At what temperature is pure water at atmospheric pressure the densest?

• A. 0 ^ext{o}C
• B. 25 ^ext{o}C
• C. 100 ^ext{o}C
• D. 4 ^ext{o}C

Answer: (D)

Verified step by step guidance

1. Understand that the density of water varies with temperature due to the unique hydrogen bonding and molecular structure of water.
2. Recall that water reaches its maximum density at a temperature slightly above the freezing point, which is unusual compared to most substances that become denser as they get colder.
3. Recognize that at 0 ^\text{o}C, water is just at the freezing point and is less dense than at some higher temperatures because ice is less dense than liquid water.
4. Know that at 100 ^\text{o}C, water is boiling and its density is much lower due to the increased kinetic energy and expansion of molecules.
5. Conclude that the temperature at which pure water is densest at atmospheric pressure is approximately 4 ^\text{o}C, where the molecular arrangement allows the closest packing of water molecules.