(13) Describe an experiment to show that the quantity of heat required to raise the temperature of one pound of water through any degree between o° C. and 100° C. is very nearly the same. To what sources of error is your experiment liable? (Sen. Camb. Local, 1897.) (14) A copper vessel contains 100 grams of water at 12° C. When 56 grams of water at 30° C. are added, the resulting temperature of the mixture is 18° C. What is the water equivalent of the calorimeter? A calorimeter with water equivalent 12 contains 100 grams of water at 12° C. When 100 grams of a metal at 100° C. are added, the resulting temperature of the mixture is 20° C. Find the specific heat of the metal. (Lond. Univ. Inter. Sci. Pass, 1894.)

PRACTICAL.

(1) Find the specific heat of the given metal. (Inter. Sci. Lond, Hon., July, 1895.)

(2) Find by the method of cooling the specific heat of the given liquid. (B. Sc. Lond. Pass, Nov., 1895.)

(3) Find the amount of heat developed when water and sulphuric acid are mixed in the proportion of 10 to 1 by volume. (B. Sc. Lond. Pass, Nov., 1895.)

(4) Find the specific heat of ice, given paraffin oil of specific heat o ̊5 and a freezing mixture. (B. Sc. Lond. Hon., Dec., 1895.)

(5) Determine the specific heat of a given body by method of mixture. (Inter. Sci. Lond. Hon., 1897.)

(6) Measure the specific heat of a given solid, applying corrections for the materials of the calorimeter, &c. (B. Sc. Lond. Pass & Hon., 1897.)

(7) Compare the specific heat of the given liquid with that of the given solid. (B. Sc. Lond. Pass, 1896.)

(8) Find the amount of heat evolved per c.c. of the mixture, when sulphuric acid and water are mixed in the proportion of 1 to 10 by volume. (B. Sc. Lond. Pass, 1896.)

CHAPTER VII

LATENT HEAT OF FUSION AND VAPORISATION

WHEN heat is communicated to a substance, a rise of temperature may not be the only result produced. Indeed, it is possible, in certain circumstances, to communicate a considerable quantity of heat to a substance without producing any alteration in temperature. In such cases the heat supplied is utilised in changing the state of the substance.

EXPT. 34.-Take a weighed calorimeter and half fill it with water, at a temperature of about 30° C. Weigh the calorimeter and its contents, and obtain the mass of the water by subtraction.

Take a piece of ice, possessing a mass about th of that of the water in the calorimeter. Wrap this ice in a piece of dry flannel. By this means any moisture on the surface of the ice will be removed, whilst the formation of more moisture by the melting of the ice will be prevented, the flannel being a bad conductor of heat.

Place a thermometer in the water contained in the calorimeter, and note the temperature which it indicates. Remove the ice from its flannel wrappings, and quickly drop it into the water in the calorimeter. Stir by means of the thermometer, and note the temperature indicated when the ice has just melted.

Remove the thermometer, and again weigh the calorimeter and its contents. Find by subtraction the mass of ice which has been melted. Now the heat yielded up by the warm water in cooling from its initial to its final temperature may be directly calculated from the mass and fall in temperature of the water. Similarly, the heat absorbed in raising the temperature of the water derived from the ice, from o° C. (the temperature of melting ice) to the final temperature of the water in the calorimeter, may be calculated.

L

It will be found that the first mentioned quantity of heat is greatly in excess of the latter quantity. Hence a certain quantity of heat has been absorbed, or rendered latent, during the change from the solid to the liquid state.

The Latent Heat of Fusion of a substance is the A quantity of heat required to convert one gram of the substance from the solid to the liquid state, no change meanwhile occurring in its temperature.

A formula for obtaining the latent heat of fusion of ice from the results of the foregoing experiment may now be easily derived.

Let w1 = mass of warm water, initially at a temperature t° C.
= mass of ice added.

Let W2

temperature (° C.) of the contents of the calorimeter when the ice has just melted.

latent heat of fusion of ice.

