An Elementary Study of Chemistry | Page 3

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 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 created nor destroyed, it is evident that it may assume many different forms. Thus the falling