# Density

Mass can be thinly distributed as in a pillow, or tightly packed as in a block of lead. The space the mass occupies is its volume, and the mass per unit of volume is its density.

Mass (m) is a fundamental measure of the amount of matter. Weight is a measure of the force exerted by a mass and this force is force is produced by the acceleration of gravity. Therefore, on the surface of the earth, the mass of an object is determined by dividing the weight of an object by 9.8 m/s2 (the acceleration of gravity on the surface of the earth). Since we are typically comparing things on the surface of the earth, the weight of an object is commonly used rather than calculating its mass.

The density (ρ) of a material depends on the phase it is in and the temperature. (The density of liquids and gases is very temperature dependent.) Water in the liquid state has a density of 1 g/cm3 = 1000kg/m3 at 4o C. Ice has a density of 0.917 g/cm3 at 0oC, and it should be noted that this decrease in density for the solid phase is unusual. For almost all other substances, the density of the solid phase is greater than that of the liquid phase. Water vapor (vapor saturated air) has a density of 0.051 g/cm3.

Some common units used for expressing density are grams/cubic centimeter, kilograms/cubic meter, grams/milliliter, grams/liter, pounds for cubic inch and pounds per cubic foot; but it should be obvious that any unit of mass per any unit of volume can be used.

 Substance Density (g/cm3) Air 0.0013 Gasoline 0.7 Wood 0.85 Water (ice) 0.92 Water (liquid) 1.0 Aluminum 2.7 Steel 7.8 Silver 10.5 Lead 11.3 Mercury 13.5 Gold 19.3