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 its energy the energy is not destroyed but is
transferred to another body or system of bodies. Just as energy cannot
be destroyed, neither can it be created. If one body gains a certain
amount of energy, some other body has lost an equivalent amount. These
facts are summed up in the law of conservation of energy which may be
stated thus: _While energy can be changed from one form into another, it
cannot be created or destroyed._

~Transformations of energy.~ Although energy can neither be creat