This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
Put some water in a saucepan or other vessel. Take its temperature with a thermometer. Set it on the stove or over a Bunsen burner, and hold the thermometer so that its bulb is below the surface of the water, but not touching the bottom of the vessel. (Fig. 2.) . Watch the sides and bottom of the saucepan.
Are the bubbles large or small at first? after a little while? What comes off from the surface of the water? Note the temperature of the water. Note it again when the bubbles begin to break at the surface. Does the mercury rise after this? Increase the heat. Can you make the water any hotter?
When water is heated the air dissolved in it expands, forming tiny bubbles. These rise, until the cold water near the surface chills them; then they contract, and sink again. When all the water has become warm they rise and escape. By this time the heat is beginning to change the water into steam, an invisible gas.1 The word vapor is applied to gases that under ordinary circumstances are liquid. Steam, therefore, is water-vapor.
Soon steam-bubbles appear. These are larger than air-bubbles. As they approach the surface of the water, they are cooled, and condensed; i.e., turned back to water. This bubbling below the surface is called simmering.2 The temperature of the water is now about 185° F. As the water grows hotter, some of the bubbles reach the surface and break there, giving off clouds of steam. Now, for the first time, the water boils. Its temperature is 212° F. By increasing the heat the water may be made to boil faster, but it will not grow hotter. All the heat is now being used in turning water into steam. If boiled long enough, all the water turns to steam and disappears in the air. If steam be cooled, by coming against a cover for instance, it gives up its heat and becomes water again.
The weight of air pressing on the surface of water prevents the steam from escaping until it gains force enough to overcome this pressure. At the sea level water boils at exactly 212°, but on a high mountain at a temperature several degrees lower. If the steam be confined, water may be raised above 212°; for the air under the lid soon takes up all the vapor it can hold, making it impossible for any more water to be changed into steam. The heat that would otherwise be used in making steam is now saved and makes the water hotter. Steam under pressure may also be raised to a temperature higher than 212° F. Such steam is called superheated. Superheated steam is utilized in canning-processes. (P. 302.)
1 Real steam is invisible, the mist we call steam being steam partially-condensed. The slow forming of water-vapor that takes place at ordinary temperatures, in the drying of clothes, the disappearance of water after a rain, and the like, is called evaporation.
2 In cooking, it is sometimes important to keep water simmering; at other times necessary to have it boiling. Learn to distinguish between these. When steam comes in jets from the spout of a teakettle, the water boils.
At 32° F. water freezes and becomes ice. At 32° ice melts and forms water. By heating ice, it may in a few minutes be changed to water and from water to steam. Do you know of anything else that is changed from solid to liquid and from liquid to gas, by heat ?
The particles of which a solid is composed hold firmly together; those of a liquid hold loosely; those of a gas tend always to go farther apart. Heat separates particles of matter; the loss of heat causes them to draw together again.1 Hence we say, "Heat expands, cold contracts." Hot air rises, because by expanding it becomes thinner and lighter.
Water is composed of hydrogen and oxygen. In what experiment was water formed? Where did the hydrogen come from ? the oxygen ?