Boiling If a quantity of fresh water is placed over a source of heat, the first effect will be the gathering of air bubbles on the sides of the dish. This comes from the air dissolved in the water. After a time, these bubbles break away from the sides and bottom and rise to the surface. All this time the temperature of the water has been rising, which means that the velocity of the molecules within the liquid has been increasing. As this velocity increases the number of impacts of the molecules up against the surface of the water increases and there is a more rapid escape of water molecules into the air. A point is soon reached when the escape of these molecules in the form of steam is very rapid; the temperature now stops rising and the liquid is said to boil. Boiling is also called ebullition.
Demonstrations. - Heat water in a beaker to about 50° and remove the flame. Fill a test tube half full of ether, insert a thermometer, and put them into the hot water, as in Fig. 1. Stir the water in the beaker with the test tube, and take the lowest temperature at which the ether boils, as its boiling point.
NOTE. -A flame must not be brought: near the ether, as its vapor is very inflammable.
Make a similar experiment with alcohol and find its boiling point.
Fill a round-bottomed flask half full of water. Boil this over a Bunsen burner, and when steam is coming freely from the neck, remove the burner and put a stopper in the mouth of the flask. Invert the flask in a ring support, as shown in Fig. 2, and pour cold water over it. This will condense the vapor above the water and reduce the pressure upon its surface. As a result the water will begin to boil vigorously, showing that if the pressure is reduced the boiling point is lowered.
The effect of reduced pressure upon the boiling point is seen upon high mountains, where water boils at so low a temperature that food cannot be cooked by boiling. Advantage is taken of this effect in the making of sugar, where vacuum pans are used to evaporate the solution without burning it. In the extraction of glue from bones and hides the pressure is increased and the boiling point correspondingly raised.
