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In brackets:

1. The reaction will not take place, unless they (to add) a catalyst. 2. Provided you wanted to obtain pure nitrogen, you (to pass) gaseous ammonia over heated copper oxide.

  1. If at ordinary temperatures molecules of a gas exhibit no attraction, Joule-Thompson effect (to be) not effective.

  2. If a liquid had been cooled gradually, it (to reach) a tempe­

rature at which crystals of the solid form (to begm^taappear.

  1. Were this laboratory equipped well, it (to be) much easier to work in it. 6. Were the oxygen in the air reduced below,a certain percentage, it (not to support) combustion. 7. Unless iron is heated red-hot, it (not to absorb) any carbon. 8. If you took a little salt and examined the grains, you (to see) that most of them are perfect cubes. 9. Provided there were no air, the piece of paper and the piece of iron (to fall) together.

  1. If one or more of the reacting substances were a weak electrolyte, the reaction (to involve) the dissociation of such substances into ions. 11. If the concentration of one of the ions involved is greater than the equivalent proportion, the concentration of the other (to be) less.

Ex. 22. Read the text and answer the questions that follow:

Dust in the air consists of solid particles of a number of substances so small that they do not readily settle. The pres­ence of dust in the air is important in that dust particles serve as nuclei for the condensation of water vapour.

If there were no dust, the air would become supersaturated with water and clouds and rain would be formed. Moisture would condense directly on the surface of solid objects.

Dust also includes living organisms, some of them lead to disease and infection.

  1. What does dust in the air consist^ of?

  2. Why is the presence of dust in the air important?

  3. What would happen if there were no dust in the air?

Ex. 23. Read the texts paying attention to the words in bold type:

1.

Air contains nearly constant proportions of three elemen­tary substances, namely nitrogen — 75.4%, oxygen — 23.2% and argon — 1.2% by weight.

The relative proportion of water vapour in the air is usually stated in terms of relative humidity. The relative humidity affects hard the rate of the evaporation.

If the relative humidity were low, evaporation would occur rapidly. If the relative humidity were high, evaporation of water would occur slowly as the air is nearly saturated.

2.

The most important source of oxygen is the air. Air may be changed into a liquid both in the laboratory and at the plant.

If liquid air were boiled, nitrogen would escape from the solution readily and more rapidly than oxygen. By the use of many devices, most of the nitrogen may be boiled out.

Ex. 24. Listen to the tape-recorded text and discuss it:

The Earth is a solid sphere. It is made up of three layers, they being called the core, the mantle, and the crust. The solid sphere is surrounded by a gaseous sphere, which is called the atmosphere.

We know most about the crust of the Earth which is the outermost sphere. This layer is very thin compared with the diameter of the whole Earth. It consists of rock which contains a lot of minerals. These are usually in compounds called oxides, containing oxygen, or sulphides, containing sulphur.

The mantle is much thicker than the crust. It consists mainly of rocks. The core seems to be divided into two parts. We believe it to be mainly composed of iron, but it also con­tains about 10% nickel.

Ex. 25. Read and render the text:

Oxygen, nitrogen, argon, and a number of other elemental gases are now produced almost exclusively from air. Oxygen ranks very high among all chemical products. Nitrogen ranks a little lower, but still high. The total production of other gases from the air is small by comparison.

Dry air consists mostly of nitrogen (75.4% by volume), almost 23.2% oxygen, almost 1% argon, and 0.03% carbon dioxide.

Some gases usually found in trace amounts in the atmos­phere. These gases sometimes result from the activities of man, but all of them are also produced naturally. For example methane, sometimes called “marsh gas”, is formed by decay brought about by anaerobic bacteria. Ozone comes from the action of solar ultraviolet radiation and of lightning on oxygen in our atmosphere. Nitrogen dioxide is formed from the spontaneous oxidation of nitric oxide, which in turn comes from the action of lightning on air.