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Unit II Properties of Matter

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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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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.

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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. Iridium 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.

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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/cm 3. 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/cm 3, is 19. 3 times more massive than an equal volume of water.

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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.

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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.

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