Man's physical universe

xanabras

THE PHYSICAL PROPERTIES OF SOLIDS 223

3. Salt is unnecessary for obtaining freezing temperatures with dry

ice, thus eUminating the salt water dripping from railroad refrigeration

cars which has produced considerable corrosion of the rails.

In the preparation of dry ice, purified carbon dioxide is liquefied by

means of pressure. The liquid carbon dioxide is then allowed to evaporate

so rapidly that a portion of the liquid is solidified as a result of

the decrease in temperature caused by the absorption of heat in this

process.

Solids Change in Volume When Heated.

We have already seen that a change in volume is always brought

about when a gas, liquid, or solid is heated. The majority of solids

increase in volume when heated, but in a few cases the solid state

occupies a greater volume than the liquid state of a substance; this is

one of the unusual properties of water. When water is cooled, it contracts

until it reaches about 4° C. Below this temperature it expands

upon cooling. The density of ice compared with water is 0.917. This

means that ice is nearly one-tenth lighter than water; only about one

tenth of an iceberg floats above the surface of the water.

In some respects it is unfortunate that water expands upon freezing

because it

causes our water pipes and automobile radiators to burst

in winter. The expansive force exerted by the freezing of water is so

great that few containers are able to withstand it.

On the other hand, the principle of the expansion of water on freezing

has many valuable applications in nature.

It has already been pointed

out that surface rocks are split apart as the moisture in them is frozen

in the winter.

If ice were more dense than water, it would sink in the

ponds and rivers as fast as it was formed in the winter ; such bodies of

water would freeze solid and most of their animal life would be destroyed.

The oceans, lakes, and rivers would all freeze from the bottom

up, and summer melting would be confined to a little slush on top.

The fact that the maximum density of water occurs at about 4° C.

also enters into this problem. When water reaches the temperature of

about 4° C. it begins to expand and therefore remains at the surface

of the body of water.

The temperature of deep bodies of water does

water cooled

not change much because both warm water and also

below 4° C. stay at the top.

Cast iron, antimony, and its alloy type-metal (82 per cent lead, 15

per cent antimony, 3 per cent tin) are among the few substances that

expand as they solidify. Coins made from copper, gold, silver, or

nickel must be stamped on a metal disk with a heavy die rather than

be cast because these metals contract as they solidify.

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