1The Physical Properties of Substances
The study of the properties of substances constitutes an important part of chem- istry, because their properties determine the uses to which they can be put. The properties of substances are their characteristic qualities.
The physical properties are those properties of a substance that can be observed without changing the substance into other substances.
Let us again use sodium chloride，common salt，as an example of a substance. We have all seen this substance in what appear to be different forms-table salt，in fine grains;salt in the form of crystals a quarter of an inch or more across. Despite
their obvious . difference，all of these samples of salt have the same fundamental prop- erties. In each case the crystals，small or large，are naturally bounded by square or rectangular crystal faces of different sizes，but with each face always at right angles to each adjacent face. The cleavage of the different crystals of salt is the same:when crushed，the crystals always break(cleave)along planes parallel to the original faces，producing smaller crystals similar to the larger ones. The different samples， dissolved in water，have the same salty taste. Their solubility is the same:at room temperature 36 g of salt can be dissolved in 100 g of water. The density of the salt is the same，2. 16g·cm-3.The density of a substance is the mass (weight) of a unit volume (1 cubic centimeter) of the substance.
There are other properties besides density and solubility that can be measured precisely and expressed in numbers. Such another property is the melting point，the temperature at which a solid substance melts to form a liquid. On the other hand， there are also interesting physical properties of a substance that are not so simple in nature. One such property is the malleability of a substance-the ease with which a substance can be hammered out into thin sheets. A related property is the ductility- the ease with which the substance can be drawn into a wire. Hardness is a similar property:we say that one substance is less hard than the second substance when it is
scratched by the second substance. The color of a substance is an important physical property.
It is customary to say that under the same external conditions all specimens of a particular substance have the same physical properties(density, hardness，color, melting point，crystalline form，et)。Sometimes，however，the word substance isused in referring to a material Without regard to its state.For example，ice，liquidwater，and water vapor may be referred to as the same substance.Moreover，a specimen containing crystals of rock salt and crystals of table salt may be cabled a mixtur，even though the specimen may consist entirely of one substance，sodium chloride.This lack of definiteness in usage seems }o cause no confusion in practice.
化学化工专业英语1、The Physical Properties of Substances
