Man's physical universe

xanabras

UNIT IV

SECTION 4

THE ABSORPTION OR EVOLUTION OF HEAT IN

PHYSICAL PROCESSES IS APPLIED IN REFRIG-

ERATION AND HEATING-SYSTEMS

Introduction.

Any change in the properties of matter is called a process.

A change

in chemical properties is a chemical process, while a change in physical

properties is a physical process. All processes involve a transformation

or redistribution of energy; but physical processes do not bring about

a change in composition, whereas chemical processes do.

Vaporization Absorbs Heat.

The pressure exerted by a vapor when it is in equilibrium with the

liquid is called the vapor pressure.

The boiling-point of a liquid depends on its vapor pressure at a given

atmospheric pressure. Thus a gasoline fraction which boils at 50° C. is

said to be more volatile {i.e., to have a higher vapor pressure at a given

temperature) than another sample which boils at 60° C. Ether and

Jiiguid

acetone vaporize very readily, while

mercury and glycerine, which boil

at much higher temperatures, have

low vapor pressures at room temperatures.

The boilmg-point of a

liquid is that temperature at which

its vapor pressure becomes equal to

the external pressure.

At sea level water boils at 100° C,

but at higher altitudes it boils at

Fig. 70. The chief physical processes.

a lower temperature because the

atmospheric pressure is less than it is at sea level. At Colorado Springs,

which is about six thousand feet above sea level, water boils at 90° C.

On the other hand, the boiling-point of water in pressure cookers and

autoclaves is higher than usual because, being sealed, the vapors produce

higher than atmospheric pressures; in fact, the boiling-point is

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