COOLING CURVES* STUART GRAVES, WILLIAM NOTTINGHAM HIGHSCHOOL, SYRACUSE, NEWYORK Among the interesting phases of the study of crystallography is the plotting of cooling curves. The student in high-school physics learns that when a crystalline substance melts a large amount of heat is absorbed. This heat is called the heat of fusion and is expressed in calories. When the substance is again cooled sufficiently to cause it to return to its solid state,
MINUTES
ALUM
"0
5
10 15 MINUTES
20
PARAFFIN
110 105
aU
$lrn
a :95 6"
%
30
85 *,I"TCI
OXALlC ACID
CARAMEL
the heat that was absorbed on melting is liberated. This interesting phenomenon can best be studied by means of cooling curves. The substance to be studied is placed in a test tube or some other suitable container and raised to a temperature considerably above its melting point. A thermometer is then placed in the liquid and readings are taken every half- or quarter-minute while i t is cooling and solidifying. When these readings are plotted on a sheet of graph paper we have our cooling curve. In the case of water or other substances which have a melting point below 'Winner of fivc-dollar award in the student contest closing March 15, 1929.
the atmospheric temperature, the test tube can be placed in a freezing mixture of salt and ice. When we examine the cooling curve of a crystalline substance, we see that the temperature falls steadily to a point several degrees below the freezingpoint, rises again to the vicinity of this point, and then continues to fall. By the upward curve we see that heat is liberated when the substance solidifies. The cooling curve of paraffin, on the other hand, shows no such rise of temperature. There is no real solidifyingpoint, but just a range of temperature through which the s~bstauce slowly hardens. This is called an amorphous substance. By merely looking at the cooling curve of caramel, we can tell that it is a substance that changes slowly and steadily from a liquid to a comparatively hard solid. It is very rarely that we find the cooling curves of two crystalline substances that are identically the same, and therefore we can often identify a substance by examining its cooling curve.
