Page 13 - Theory and Problems of BEGINNING CHEMISTRY

2 BASIC CONCEPTS [CHAP. 1

the earth. It is interesting to note that different conditions on the moon—for example, the lack of water and air
on the surface—might well cause a different sort of distribution of elements on the earth’s satellite.

1.3. MATTER AND ENERGY
Chemistry focusses on the study of matter, including its composition, its properties, its structure, the changes
that it undergoes, and the laws governing those changes. Matter is anything that has mass and occupies space.
Any material object, no matter how large or small, is composed of matter. In contrast, light, heat, and sound
are forms of energy. Energy is the ability to produce change. Whenever a change of any kind occurs, energy is
involved; and whenever any form of energy is changed to another form, it is evidence that a change of some kind
is occurring or has occurred.
The concept of mass is central to the discussion of matter and energy. The mass of an object depends on
the quantity of matter in the object. The more mass the object has, the more it weighs, the harder it is to set into
motion, and the harder it is to change the object’s velocity once it is in motion.
Matter and energy are now known to be somewhat interconvertible. The quantity of energy producible from
a quantity of matter, or vice versa, is given by Einstein’s famous equation

E = mc 2

2
where E is the energy, m is the mass of the matter that is converted to energy, and c is a constant—the square
2
of the velocity of light. The constant c is so large,
2
2
90 000 000 000 000 000 meters /second 2 or 34 600 000 000 miles /second 2
that tremendous quantities of energy are associated with conversions of minute quantities of matter to energy. The
quantity of mass accounted for by the energy contained in a material object is so small that it is not measurable.
Hence, the mass of an object is very nearly identical to the quantity of matter in the object. Particles of energy
have very small masses despite having no matter whatsoever; that is, all the mass of a particle of light is associated
with its energy. Even for the most energetic of light particles, the mass is small. The quantity of mass in any
material body corresponding to the energy of the body is so small that it was not even conceived of until Einstein
published his theory of relativity in 1905. Thereafter, it had only theoretical significance until World War II,
when it was discovered how radioactive processes could be used to transform very small quantities of matter into
very large quantities of energy, from which resulted the atomic and hydrogen bombs. Peaceful uses of atomic
energy have developed since that time, including the production of the greater part of the electric power in many
countries.
The mass of an object is directly associated with its weight. The weight of a body is the pull on the body by
the nearest celestial body. On earth, the weight of a body is the pull of the earth on the body, but on the moon,
the weight corresponds to the pull of the moon on the body. The weight of a body is directly proportional to its
mass and also depends on the distance of the body from the center of the earth or moon or whatever celestial
body the object is near. In contrast, the mass of an object is independent of its position. At any given location,
for example, on the surface of the earth, the weight of an object is directly proportional to its mass.
When astronauts walk on the moon, they must take care to adjust to the lower gravity on the moon. Their
masses are the same no matter where they are, but their weights are about one-sixth as much on the moon as
on the earth because the moon is so much lighter than the earth. A given push, which would cause an astronaut
to jump 1 ft high on the earth, would cause her or him to jump 6 ft on the moon. Since weight and mass are
directly proportional on the surface of the earth, chemists have often used the terms interchangeably. The custom
formerly was to use the term weight, but modern practice tends to use the term mass to describe quantities of
matter. In this text, the term mass is used, but other chemistry texts might use the term weight, and the student
must be aware that some instructors still prefer the latter.
The study of chemistry is concerned with the changes that matter undergoes, and therefore chemistry is also
concerned with energy. Energy occurs in many forms—heat, light, sound, chemical energy, mechanical energy,
electrical energy, and nuclear energy. In general, it is possible to convert each of these forms of energy to others.