The temperature of 4 C was selected because water has its maximum density at this temperature. In the metric system, the density of water is 1 gram per cubic centimeter. The standard temperature of 4 C is used whenever the density of liquids and solids is measured. Changes in temperature will not change the weight of a substance but will change the volume of the substance by expansion or contraction, thus changing the weight per unit volume. In physics, the word specific implies a ratio.
Weight is the measure of the earth s attraction for a body.
The earth s attraction for a body is called gravity. Thus, the ratio of the weight of a unit volume of some substance to the weight of an equal volume of a standard substance, measured under standard pressure and temperature con-ditions, is called specific gravity. The terms specific weight and specific density are sometimes used to express this ratio. If a cubic foot of a certain liquid weighs 68.64 pounds, then its specific gravity is 1.1, Thus, the specific gravity of the liquid is the ratio of its density to the density of water. If the specific gravity of a liquid or solid is known, the density of the liquid or solid maybe obtained by multiplying its specific gravity by the density of water.