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.

Workbook

MP