Refrigeration & Air Conditioning Technology, 8e - 31

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Unit 2 Matter and Energy
Figure 2.3 The water in the container exerts pressure outward and
downward. The outward pressure is what makes water seek its own level.
The water molecules still have a small amount of adhesion to each other.
The pressure is proportional to the depth.

Figure 2.5 Table of density and specific gravity.
SUBSTANCE

DENSITY LB/FT 3

ALUMINUM

171

2.74

BRASS (RED)

548

8.78

COPPER

WATER

SPECIFIC GRAVITY

GOLD
ICE @ 32°F
TUNGSTEN
WATER
MARBLE

556

8.91

1208

19.36

57.5
1210
62.4
162

0.92
19.39
1
2.596

mass. A liquid such as water is said to have mass. The air in
the atmosphere has weight and mass.
Figure 2.4 Gas molecules travel at random. When a container with a
small amount of gas pressure is opened, the gas molecules seem to repel
each other and fly out.

2.3 DENSITY
The density of a substance describes its mass-to-volume relationship. The mass contained in a particular volume is the
density of that substance. Sometimes it is advantageous to
compare different substances according to weight per unit
volume or pounds per cubic foot. Water, for example, has a
density of 62.4 lb/ft3. Wood floats on water because the density (weight per volume) of wood is less than the density of
water. In other words, it weighs less per cubic foot. Iron, on
the other hand, sinks because it is more dense than water.
Figure 2.5 lists some typical densities. Using Figure 2.5 as a
guide, can you figure out why ice floats on water?

2.4 SPECIFIC GRAVITY

for each other. The vapor exerts pressure evenly in all directions, Figure 2.4.
The study of matter leads to the study of other terms
that help to understand how different substances compare
to each other.

Specific gravity is a unitless number because it is the density
of a substance divided by the density of water. Water is simply used as the standard comparison. The density of water
is 62.4  lb/ft3. So, the specific gravity of water is 62.4  lb/ft3
4 62.4 lb/ft3 5 1. Notice that the units cancel because of the
division. The density of red brass is 548 lb/ft3. Its specific gravity is then 548 lb/ft3 4 62.4 lb/ft3 5 8.78. Figure 2.5 lists some
typical specific gravities of substances. Note that a substance
with a specific gravity greater than 1 will sink in water, and a
substance with a specific gravity less than 1 will float on water.

2.2 MASS AND WEIGHT

2.5 SPECIFIC VOLUME

Mass is the property of matter that responds to gravitational
attraction. Weight is the force that matter (solid, liquid, or
gas) applies to a supporting surface when it is at rest. Weight
is not a property of matter but is dependent on the gravitational attraction. The stronger the force of gravity, the more
an object will weigh. The earth has stronger gravitational
attraction forces than the moon. This is why objects weigh
more on the earth than on the moon. All solid matter has

Specific volume indicates the volume that each pound of a
gas occupies. The measurement requires that there must be
only 1 lb of the gas. The units are ft3/lb, which differs from
the units for total volume, which are simply ft3. One pound
of clean, dry air has a total volume of 13.33 ft3 at standard
atmospheric conditions. Its specific volume would then be
13.33 ft3/lb. As air is heated it becomes lighter and its specific volume increases. As air is cooled, its specific volume

```

Refrigeration & Air Conditioning Technology, 8e

Contents
Refrigeration & Air Conditioning Technology, 8e - Cover1
Refrigeration & Air Conditioning Technology, 8e - Cover2
Refrigeration & Air Conditioning Technology, 8e - i
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