sition 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