The specific gravity of a liquid or solid is based on the density of water at this temperature.

• A. 25 °C
• B. 0 °C
• C. 4 °C
• D. 100 °C

Answer: (C)

Specific gravity is the ratio of a substance’s density to the density of water measured at a fixed reference temperature. Water is densest near 4°C, around 1.0 g/mL, so using 4°C as the reference makes SG values consistent and not influenced by how water’s density changes with temperature. If you used water at other temperatures, the reference density would shift and distort the comparison. That’s why 4°C is the standard reference temperature. Among the options, 4°C is the temperature that matches this standard, while the others correspond to water that is less dense than at 4°C.