# Man's physical universe

THE PHYSICAL PROPERTIES OF SOLIDS 221

Heat of Fusion

Calories

PER Gram

Sodium chloride 124

Ice 79.8

Aluminum 76.8

Copper 43

Tin 14

Le Chatelier's Law Is the Fundamental Law of Equilibria.

When a mixture of ice and water is heated, the ice melts; and the

water freezes when the mixture is cooled. Both processes successfully

resist a change in temperature until either the ice or water is used up.

This is an illustration of a general principle formulated by Le Chatelier,

which in simple language merely states that when a system that is in

equilibrium is subjected to a stress, the equilibrium is disturbed and a

change takes place in the direction that tends to relieve the stress until

the system once more reaches a state of equilibrium.

An equilibrium can

be subjected to a stress by altering the temperature or pressure of the

system or the concentration of one or more of the substances contained

in it.

The application of this principle to physical equilibria may be illustrated

as follows:

1. When a liquid in equilibrium with its vapor has heat added, part

of the liquid evaporates, inasmuch as vaporization absorbs part of the

added heat. If heat is removed from the system, part of the vapor condenses;

and the heat thus evolved tends to compensate for the heat

removed.

2. When a solid is in equilibrium with its liquid, the temperature

of the system cannot be permanently altered without completely

removing either the solid or the liquid. Thus ice and water will remain

together without an increase in the amount of either phase as long as

no heat is gained nor lost by the system. Ice will remain in water in a

good thermos bottle for some time because little heat is allowed to enter

the system. The temperature of a mixture of ice and water is 0° C,

and it cannot be changed by either adding heat to, or removing it from,

the mixture; when heat is applied, a portion of the ice melts and thus

absorbs the heat; when heat is removed, a portion of the water freezes

and thus evolves heat.

A tub of water may be placed in a basement to prevent the freezing

of nearby vegetables. The principle of this procedure is that the heat

liberated as the water is first cooled and is then frozen will help to

prevent the freezing of the vegetables, which should be protected in

part by being covered with cloth or earth.

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