Text 18

( - 3 ). The Nature of a Gas

That the molcculcs of a gas arc not held together, but arc moving freely in a volume rather large compared with the volumes of the molcculcs themselves is the characteristic feature of a gas. The Van dcr Waals attractive forces between the molcculcs still operate whenever two molcculcs come close together, but usually these forces arc negligibly small because the molcculcs are far apart. Because of this freedom of molecular motion, a specimen of a gas docs not have either definite shape or definite size. A gas shapes itself to its container.


Gases at ordinary pressure arc very dilute: the molcculcs themselves constitute only about one-thousandth of the total volume of the gas, the rest being empty space. Thus, I g of solid iodine has a volume of about 0.2 cm * (its density is 4.93 g / cm3), Whereas 1 g of iodine gas at 1-atm pressure and at the temperature of I84 C (its boiling point) has a volume of 148 cm ', over 700 times greater. The volume of all the molcculcs in a gas at ordinary pressure is accordingly small compared with the volume of the gas itself. On the other hand, the diameter of a gas molcculc is not extremely small compared with the distance between molcculcs; in a gas at room temperature and 1-atm pressure the average distance from a molcculc to its nearest neighbours is about ten times its molecular diameter.

. 3. .

. 4. .

. 5. , . , .

1. Molcculcs of a gas move freely in a volume of its container. 2. The Van der Waals forces do not act between the molecules of a gas. 3. The molecules of a gas are rather close together at ordinary pressure. 4. The volume occupied by a gas is considerably greater than the volume of a solid of the same element. 5. The distance between the molecules of a gas at 1 -atm pressure is greater than the diameter of the molecule itself. 6. The temperature has no influence on the gas volume.

. 6. .

Section III

Ex. 1. Respond to the following statements using the expressions: that's right, exactly so; you are mistaken; on the contrary; I wouldn 7 say so.

1. The nature of a gas is quite similar to the nature of a liquid. 2. History is his unfavourite subject. 3. Nick does not like to make experiments. 4.1 think theoretical physics is much more interesting than practical physics. 5. In my opinion, Professor N is not a good lecturer. 6. There is no need going to the library, you may find everything in this textbook. 7. You should have finished your measurements first and then made calculations.

Ex. 2. Translate the sentences into English.

1. , . - , . 2. . - , , . 3. , 1 0,2 *, ? - . 4. ? - , .

5. -, - . - , , . 6. . - , .

Ex. 3. Make up short dialogues according to the model.

Model: - A statement.

- A question.

Example:

- An answer.

- I like reading vciy much.

- What do you like?

- Reading.

Ex. 4. Give detailed answers to the following questions:

1. What happens to a crystalline substancc when it is heated? 2. What is the melting point of a substancc? 3. What are the characteristic features of a liquid? 4. What are the characteristic features of a gas? 5. What docs the physical state of a substancc depend on?

Ex. 5. Discuss the following topics:

1. Factors that Influcncc the Physical State of a Substancc.

2. Solids, Liquids and Gases from the Molecular Point of View.

3. An Example of Transformation of the Same Substancc into Different States.

WHAT IS IT?

The measure of the degrees of hotness or coldness of a substancc or a body.

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Text 9 | Fluorine | Lesson 10 | Text 10 | Lesson 11 | Fascinating Phosphorus | Modifications of Phosphorus | Lesson 12 | Chemical Symbols for Representing Compounds | Weight of 12 atom of carbon 12 |

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