Which term describes the ratio of the weight of a liquid to the weight of equal volume of water at the same temperature?

• A. Density
• B. Viscosity
• C. Specific gravity
• D. Refractive index

Answer: (C)

When you compare the weight of a liquid to the weight of an equal volume of water at the same temperature, you’re looking at a ratio that is independent of units and volume—this is called specific gravity. It is defined as the weight (or density) of a given volume of liquid divided by the weight (or density) of the same volume of water at the same temperature. Because the volumes are the same, the ratio yields a dimensionless number. Water is the reference with a specific gravity of 1; liquids denser than water have a value greater than 1, while lighter liquids have a value less than 1.

Density is related but is a property with units (mass per volume) rather than a pure ratio to water. Viscosity and refractive index describe entirely different properties (flow resistance and how light bends, respectively) and don’t express a weight-to-water ratio.