CHAPTER VIII

CHANGE OF STATE

FUSION

Cooling Curves.-EXPT. 40.-Take a thin-walled boiling tube and half fill it with distilled water. Obtain a thin-walled test tube, of such a diameter that it will just admit of a thermometer being placed in it, and fill the free space surrounding the thermometer bulb with mercury. Place the test tube and thermometer in the boiling tube (Fig. 73), and then place the whole arrangement in a beaker containing a freezing mixture made from ice and salt. Read the temperature indicated by the thermometer every half minute, and finally plot your observations, measuring time horizontally and temperature vertically.

You will thus obtain a cooling curve of the general form shown in Fig. 74. It will be noticed that, after falling to o° C., the temperature remains constant for a considerable interval, and then recommences to fall.

The meaning of this stationary temperature at o° C. may be easily explained. When the distilled water has cooled to o ̊C., solidification commences. But for every gram of water which solidifies, 80 therms of heat will be given up. On the other hand, the colder mixture of ice and salt is continually abstracting heat from the water in the test tube. The water will solidify at such a rate that the heat given up during solidification is just balanced by the heat abstracted by the freezing mixture, the temperature of the freezing water meanwhile remaining stationary.

When the whole of the water has solidified, cooling will re

to cool, and to draw a cooling FIG. 73.- Arrangement for determining the curve from the observations

of the temperature made at short intervals of time. Crystalline

may be observed, indicating that the different metals solidify at different temperatures.

EXPT. 41.-Obtain the cooling curve for paraffin wax.-Heat some wax contained in a test tube to about 60° C.; place a thermometer in it, and regularly observe the temperature

at short intervals. Plot the curve in the manner already explained.

To determine the Melting Point of a Substance.

EXPT. 42. To determine the melting point of sulphur.-Introduce a little powdered sulphur into a piece of capillary tube closed at its lower end. Fasten this piece of capillary tube by the side of the bulb of a thermometer reading to 200° C., by the aid of pieces of thin wire. Support the thermometer within a rather wide boiling tube,

FIG. 75.-Method of supporting the test
tube shown in Fig. 75.

the upper end of which is filled up with a plug of glass wool. The air in the tube is heated by means of a gas flame. The wider tube (Fig. 76) serves to equalise the temperature along the whole height of the thermometer. The thermometer and test tube are supported by wires in the manner indicated in Fig. 75. The lower end of the outer tube is preferably covered with a piece of iron wire gauze.

Notice the temperature when the first indications of melting are exhibited. This will be the melting point of the sulphur.

Overcooling. - Under certain conditions it is possible to cool water

to a temperature considerably below o° C. without solidification occurring. Fahrenheit found that water, enclosed in a thermometer-shaped vessel which had been freed from air, could be cooled below the freezing point of water without any ice being formed. When, however, the stem was broken, solidification rapidly set in. Gay-Lussac cooled water to -12° C. without solidification taking place, the surface of the water being covered with a layer of oil. A small fragment of ice dropped into the overcooled water caused solidification to commence, the temperature quickly rising to o° C.

This property is not peculiar to water, but is exhibited by most other substances. If, for instance, phosphorus is melted in a U tube, its free surfaces being covered with water, it may be cooled considerably below 44° C., which is its normal melting point, without solidification occurring. If a small fragment of solid yellow phosphorus is then dropped into one of the limbs of the U tube, solidification will commence at this fragment, and spread along the tube, the temperature of the phosphorus rising meanwhile to 44° C. A fragment of amorphous red phosphorus will not start solidification. The rate at which the solidification travels along the tube is greater in proportion to the degree of overcooling.

From this experiment we learn two facts of great import

ance

1. Solidification takes place only at the surface of the solid already formed.

2. The rate at which solidification takes place at a given surface is proportional to the degree of over-cooling.

Beckmann's Freezing Point Apparatus.-The fact that the temperature of an over-cooled liquid rises to the freezing point when solidification occurs, has been made the basis of a method of determining freezing points. Beckmann's apparatus is shown in Fig. 77. It consists of a test tube A, provided with a side inlet tube D, and a cork through which a thermometer T and a platinum wire stirrer S, pass. This test tube is surrounded by a larger tube B, the space between the two forming an air jacket. Both are surrounded by a larger vessel C, which can be filled with a suitable freezing mixture. The latter is kept in circulation by means of the wire stirrer S1.

The method of using the above apparatus is as follows:

Some of the liquid of which the freezing point is required, is introduced

into A, by way of the inlet tube D, which is afterwards closed by a cork. A suitable freezing mixture is introduced into C, and the whole apparatus

is allowed to stand till the thermometer T indicates a temperature one or two degrees below the probable freezing point of the liquid. The platinum wire stirrer S is then moved briskly up and down, when solidification will generally Occur. The temperature indicated by T rises and shortly attains a value which remains constant for some time. This temperature is the freezing point of the liquid.

The Freezing Point of a Solution is always lower than that of the pure Solvent.

A Common method of removing ice from pavements is to sprinkle salt upon it. The mixture of salt and ice is incapable of remain

ing solid unless its temperature is much below o° C. Consequently the ice melts, and the heat absorbed in this process causes the temperature to fall till the freezing point of the mixture is attained.