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).

Text B

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.