X. Writing

Choose one of the topics and write a composition (200-250 words).

a) Impact of water quality on our health.

b) Water quality in Voronezh Reservoir.

c) Organic chemicals in the urban water cycle.

d) Chemical and Physical properties of water.

XI. Grammar: Modal Verbs

a. Underline the correct word(s).

1. A. Could/May/ Shall you tell me the time, please?

B. Yes. It is quarter past ten.

2. We might/ have to rehearse the play every night until Friday, because everything must/ shall be perfect for the performance.

3. A. Would/ Shall/ Will we go shopping tonight?

B. Yes. We need to buy some bread.

4. May/ Should/ Would I help you, madam?

5. We couldn’t / mustn’t make any mistakes in the test.

6. Can/ Shall/ May you post this letter for me, please?

7. A. Shall/ Would/ Could I speak to Jerry, please?

B. I am sorry, but he is not here at the moment.

8. May/ Will/ Would I have a cup of coffee, please?

9. A. You mustn’t / needn’t/ couldn’t lock the door.

B. Oh, will you do it when you leave?

10. I have a lot of lines to learn by heart and I must/might learn them all before midday.

b. Rephrase the following sentences using modal verbs.

1. I advise you to buy this book.

2. It is not necessary for him to take the exam again.

3. It is possible that Jane will call me this evening.

4. You aren’t allowed to eat and drink in the classrooms.

5. We are obliged to clock in and out every day.

6. Would you like me to help you?

7. Perhaps he will go for a picnic on Sunday afternoon.

8. How about throwing a party on your birthday?

9. I am sure Terry isn’t in the office.

10. You had better book your flight early.

UNIT 9 Molecules and chemical bonding I. Lead-in Before you read give definition to the word “molecule” using these words: particle, compound, substance, composed, interactions, properties, atoms, chemical forces.

1. Reading

Text A

1. Read the text and check if your definition is correct.

Molecule

Molecule is the smallest identifiable unit into which a pure substance can be divided and still retain the composition and chemical properties of that substance. Further subdivision of the substance leads to still smaller parts that usually differ from the original substance in composition and always differ from it in chemical properties.

Atoms consist of a single nucleus with a positive charge surrounded by a cloud of negatively charged electrons. When atoms approach one another closely, the electron clouds interact with each other and with the nuclei, then the atoms bond together to form a molecule. Thus, from a structural point of view, a molecule may consist of a single atom, as in a molecule of a noble gas such as helium (He), or it may consist of an aggregation of atoms held together by valence forces. Diatomic molecules contain two atoms that are chemically bonded. If the two atoms are identical, as in, for example, the oxygen molecule (O₂), they compose a homonuclear diatomic molecule, while if the atoms are different, as in the carbon monoxide molecule (CO), they make up a heteronuclear diatomic molecule. Molecules containing more than two atoms are termed polyatomic molecules, e.g., carbon dioxide (CO₂) and water (H₂O). Polymer molecules may contain many thousands of component atoms.

The ratio of the numbers of atoms that can be bonded together to form molecules is fixed; for example, every water molecule contains two atoms of hydrogen and one atom of oxygen. It is this feature that distinguishes chemical compounds from solutions and other mechanical mixtures. Thus hydrogen and oxygen may be present in any arbitrary proportions in mechanical mixtures but when sparked will combine only in definite proportions to form the chemical compound water (H₂O). It is possible for the same atoms to combine in different but definite proportions to form different molecules; for example, two atoms of hydrogen will chemically bond with one atom of oxygen to yield a water molecule, whereas two atoms of hydrogen can chemically bond with two atoms of oxygen to form a molecule of hydrogen peroxide (H₂O₂).

b. Answer the following questions:

1. What does further subdivision of the substance lead to?

2. What happens when atoms approach to one another closely?

3. Which types of molecules are mentioned in the text?

4. Which feature distinguishes chemical compounds from solutions?

5. Is it possible for the same atoms to form different molecules? Give examples.

c. Combine the words in bold from the text with their Russian equivalents: отрицательно заряженный, чистый, черта, взаимодействовать, приближаться, скопление, производить, перекись водорода, соотношение произвольный.