At which temperature is pure water most dense?

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

Answer: (C)

Verified step by step guidance

1. Understand that the density of water varies with temperature due to the way water molecules arrange themselves as they gain or lose energy.
2. Recall that water reaches its maximum density at a specific temperature because of the balance between thermal expansion and hydrogen bonding.
3. Recognize that as water cools from higher temperatures, it becomes denser until it reaches about 4 °C, where it is most dense.
4. Below 4 °C, water begins to expand again as it approaches the freezing point (0 °C), which is why ice floats on liquid water.
5. Therefore, the temperature at which pure water is most dense is 4 °C.