20. Boiling. _Heat absorbed in Boiling_. If a kettle of water is placed above a flame, the temperature of the water gradually increases, and soon small bubbles form at the bottom of the kettle and begin to rise through the water. At first the bubbles do not get far in their ascent, but disappear before they reach the surface; later, as the water gets hotter and hotter, the bubbles become larger and more numerous, rise higher and higher, and finally reach the surface and pass from the water into the air; steam comes from the vessel, and the water is said to _boil_. The temperature at which a liquid boils is called the boiling point. While the water is heating, the temperature steadily rises, but as soon as the water begins to boil the thermometer reading becomes stationary and does not change, no matter how hard the water boils and in spite of the fact that heat from the flame is constantly passing into the water. If the flame is removed from the boiling water for but a second, the boiling ceases; if the flame is replaced, the boiling begins again immediately. Unless heat is constantly supplied, water at the boiling point cannot be transformed into steam. _The number of calories which must be supplied to 1 gram of water at the boiling point in order to change it into steam at the same temperature is called the heat of vaporization_; it is the heat necessary to change 1 gram of water at the boiling point into steam of the same temperature. 21. The Amount of Heat Absorbed. The amount of heat which must be constantly supplied to water at the boiling point in order to change it into steam is far greater than we realize. If we put a beaker of ice water (water at 0 deg. C.) over a steady flame, and note (1) the time which elapses before the water begins to boil, and (2) the time which elapses before the boiling water completely boils away, we shall see that it takes about 5-1/4 times as long to change water into steam as it does to change its temperature from 0 deg. C. to 100 deg. C. Since, with a steady flame, it takes 5-1/4 times as long to change water into steam as it does to change its temperature from 0 deg. C. to the boiling point, we conclude that it takes 5-1/4 times as much heat to convert water at the boiling point into steam as it does to raise it from the temperature of ice water to that of boiling water. The amount of heat necessary to raise the temperature of 1 gram of water 1 deg. C. is equal to 1 calorie, and the amount necessary to raise the temperature 100 deg. C. is equal to 100 calories; hence the amount of heat necessary to convert 1 gram of water at the boiling point into steam at that same temperature is equal to approximately 525 calories. Very careful experiments show the exact heat of vaporization to be 536.1 calories. (See Laboratory Manual.)