~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_.