Substances

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The States of Matter

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That mattermay existin three physical states: solid, liquidor gas is a common knowledge. It is usuallypossible to change matter from one state to the other by changingits temperature. For instance, a piece of iceis called a solid; it may meltand form a liquid; as it evaporates,liquid water changes into a vapour,i.e. into the gaseous state.

Many kinds of matter, likewater, can be obtainedin each of the three states; for some, however, extraordinary meanshave to be used in order to produceone, or even two of the states; and for others, only two states are known or can be produced.

Common saltfor example, exists normally as a solid; at a temperature of several hundred degrees,it can be liquefied;and at stillhigher temperature it is converted into vapour. Carbon, a solid under normal conditions, can be vaporized,but it has never been liquefied.

Solids have both a definite volume and a definite shape. Liquids too, have a definite volume, but they take the shapeof their containers. Gases have neither a definite shape nor a definite volume. A chemist must have a thorough knowledge of the states of matter and of the physical laws which govern the behaviourof matter in various states.

That all matter iscomposed ofmolecules is known to everybody. The question which must be answered, then, is: if all matter is composed of molecules, what is the essentialdifference between the states of matter? The answer to this question is that the essential difference between these states is the relative quantitiesof energy molecules possess in different states.

Vocabulary List

to exist – существовать;

solid – твёрдое состояние;

liquid – жидкость;

to evaporate – выпаривать;

to obtain – получать;

to be liquefied – быть превращённым в жидкость;

a chemist must have a thorough knowledge – химик должен хорошо знать.

Substances are distinguished by their properties – сolour, smell, taste, specific gravity, greater or lesser hardness, melting and boiling points, volatility, etc.

For example, in describing the properties of sugar, one can state that sugar is a hard, brittle substance, white in colour, sweet to the taste, without odour, easily soluble in water, heavier than water and it turns brown when it is heated, etc.

In order to learn the properties of a substance one must have it in its pure form. Even small admixtures of foreign substances may change the properties of a substance. For example: pure water is both colourless and transparent, but if a drop of milk is added to a glass of water, the water becomes clouded; if a drop of ink is added, the water becomes coloured. All the enumerated properties are not those of water but they are the properties of the admixtures.

In some cases, one may see at once that a substance is heterogeneous, that is, a mixture of different substances.

Granite, cement, petroleum are examples of non-homogeneous materials; they consist of mixtures of substances. Thus, granite is a mixture of varying quantities of silica, feldspar, and mica, each of which possesses its own set of properties. Coal is not a substance too because different samples contain different relative amounts of ash, water, carbon, and other components.

Every material, therefore, consists of a single (pure) substance, or it is a mixture of two or more substances, each of which retains in the mixture its own characteristic properties.

Vocabulary List

smell – запах;

taste – вкус;

melting and boiling points – точки плавления и кипения;

volatility – летучесть, изменчивость;

brittle – хрупкий;

odour – запах;

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