Then during the cooling of w1 grams of water from t° C. to t° C., a quantity of heat, equal to w1 (t1 − t) therms, has been given up. This heat has been utilised :

I. In converting w, grams of ice, at o° C., into w2 grams of water at the same temperature. The quantity of heat thus used is Lw2.

2. In heating w, grams of water from o° C. to t° C. The quantity of heat necessary for this purpose is W2 (t2-0).

In order to take account of the heat rendered up by the calorimeter, it is sufficient to notice that the thermal capacity, or water equivalent, of the calorimeter is equal to w3s, where wg is the mass of the calorimeter, and s is the specific heat of the substance of which it is composed (p. 126). Hence the correct equation will stand as

Calculate the value of the latent heat of fusion of ice from the results of the foregoing experiment.

The correct value of L for ice is 80 therms per gram.

The foregoing experiment will render it clear that before the latent heat of fusion of a substance can be determined some other constants of the substance must be obtained. Thus, if we know the melting point of paraffin wax, together with its specific

heat, both when in the liquid and the solid state, we might pour some melted paraffin wax, which had been heated to about 70° C., into a beaker containing cold water, stirring the whole, and noting the final temperature arrived at.

Then the heat given up by the wax is as follows:

1. Heat given up while the temperature falls from 70° C. to the melting point.

2. Heat given up as the liquid wax becomes solid, the temperature remaining constant.

3. Heat given up as the solid wax cools from its melting point to the final temperature.

EXPT. 35. To determine the melting point of wax.-This may be determined by dipping the bulb of a thermometer into some melted wax, the temperature of which is some degrees above the melting point. When the film of solid wax at first formed has become liquefied, remove the thermometer, and rotating it quickly in the hand, notice the temperature indicated when a film of the opaque wax makes its appearance on the outside of the bulb. Then place the bulb in some water, slowly heat this, and notice the temperature when the film disappears. The mean of these two temperatures may be taken as the melting point of the wax.

EXPT. 36. To determine the specific heat of liquid paraffin wax.— Take a large beaker, or, better still, a large copper calorimeter, half filled with water at a temperature one or two degrees above the melting point of the wax. Determine the mass and temperature of this water. Take a large test tube containing a known mass of melted paraffin wax at a temperature of about 80° C, and immerse this in the water, stirring both the wax and the water, and noting the common temperature which both finally attain. The specific heat of the melted wax may be calculated in a manner similar to that previously described.

EXPT. 37. To determine the specific heat of solid paraffin wax.— Cut up some solid paraffin wax into shavings, weigh them, and leave them for about a quarter of an hour in a beaker with the bulb of a thermometer placed in their midst. After taking their temperature in this manner, shoot the shavings into a beaker half full of water at about 45° C., and stir so as to be sure, as far as possible, that the wax and water attain the same final temperature. Note this common temperature. The calculation of the specific heat of the wax may now be performed. This stage of the experiment is one of considerable difficulty, since the wax is a very bad conductor of heat, and also floats on the water.

With sufficient care, however, a tolerably accurate result may be obtained.

You may now obtain the latent heat of the wax, either by the method already sketched out, or by placing solid wax at a known temperature in a known mass of melted wax at about 80°, carefully noting the initial temperature of the latter,. and the final temperature attained by the

mixture.

Determination of Specific Heat by Black's Ice Calorimeter.

EXPT. 38.-Take a block of ice, of dimensions about 4" x 4" x 3", and cut an approximately hemispherical hollow through its upper

surface (Fig. 64). The surface of another block of ice is smoothed so as to form a cover for this arrangement. A piece of metal is heated to 100° C., either by placing it directly in boiling water, or, better, in a steam jacket (see Chapter VI., p. 124). When this is ready, dry out the cavity in the ice with a towel and quickly transfer the metal to it, immediately afterwards placing the ice cover in position. After about five minutes the metal will have attained the temperature of the ice, a certain amount of the latter having been converted into water at o° C. by the heat given up during the process.

Pour this water into a weighed evaporating basin, drying the cavity by means of some weighed filter paper, and determine the mass of the ice which has been melted, by weighing and subtraction.