A hydrometer measures the relative density of a liquids relative to the density of water. This is referred to as Specific Gravity. Sugar water will have a density that is greater than pure water. Ethanol has a density that is less than water. The hydrometer will sink further into a solution of alcohol and not sink as far into a sugar solution. Specific Gravity is unitless. It defines the ratio of the density of a solution to the density of water. Specific Gravity is calculated like so, Specific Gravity = (Density g/cm)/(1 g/cm).
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