# Man's physical universe

THE KINETIC-MOLECULAR THEORY 239

cules in such solids. The surfaces of small blocks of metals may be

made so true and may be polished so highly that when placed next to

each other they will stick together ; both blocks can be lifted as a unit

by picking up either block. This intimate contact is supposed to bring

enough molecules close enough together to bring about considerable

attraction between them. The presence of moving molecules is demonstrated

by the fact

that such blocks of different metals as lead and

gold, when left in contact with each other for some time and then

tested, show that appreciable amounts of gold have diffused from the

gold blocks into the lead blocks and vice versa.

STUDY QUESTIONS

1. State the main points of the kinetic-molecular theory.

2. What does the kinetic-molecular theory explain?

3. Why are the sides of a football kept distended after it is blown up?

4. Explain Boyle's law in terms of the kinetic-molecular theory.

5. Why is it difficult to compress solids?

6. Explain in terms of the kinetic-molecular theory why a tire is more likely to

blow out on a hot day than on a cold day.

7. Show how the kinetic-molecular theory explains: (c) the states of matter,

(fe) gas pressure, (c) why a liquid has surface tension, (d) the true nature of

evaporation, (e) what the boiling-point is, (/) what the melting-point is,

(g) compressibility, (h) diffusibility, (i) liquefaction, (j) why gases expand

when heated.

8. Account for the following phenomena in terms of the kinetic-molecular

theory: (a) the effect of temperature and pressure changes on the viscosity

of a liquid, (6) the difference between the two classes of solids, (c) solids do

not take the shapes of vessels in which they are placed, while liquids do,

(d) the cooling of a gas on expansion, (e) the effect of temperature and pressure

on the volume of a gas.

9. Explain why liquids and solids are less compressible than gases.

10. Mention at least one line of evidence which supports the kinetic-molecular

theory.

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