Which of the following temperatures is used as the reference for water density when calculating specific gravity?

• A. 0°C
• B. 20°C
• C. -40°C
• D. 4°C

Answer: (D)

Specific gravity uses a standard reference density for water, and that reference is taken at the temperature where water is densest. Water reaches its maximum density at about 4°C, roughly 1.0 g/cm^3. Using this temperature as the reference keeps SG values consistent and minimizes how much they change with temperature, since water would be slightly less dense at other temperatures. At 0°C or 20°C, the density of water is not 1.0 g/cm^3, which would tilt SG values, and at -40°C water can’t even be a liquid. So the reference temperature is 4°C.