Chemistry Reference
In-Depth Information
1.3
Matter and Energy
Matter
is anything that has mass and occupies space. All the material things in
the universe are composed of matter, including anything we can touch as well
as the planets in the solar system and all the stars in the sky.
The
mass
of an object measures how much matter is in the object. Mass is
directly proportional to weight at any given place in the universe. If we leave
the surface of the Earth, our mass remains the same, but our weight changes. An
astronaut positioned between two celestial bodies such that their gravitational
attractions pull equally in opposite directions is weightless, but the astronaut's
mass remains the same as it is on Earth. Because chemists ordinarily do their
work on the Earth's surface and because mass and weight are directly propor-
tional here, many chemists use the terms
mass
and
weight
interchangeably, but
we must remember that they differ.
Energy
is the capacity to do work. We cannot hold a sound or a beam of
light in our hands; they are not forms of matter but forms of energy. Some of
the many forms of energy are outlined in Table 1.3. Energy cannot be created
or destroyed, but it can be converted from one form to another. This statement
is known as the
law of conservation of energy.
Table 1.3
Forms of Energy
Heat
Chemical
Nuclear
Mechanical
Kinetic (energy of motion)
Potential (energy of position)
Electrical
Sound
Electromagnetic (light)
Visible light
Ultraviolet
X-rays
Gamma rays
Infrared
Radio waves
Microwaves
Solar*
EXAMPLE 1.6
What desired energy conversion is exhibited by (a) use of a flashlight and
(b) an automobile consuming gasoline?
Solution
(a)
Chemical energy is converted to electrical energy, which is converted to light.
*Solar energy is a combination of several
forms of light.
(b)
Chemical energy is converted to kinetic energy.
Practice Problem 1.6
What desired energy conversion is exhibited by
(a) an alternator in a car recharging the battery and (b) automobile brakes in use?
ENRICHMENT
In 1905, Albert Einstein (1879-1955)
published his theory that the mass of a sample of
matter is increased as the energy of the sample is in-
creased. For example, a baseball in motion has a
very slightly greater mass than the same baseball at
rest. The difference in mass is given by the famous
equation
c
2
(300,000 kilometers/second)
2
90,000,000,000 kilometers
2
/second
2
(186,000 miles/second)
2
34,600,000,000 miles
2
/second
2
For macroscopic bodies such as a baseball, the in-
crease in mass because of the added energy is so small
that it is not measurable. It was not even discovered un-
til the beginning of the twentieth century. At atomic and
subatomic levels, however, the conversion of a small
quantity of matter into energy is very important. It is the
energy source of the Sun and the stars, the atomic bomb,
the hydrogen bomb, and nuclear power plants.
mc
2
E
In this equation,
E
is the energy of the object,
m
is the
mass difference,
and
c
2
is a very large constant—the
square of the velocity of light:
