Which statement is true about water density?

• A. Water is densest at 0°C
• B. Water is densest at 4°C
• C. Water is densest at 10°C
• D. Water is densest at 20°C

Answer: (B)

Water behaves differently from most liquids because of hydrogen bonding. As it cools from room temperature, the molecules pack a bit more tightly and density increases until about 4°C. At this temperature the liquid is most compact. If the water cools further toward freezing, the hydrogen bonds arrange into a more open, ice-like structure, causing the liquid to occupy more space and its density to drop. So the densest liquid water is at roughly 4°C. At 0°C, 10°C, or 20°C, the density is less than at 4°C because either the water is not as tightly packed (near freezing) or is more thermally expanded (warmer temperatures).