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

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.

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.