What substance is commonly used as a reference to measure specific gravity of solids?

The measurement of specific gravity for solids is commonly referenced against water at standard conditions. Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, which in most cases is water because of its abundance and convenience in laboratory settings.

Water has a defined density at standard conditions, which makes it an ideal standard for comparison. When determining the specific gravity of a solid, the solid's density is measured, and the result is divided by the density of water. The specific gravity value indicates whether the solid will float or sink when placed in water.

Using other substances like alcohol, mercury, or air introduces variability that can complicate the measurement. For example, while mercury is denser than water and can be used for liquids, it is not a suitable reference for solids. Similarly, air's density can vary with temperature and pressure, making it unreliable as a consistent reference for solid materials. Thus, water at standard conditions remains the most practical and widely used reference for measuring the specific gravity of solids.