An Elementary Study of Chemistry | Page 3

two sciences can be given.
~Physical changes.~ One class of changes is not accompanied by an

alteration in the composition of matter. When a lump of coal is broken
the pieces do not differ from the original lump save in size. A rod of
iron may be broken into pieces; it may be magnetized; it may be heated
until it glows; it may be melted. In none of these changes has the
composition of the iron been affected. The pieces of iron, the
magnetized iron, the glowing iron, the melted iron, are just as truly iron
as was the original rod. Sugar may be dissolved in water, but neither
the sugar nor the water is changed in composition. The resulting liquid
has the sweet taste of sugar; moreover the water may be evaporated by
heating and the sugar recovered unchanged. Such changes are called
physical changes.
DEFINITION: Physical changes are those which do not involve a
change in the composition of the matter.
~Chemical changes.~ Matter may undergo other changes in which its
composition is altered. When a lump of coal is burned ashes and
invisible gases are formed which are entirely different in composition
and properties from the original coal. A rod of iron when exposed to
moist air is gradually changed into rust, which is entirely different from
the original iron. When sugar is heated a black substance is formed
which is neither sweet nor soluble in water. Such changes are evidently
quite different from the physical changes just described, for in them
new substances are formed in place of the ones undergoing change.
Changes of this kind are called chemical changes.
DEFINITION: Chemical changes are those which involve a change in
the composition of the matter.
~How to distinguish between physical and chemical changes.~ It is not
always easy to tell to which class a given change belongs, and many
cases will require careful thought on the part of the student. The test
question in all cases is, Has the composition of the substance been
changed? Usually this can be answered by a study of the properties of
the substance before and after the change, since a change in
composition is attended by a change in properties. In some cases,
however, only a trained observer can decide the question.

~Changes in physical state.~ One class of physical changes should be
noted with especial care, since it is likely to prove misleading. It is a
familiar fact that ice is changed into water, and water into steam, by
heating. Here we have three different substances,--the solid ice, the
liquid water, and the gaseous steam,--the properties of which differ
widely. The chemist can readily show, however, that these three bodies
have exactly the same composition, being composed of the same
substances in the same proportion. Hence the change from one of these
substances into another is a physical change. Many other substances
may, under suitable conditions, be changed from solids into liquids, or
from liquids into gases, without change in composition. Thus butter and
wax will melt when heated; alcohol and gasoline will evaporate when
exposed to the air. The three states--solid, liquid, and gas--are called
the three physical states of matter.
~Physical and chemical properties.~ Many properties of a substance
can be noted without causing the substance to undergo chemical change,
and are therefore called its physical properties. Among these are its
physical state, color, odor, taste, size, shape, weight. Other properties
are only discovered when the substance undergoes chemical change.
These are called its chemical properties. Thus we find that coal burns
in air, gunpowder explodes when ignited, milk sours when exposed to
air.
~Definition of physics and chemistry.~ It is now possible to make a
general distinction between physics and chemistry.
DEFINITION: Physics is the science which deals with those changes in
matter which do not involve a change in composition.
DEFINITION: Chemistry is the science which deals with those changes
in matter which do involve a change in composition.
~Two factors in all changes.~ In all the changes which matter can
undergo, whether physical or chemical, two factors must be taken into
account, namely, energy and matter.
~Energy.~ It is a familiar fact that certain bodies have the power to do

work. Thus water falling from a height upon a water wheel turns the
wheel and in this way does the work of the mills. Magnetized iron
attracts iron to itself and the motion of the iron as it moves towards the
magnet can be made to do work. When coal is burned it causes the
engine to move and transports the loaded cars from place to place.
When a body has this power to do work it is said to possess energy.
~Law of conservation of energy.~ Careful experiments have shown that
when one body parts with