What is the specific gravity defined as?

Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, typically water for liquids and air for gases at a specific temperature and pressure. When units are involved, specific gravity is often expressed as the mass (in grams) of a fixed volume (in milliliters) of the substance, which aligns with the definition of gram per milliliter.

Thus, the definition of specific gravity as gram per milliliter directly connects the concept of density (mass per unit volume) to the comparison with water, reinforcing that it's a dimensionless quantity. The mass of the substance relative to the same volume of water gives us this critical relationship.

Other unit options do not correctly reflect the measurement of specific gravity. Milliliter per gram, for instance, would not provide a direct comparison of the substance's density against water. Liter per kilogram would reference a different scale that doesn't apply here, and gram per liter describes density rather than specific gravity, making it unsuitable as a definition for specific gravity.