recall that water exhibits anomalous behavior near 0 °C, where it reaches its maximum density at approximately 4 °C, rather than continuously increasing as temperature decreases.
3. Recognize that at 0 °C, water is close to freezing and begins to form ice, which has a lower density than liquid water, so the density at 0 °C is less than at 4 °C.
4. At higher temperatures like 25 °C and 100 °C, water expands due to increased molecular motion, decreasing its density compared to the density at 4 °C.
5. Therefore, by comparing these temperature points, the density of pure water at atmospheric pressure is greatest at 4 °C because this is the temperature where water molecules are most closely packed before expansion occurs.