# GASES LIQUID AND DENSITY

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A fluid is any substance that can flow and change the shape of the volume that
it occupies. (By contrast, a solid tends to maintain its shape.) We use the term
“fluid” for both gases and liquids. The key difference between them is that a
liquid has cohesion, while a gas does not. The molecules in a liquid are close
to one another, so they can exert attractive forces on each other and thus tend to
stay together (that is, to cohere). That’s why a quantity of liquid maintains the
same volume as it flows: If you pour 500 mL of water into a pan, the water
will still occupy a volume of 500 mL. The molecules of a gas, by contrast,
are separated on average by distances far larger than the size of a molecule.
Hence the forces between molecules are weak, there is little or no cohesion,
and a gas can easily change in volume. If you open the valve on a tank of
compressed oxygen that has a volume of 500 mL, the oxygen will expand to
a far greater volume.

An important property of any material, fluid or solid, is its density, defined as its mass per unit volume. A homogeneous material such as ice or iron has the same density throughout. We use Þ (the Greek letter rho) for density. For a homogeneous material

Two objects made of the same material have the same density even though they
may have different masses and different volumes. That’s because the ratio of
mass to volume is the same for both objects (Fig. 12.1).
The SI unit of density is the kilogram per cubic meter 11 kg/m3. The cgs
unit, the gram per cubic centimeter 11 g/cm3, is also widely used:

1 g/cm3 = 1000 kg/m3

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The densities of some common substances at ordinary temperatures are given in
Table 12.1. Note the wide range of magnitudes. The densest material found on
earth is the metal osmium 1r = 22,500 kg/m3, but its density pales by comparison
to the densities of exotic astronomical objects, such as white dwarf stars
and neutron stars.

The specific gravity of a material is the ratio of its density to the density of
water at 4.0°C, 1000 kg/m3
; it is a pure number without units. For example, the
specific gravity of aluminum is 2.7. “Specific gravity” is a poor term, since it has
nothing to do with gravity; “relative density” would have been a better choice.
The density of some materials varies from point to point within the material.
One example is the material of the human body, which includes low-density fat
) and high-density bone (from 1700 to 2500 kg/m3
). Two others
are the earth’s atmosphere (which is less dense at high altitudes) and oceans
(which are denser at greater depths). For these materials, Eq. (12.1) describes the
average density. In general, the density of a material depends on environmental
factors such as temperature and pressure.

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