Compounds and elements

All the substances which occur in nature consist of one or more basic elements; a substance containing more than one element is known as a compound. An element is a substance that can neither be decomposed (broken into a number of other substances) by ordinary chemical action, or made by a chemical union of a number of other substances. A compound consists of two or more different elements in combination and has properties different from the properties of its constituent parts. Water, for example, is a compound of oxygen and hydrogen . A molecule is the smallest amount of a substance that can retain the characteristic properties of that substance and may consist, for example, of two atoms of hydrogen and two atoms of oxygen for hydrogen peroxide, of one atom of oxygen and one atom of carbon for carbon monoxide and of two atoms of oxygen and one atom of carbon for carbon dioxide. An atom is the smallest unit of which a chemical element is built. The atoms of any particular element all have the same average mass and this average mass differs from the average mass of the atoms of any other element.

Exercises

1. Переведите предложения на русский язык, обращая внимание на различные способы выражения долженствования:

1. In scientific work we must measure in units of the metric system.

2. One should turn off the gas when one leaves home.

3. This part is very complex and the worker has to use special tools to cut it.

	to have to
must	ought to
	to be to
	should

4. We ought to say that the discovery of atomic energy is as impor­tant as the discovery of fire.

5. We should be prepared for scientific study of life on other planets.

6. Atoms must serve mankind.

7. The lecture is to begin at 6 o'clock. We ought to come in time.

8. You were to replace the tool by another one, why didn't you do it?

9. The engineer will have to improve the accuracy of this machine tool.

10. He had to work hard before he was able to submit his paper to a scientific journal.

11. The use of new machinery should increase the output of coal by 25 per cent per month.

2. Переведите предложения, обращая особое внимание на значения слова one:

1. The idea of automation is one of the most important ideas for modern industry.

2. He knew that no one could help him.

3. Our old laboratory equipment was much worse than the new one.

4. Due to radioactive elements one can measure the thickness of var­ious materials.

5. One should always inspect the machine tool before he turns it on.

6. One cannot read such technical papers without a dictionary.

7. The new power stations are much more powerful than the old ones.

8. One must know that these rays produce a harmful effect on man.

9. One should always be careful when one operates this machine tool. 10. One can easily regulate the speed of this machine.

3. Выберите перевод выделенных модальных глаголов из данных ниже:

1. You may use the new devices for your research.

2. The solar batteries must heat and light homes.

3. They could visit this new automation shop.

а) должны; Ь) можете; с) могли; d) сможете VII. Замените слова в скобках эквивалентами модальных глаголов, данными ниже:

4. You (должны) to replace this old lathe.

5. In future we (сможем) to use solar energy more effectively.

6. They (было разрешено) to test the machine tool.

7. The students (смогли) to make the experiment.

a) were allowed; b) will be able; c) have; d) were able; e) were