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Procedure: Boil the water in the flask for about ten minutes, then put the cork, with the thermometer in the hole (the thermometer must extend into the

water), into the

flask, and at once

turn out the light, grasp the

FIG. 39. MANNER OF POURING COLD
WATER RUN OVER FLASK IN ORDER
TO REDUCE THE PRESSURE ABOVE
THE WATER WITHIN IT.

What happens?

Why?

upper part of the neck of the flask with the folded towel and run cold water over the upper part of the flask, above, not over, the part containing the water. See Fig. 39. Watch the thermometer.

Endeavor to answer these questions before reading the following paragraph.

Explanation of experiment. The air in the flask was heated, as well as the water, while the latter was boiling, therefore it expanded and, consequently, passed from the flask and its place was taken by the steam produced as the water boiled. When the flask was held under the cold water, the steam was condensed and thus there was a partial vacuum above the water; this being the case, the pressure was, of course, reduced and the vapor, having less pressure to overcome, bubbled at a lower temperature.

It is because the pressure on high mountains is less than at sea level that water boils there at a lower temperature than it does in lower altitudes. At the top of Mount Blanc, water boils at 84° C.

Why pressure makes it possible for some bodies to float. Due to the fact that the pressure of liquids is equal in all directions, upwards as well as downwards, any body that does not weigh more than the amount of liquid which it displaces will float since its weight is not more than that of the amount of liquid that would otherwise have been in that part.

The nature of specific gravity or density and its influence on floating bodies.-First what is specific gravity or density? These two words are used interchangeably to signify the weight of a substance as compared with that of another substance which is taken as a standard. Distilled water is the standard usually employed, especially for liquids. A liquid in which solids heavier than water are in solution or suspension will, of course, have a higher specific gravity than water. Naturally, the greater the density of the liquid, the heavier the body that will be able to float in it. It is impossible for people to sink in the Great Salt Lake of Utah.

How the specific gravity of liquids is ascertained.The specific gravity of such liquids as water, milk, sugar solutions, acids, urine, blood, and the like is usually gaged with a hydrometer. This consists of a glass cylinder on one end of which is a bulb containing mercury or shot and on the other a slender stem within which is a scale (see Fig. 23). The scale varies slightly in different forms of hydrometer. The scale of an instrument intended for measuring the density of liquids lighter than water is usually marked in such

a manner that when the instrument is put into distilled water, it sinks until the mark I is on a line with the water, and when put into pure alcohol, it sinks until the mark 100 is reached. The scale of another variety of hydrometer is marked so that the instrument will sink to I in distilled water and the markings above the I indicate weights less than I and those below that point show densities greater than that of water. Thus the instrument will not sink to as great an extent in milk, blood, and other dense liquids as in water. The principle involved is that a floating body sinks until it displaces its own weight. Determination of the sp. g. of a liquid is of value in testing for its purity.

Experiment 6. Object, to show the effect of specific gravity on the boiling point of liquids.

Procedure: (a) Ascertain the specific gravity of tap water, boil, and notice the temperature at which boiling occurs. (b) Add three tablespoonfuls of salt to the water, cool, ascertain the sp.g., boil, and note the temperature at which boiling occurs. (c) Take equal parts of water and alcohol, find the sp.g., boil, and note the temperature.

From this experiment it can be realized that liquids with a lower sp.g. than water boil before the temperature reaches 100° C., while those of greater density than water have a higher boiling point than the latter. For example:

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CHAPTER IV

SOME COMMON PHYSICAL PROCESSES AND THEIR

RESULTS

Evaporation-Condensation-Humidity-Dew-Fog-Frost

Rain-Hail-Snow-Artificial Ice-Distillation-Sublimation-Diffusion-Osmosis-Dialysis

Evaporation compared with vaporization.-Evaporation, like vaporization, is a process in which liquids pass to a gaseous state, but the term vaporization is used when the formation of vapor takes place throughout the entire mass of the liquid and evaporation when it occurs only at the surface of the liquid. In order for vaporization to occur, a liquid must be heated to its boiling temperature, evaporation takes place to some extent at all temperatures, but is, of course, increased by a high temperature.

Why evaporation takes place. As the molecules composing a liquid are in constant motion and their attraction for each other, which holds them together, is only fairly strong, some of the molecules at the surface escape from the liquid into the air. Many of these are drawn back again by the attraction of the molecules at the surface, but more molecules escape than fall back and thus after a time all the molecules of the liquid escape and move more quickly and fly apart and thus become gaseous.

Factors which influence the rate of evaporation.Important factors in influencing the rate of evaporation are: (1) The nature of the liquid. To see this, expose in separate dishes a little water, alcohol, and ether. It will be seen that the liquids which have the lowest boiling point evaporate the quickest. (2) The temperature of the liquid. Heating a liquid, by increasing the rate of molecular motion, hastens evaporation. (3) The amount of surface exposed. Evaporation takes place from the surface of a liquid, consequently, the greater the extent of free surface, the more rapid the rate of evaporation. (4) Pressure. In order to pass into the air, the molecules from a liquid must overcome the pressure of the air upon the surface of the liquid, if therefore there is no air, as in a vacuum, evaporation will occur much more rapidly at a low temperature than under ordinary circumstances. For this reason, vaccuum pans are used in the preparation of condensed milk, much of the nutrient of which is lost if it is subjected to a high temperature. (5) The amount of vapor already in the air, i. e., the degree of humidity under ordinary conditions of temperature and pressure, the atmosphere can hold only a certain amount of vapor; for this reason, evaporation will not take place as quickly on a damp, as on a dry, day; e. g., wet clothes dry more slowly on a damp day. (6) The rapidity with which moist air is driven away from around the evaporating liquid-e. g., on a windy day, clothes will dry more quickly than when there is no wind, and fanning a moist surface hastens its drying.

Result of evaporation on temperature.-A surface from which evaporation takes place and the air around it are cooled by evaporation. Thus the air is cooled after rain while the moisture is evaporating. The

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