Substances and their transformations

All objects surrounding us in nature are composed of different substances. Iron, glass, wood, water, sugar, etc. are all examples of such substances., Chemistry is the study of substances and their transformations. Therefore, first problem we take up upon beginning a course in chemistry is how to distil guish and recognize substances.

Substances are distinguished by their properties — colour, smell, taste, specific gravity , greater or less hardness, melting and boiling points, etc. For example, in describing the properties of sugar, we can state that sugar is a hard brittle substance, white in colour, sweet in taste, without odour, easily soluble it water, heavier than water, with a specific gravity of 1.58, 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 properties of a substance. For example, pure water is colourless, tasteless and transparent, but if a drop of milk is added to a glass of water, the water becomes clouded. All the enumerated properties are not those of water, but of the admixtures.

In some cases we may see at once that a substance is heterogeneous, that is a mixture of different substances. Take granite, for example. .We notice in it pink particles of field spar, semi-transparent crystals of quartz and dark shining scales of mica.² In other cases it cannot be seen at a glance that a given substance is heterogeneous, but this may be detected by various methods. Thus, the heterogene of milk may be shown if we let the milk stand: the thicker layer of cream rises the top. This means that milk is heterogeneous. We can also use a microscope, Under the microscope it will be clearly seen that milk is a liquid which consists of tiny suspended globules of fat.

Ex. 1. Find the English equivalent to the following Russian sentence:

Чистая вода бесцветна, не имеет вкуса и прозрачна.

Ex. 2. Point out the sentence which expresses the main idea of the text:

1. Sugar is a hard, brittle substance, white in colour, sweet in taste, without odour, easily soluble in water, etc.

2. Chemistry is the study of substances and their transformations and the main problem in chemistry is how to distinguish and recognize substances.

3. 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.

Ex. 3. Point out which of these sentences contains the information from the text.

1. It is known that matter exists in three states or forms: solids, liquids {and gases.

2. Compounds consist of two or more elements and can be decomposed [into these constituent elements.

3. Substances are distinguished by their properties — colour, smell, [taste, specific gravity, etc.

Ex. 4. Rearrange the sentences to make a short summary of the text, use 'since' or 'therefore' wherever possible:

1. Substances are distinguished by their properties.

2. In order to learn the properties of a substance one must have it in its pure form.

3. Even small admixtures of foreign substances may change properties of a substance.

4. Chemistry is the study of substances and their transformations.