specific gravity

Meaning: Specific gravity (SG) compares a material's density with the density of water, the usual reference for liquids and solids. Because it is a ratio of two densities, the units cancel and SG is dimensionless. Water has SG = 1, so the number tells you how many times as dense the material is as water. Since the density of water is about 1000 kg/m³ (1 g/cm³), the SG of a substance equals its density in g/cm³ almost exactly. Mercury, at about 13.6 g/cm³, has an SG of about 13.6.

Equation: SG = ρsubstance / ρwater, where ρsubstance is the density of the material (kg/m³) and ρwater is the density of water (kg/m³, 1000 at 4 °C). SG itself has no unit.

On the MCAT: Expect fluid passages on buoyancy. A freely floating object displaces its own weight of fluid, so the fraction submerged equals ρobject / ρfluid. An object with SG = 0.9 relative to water floats with 90% of its volume submerged. You may also be asked to compare two fluids or predict whether an object sinks or floats in each.

Common confusion: Students think SG carries units or that it equals density in kg/m³. It does not. A substance with a density of 800 kg/m³ has an SG of 0.8, not 800. Also, SG depends on the reference chosen, so check that the reference is water unless the passage says otherwise.