The most important chemical substance

Text B

PROPERTIES OF WATER

The older chemists considered water to be an element. They were quite right because they did not know how to decompose it into simpler substances. Now it is known that water consists of hydrogen and oxygen only, united in the proportions of two to one by volume approximately. At ordinary temperatures pure water is a tasteless and odourless liquid; it is colorless in moderately thin layers, but appears greenish-blue when viewed in thick layers. Water boils at 100° C under 760 mm. pressure. The greater is the pressure, the higher is the boiling point; and conversely, the less is the pressure, the lower is the boiling point. Liquid water freezes at 0° C into crystalline ice. Water vapour freezes into hoar frost and snow. The chemical properties of water can be classified broadly under three main headings, viz.:

1) reactions in which water undergoes decomposition;

2) reactions in which water acts as a catalyst;

3) reactions in which water forms addition compounds.

The combination of hydrogen and oxygen to form water is attended by evolution of a large quantity of heat as indicated in the equation 2H₂ + O₂ = 2 H₂O + 116.2 Cals, the water formed remains as steam. Many elements will decompose water at a suitable temperature. The alkali metals (sodium, potassium, etc.) attack water readily at the ordinary temperature. Non-metals for the most part do not react with water, the exceptions being carbon and silicon, fluorine and chlorine.

3. Answer the questions:

1. What does water consist of?

2. What kind of liquid is water?

3. What temperature does it boil at?

4. What temperature does it freeze at?

5. What metals attack water at the ordinary temperature?

6. Do non-metals react with water?

4. Complete the following sentences using some active words and word combinations (see below):

1. Water consists of ………… and ………….

2. Pure water is a ………… …………

3. Water is………… …………

4. Water acts as ………… …………

5. Many elements decompose water at a …………

6. The older chemists considered water to be …………

a catalyst, oxygen, a suitable temperature, an element, tasteless and odourless liquid, greenish- hydrogen, blue in thick layers.

5. Retell the text using the scheme (see Un.1).

1. Match the English words and word combinations in A with their Russian equivalents in B.

droplet насыщать

greasy среда, окружение

fatty точка кипения

dust капелька

saturate единица объема

define жирный, сальный

environment определять

the boiling point жирный

the unit of volume пыль

2. Read the text trying to understand its main idea.

Water is hydrogen oxide, a compound of hydrogen and oxygen. It can be made if hydrogen or hydrogen containing substances are burnt in air or oxygen. Most of the world’s water is liquid but an important fraction is solid, as ice or snow. Many mineral substances contain water of crystallization (e.g. copper sulfate) and in the atmosphere there are millions of tons of water vapour.

Fresh water which is accumulated on the earth's surface is known as surface water. Lakes, rivers, reservoirs, streams, swamps and any other natural storage basin contain surface water. Not all surface water areas are natural as there are many man-made lakes and reservoirs. On the other hand, fresh water (from rain, melting snow or ice) which soaks into the soil is known as ground water.

Clouds consist of minute droplets of water or crystals of ice. Water dissolves a very large number of substances and it is the most important solvent. It does not dissolve greasy, fatty substances or most plastics. It was established that ordinary water is impure, it contains dissolved salts and gases and sometimes organic matter. For chemical work water is to be purified by distillation. Pure water is colourless, tasteless, and odourless. Rain water is nearly pure water, containing only small proportions of dust and dissolved gases.

When the chemists had examined the properties of water, they found that physical properties of water could be used in many physical constants and units. The freezing point of water (saturated with air at 1 atm pressure) is taken as 0 °C and the boiling point is of water at 1 atm is taken at 100 °C. The unit of volume in metric system is chosen so that 1 ml of water at 3.98 °C (the temperature of its maximum density) weighs 1.0000 g/cm³.

So water is one of the most important of all chemical substances. It is a main constituent of living matter and of the environment in which we live.

3. Divide the text into some logical parts and entitle them.

4. Translate the following sentences into English using lexical material of the text:

1. Вода является основным растворителем большого количества веществ.

2. Обычная вода содержит примеси.

3. Вода может быть очищена методом дистилляции.

4. Дождевая вода практически очищена.

5. Вода – главная составляющая всех живых веществ.

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