The large amount of heat that must be liberated before water freezes accounts for the slowness of the formation of ice. It is also the reason why the temperature never falls so low in the vicinity of large lakes as it does far inland, the heat given out by the freezing water warming the surrounding air.

The heat that disappears on melting and reappears on solidifying is called the heat of fusion. It is sometimes called latent heat since the heat seems to become hidden or latent. It is now believed that the heat energy that disappears when a body melts has been transformed into the potential energy of partially separated molecules. The heat of fusion therefore represents the work done in changing a solid to a liquid without a change of temperature.

182. Melting of Crystalline and Amorphous Substances.—If a piece of ice is placed in boiling hot water and then removed, the temperature of the unmelted ice is still 0°C. There is no known means of warming ice under atmospheric pressure above its melting point and maintaining its solid state. Ice being composed of ice crystals is called a crystalline body. All crystalline substances have fixed melting points. For example, ice always melts at 0°C. The melting points of some common crystalline substances are given below:

Melting Points of Some Crystalline Substances

1. Aluminum658 C.
2. Cast iron1200 C.
3. Copper1083 C.
4. Ice0 C.
5. Lead327 C.
6. Mercury-39 C.
7. Phenol (carbolic acid)43 C.
8. Platinum1755 C.
9. Salt (sodium chloride)795 C.
10. Saltpeter (potassium nitrate)340 C.
11. Silver961 C.
12. Sodium hyposulphite (hypo)47 C.
13. Zinc419 C.

Non-crystalline or amorphous substances such as glass, tar, glue, etc., do not have well defined melting points as do crystalline bodies. When heated they gradually soften and become fluid. For this reason glass can be pressed and molded.

183. Change of Volume During Solidification.—The fact that ice floats and that it breaks bottles and pipes in which it freezes shows that water expands on freezing. How a substance may occupy more space when solid than when liquid may be understood when we learn that ice consists of masses of star-shaped crystals. (See Fig. 151.) The formation of these crystals must leave unoccupied spaces between them in the ice. When liquefied, however, no spaces are left and the substance occupies less volume. Most substances contract upon solidifying. Antimony and bismuth, however, expand on solidifying while iron changes little in volume. Only those bodies that expand, or else show little change of volume on solidifying, can make sharp castings, for if they contract they will not completely fill the mold. For this reason gold and silver coins must be stamped and not cast. Type metal, an alloy of antimony and lead, expands on solidifying to form the sharp outlines of good type. Several important effects of the expansion of water when freezing should be noted. (a) Ice floats, (b) if it sank as soon as formed, lakes and rivers would freeze solid, (c) freezing water is one of the active agents in the disintegration of rocks.

Fig. 151.—Ice crystals.