IV. Comprehension.

Exercise 10.Answer the following questions:

1.When are the molecules able to break out of the lattice?

2.When does the substance lose its definite shape ?

3.Why do liquids give off smell ?

4.What happens when a liquids is heated ?

5.What is evaporation ?

6.What is condensation ?

V. Oral practice

Exercise 11. Describe the process of evaporation.

Exercise 12 .Compare the process of condensation with the process of evaporation .

VI. Reading and comprehension.

Exercise 13. Read text B “Water purification ” without a dictionary for 8 min.

Text b Water purification

1. The abundance of water in liquid, solid and gaseous state is a matter of common observation. Not only is water the most abundant, compound, but it is also very important for life. To be sure life would be impossible without water.

2. From the chemical point of view water has many points of interest, for it enters into chemical reactions which are of fundamental importance. Not only does water react with many substances, but it has also a marked influence upon many chemical reactions.

3. For many purposes it is desirable that water should be pure. The purest natural water is rain. It would be wrong, however, to consider it as being really pure. Nor is ground water pure. It contains a great deal of impurities which fail to settle.¹ Dissolved substances do not settle, nor do they evaporate with water, which makes their removal difficult.

4. One of the most important problems is to obtain water sufficiently pure to meet our needs. The choice as to what process is to be used for purification of water depends upon the uses for which it is intended as well as the impurities it contains. Water used for steam boilers should be free from substances that cause corrosion and scale formation. Water for washing should not contain substances that react with soap. When water is to be used for drinking it is necessary that the microbes which it may contain should be killed. To achieve this, water which is to be purified is thoroughly filtered. Another way to purify water is to boil it.

5. None of these methods is fit for producing pure water in the chemical sense, since most of the soluble salts are unaf­fected by the treatment. To remove these¹¹ and to prepare chemically pure water suitable for scientific use, we take advantage of the fact that water is easily changed to steam while most of the dissolved substances as already mentioned are not volatile. By condensing the steam, we are thus able to remove all the impurities except volatile ones. This pro­cess is called distillation. Distilled water has many uses, both in the laboratory and in industry, when even small quantities of impurities would be undesirable.

6. Water is a compound substance which consists of two ele­ments hydrogen and oxygen. It can be decomposed by the electric current. By decomposing water two volumes of hy­drogen and one volume of oxygen are obtained. That water is a compound substance may be shown by the reaction be­tween water and some metals. At ordinary temperature, pure water is a tasteless, odourless and colourless liquid. We know that water boils at 100°C under 760 mm pressure. The greater the pressure, the higher the boiling point, the less the pres­sure, the lower the boiling point. Steam or water vapour is an invisible colourless gas that condenses to a visible cloud of small particles when it comes in contact with the atmos­phere.

Notes:

a matter of common - добре відоме ;

point of view - точка зору;

to be sure - без сумнiву;

to meet the needs - вiдповiдати потребам;

to depend upon - .залежити вiд;

undesirable -.небажаний.

Ex.14. Answer the following questions .

1.In what states does water occur ?

2.Why is ground water not pure ?

3. What is one of the most important problems as regards water?

4. What kind of water should be used for drinking?

5. What methods are used for purifying water?

6. Where is distilled water used ?

Ex. 15 .Explain : why water is so important for life .

Ex . 16.Speak about the main properties of water .

Ex.17 .Read and translate the following text orally without a dictionary for 8 min. List the main points in Ukrainian.