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Unit II. Properties of Matter. One of the properties of solids, as well as liquids and even gases, is the measure of how tightly the material is packed together: density. Density is a measure of how much matter occupies a given space; it is the amount of mass per unit volume :.
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Unit II Properties of Matter
One of the properties of solids, as well as liquids and even gases, is the measure of how tightly the material is packed together: density. • Density is a measure of how much matter occupies a given space; • it is the amount of mass per unit volume:
Density is not mass and it is not volume. • Density is a ratio; it is the amount of mass per unit volume. • Density is a property of a material; it doesn’t matter how much you have. • A pure iron nail has the same density as a pure iron frying pan. • The frying pan may have 100 times as many iron atoms and have 100 times as much mass, but its atoms will take up 100 times as much space. • The mass per unit volume for the iron nail and the iron frying pan is the same.
The density of a material depends upon the masses of the individual atoms that make it up, and the spacing between those atoms. • Iridium, a hard, brittle, silvery-white metal in the platinum family, is the densest substance on Earth. • Even though an individual iridium atom is less massive than individual atoms of gold, mercury, lead, or uranium. • The close spacing of iridium atoms in an iridium crystal gives it the greatest density. • A cubic centimeter of iridium contains more atoms than a cubic centimeter of gold or uranium.
Density varies somewhat with temperature and pressure, so, except for water, densities are given at 0°C and atmospheric pressure. • Note that water at 4°C has a density of 1.00 g/cm3. • The gram was originally defined as the mass of a cubic centimeter of water at a temperature of 4°C. • A gold brick, with a density of 19.3 g/cm3 , is 19.3 times more massive than an equal volume of water.
Weight Density • A quantity known as weight density can be expressed by the amount of weight a body has per unit volume: • Weight density is commonly used when discussing liquid pressure.
Specific Gravity • A standard measure of density is specific gravity —the ratio of the mass (or weight) of a substance to the mass (or weight) of an equal volume of water. • For example, if a substance weighs five times as much as an equal volume of water, its specific gravity is 5. • Or put another way, specific gravity is a ratio of the density of a material to the density of water. • So specific gravity has no units (density units divided by density units cancel). • If you want to know the specific gravity of any material listed in Table 18.1, it’s there; the magnitude of its density is its specific gravity.
