The temperature at which water has its maximum density, used as a reference for specific gravity, is what?

• A. The temperature at which air expands most.
• B. The temperature at which water boils at sea level.
• C. The temperature at which water freezes.
• D. The temperature at which water density is maximum, used for SG reference.

Answer: (D)

Water’s density isn’t constant with temperature; it reaches a maximum at about 4°C. That happens because the structure of water and hydrogen bonding allow the liquid to become most tightly packed at that temperature, before it expands again as it gets warmer or begins to approach freezing. Because of this unique point, 4°C is used as the reference density for specific gravity. Specific gravity is the ratio of a liquid’s density to the density of water at 4°C, so pure water has SG ≈ 1.0. This gives a consistent baseline for comparing densities of different liquids. The other statements describe unrelated properties, such as how air expands, water’s boiling point, or water’s freezing point, and aren’t used as the SG reference.