Ratio of the density of a material to the density of a standard material.

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Specific gravity is the ratio of the density of a material to the density of a standard reference material, making it a dimensionless value. For liquids and solids, the standard is water (density about 1 g/cm³ at 4°C). This means if a substance has the same density as water, its specific gravity is 1; if it is denser, its specific gravity is greater than 1; if it is lighter, it is less than 1. This ratio helps you quickly judge how heavy a material is relative to water and predicts whether it will sink or float in water—an important consideration in hazmat scenarios and spill behavior. For gases, the same idea is used with air as the reference, so the specific gravity indicates whether a gas is heavier or lighter than air. Other terms like pressure or vapor pressure describe other properties (force per area, volatility), and molecular weight is about mass per mole, not the density ratio, so they don’t express this density comparison.