Specific gravity equals the ratio of a fluid's density to the density of which substance?

• A. Pure water.
• B. Dry air
• C. Sea water
• D. The same fluid at standard temperature

Answer: (A)

Specific gravity is the ratio of a fluid’s density to a standard reference density. The standard reference is pure water, usually at 4°C, which has a defined density of about 1.000 g/cm³. Because water provides a stable, well-known baseline, SG becomes a dimensionless number that shows how dense a liquid is relative to water. Dry air isn’t used because its density varies greatly with temperature and pressure, so it wouldn’t provide a consistent standard. Sea water has a higher density than freshwater, but SG is defined against freshwater, not seawater. If you compared a fluid to itself at standard temperature, the ratio would be 1, which isn’t useful for comparing different liquids. So the reference substance is pure water.