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PHYSICAL AND CHEMICAL CHANGESTransformations are of two kinds, known as physical and chemical changes.Physical changes are those which effect the state or condition of matter withoutchanging its composition. There is no change in the composition of a substancewhen it is transformed from the liquid to the gaseous state by heating, or from theliquid to the solid state by cooling, and vice versa. Thus, in all three states, stream,water, and ice are all the same substance, made up of the simpler substances,hydrogen and oxygen. The state, but not the composition is effected by the process ofheating or cooling.By changes in the conditions of a substance are meant changes such as when asolid mass is made to fine powder, salt or sugar are dissolved in water. It is clear thatsuch transformations like the changes from one state of matter to another, do notaffect the composition of the material. The fine power of a substance may be madesolid again, and we get back the sail or sugar unchanged by evaporating the water.Chemical changes, however, are those matter changes in which changes ofcomposition does take place and the new substance formed will not return to itsoriginal state, unless it is subjected to another chemical change. Thus iron, when it iseffected by moist a ir, is slowly transformed into iron rust entirely different in itsproperties from the original iron. The new substances will not become iron againwhen it is allowed to stand, although the iron can be obtained from it if it issubjected to the proper conditions
PHYSICAL PROPERTIES OF GASES, LIQUIDS AND SOLIDSThe characteristic property of a gas is that of diffusion; a gas distributes itselfuniformly in any space into which it is put; its volume is determined by the volume ofthe vessel which contains it. Gases are very compressible; a relatively small change involume in which they expand when heated is great compared with that of liquids andsolids.Liquids are characterized by possessing a definite volume but no fixed form;they take the shape of the vessel containing them, except when in minute quantities,as drops, they assume a form more or less spherical. According to the kinetic theory,the molecules of a liquid are packed close together and attract one another; theattraction is not great enough, however, to prevent their motion. There is not muchfree space between the molecules, because liquids are very incompressible – a greatpressure has little effect in decreasing the volume of a liquid.Solids possess a definite form. According to the kinetic theory the moleculesare closely packed, and the attraction between them is great enough to prevent freemotion. It is possible in a solid, therefore, to have a definite arrangement of themolecules which remain fixed; this is seer in the fact that many solids assume definiteforms which are characteristic. For example, when sodium chloride separates from asolution in water, it appears as cubes. Other substances appear in other geometricforms. The study of the forms of crystals has developed into a science calledcrystallography, a knowledge of which is of service to the chemist.