Here is a very practical fact about boiling water that many people do not know; and their gas bills would be much smaller if they knew it. Try this experiment:
Fig. 54. Will boiling water get hotter if you make it boil harder?
Experiment 38. Heat some water to boiling. Put the boiling-point thermometer into the water (the thermometer graduated to 110° Centigrade and 220° Fahrenheit), and note the temperature of the boiling water. Turn up the gas and make the water boil as violently as possible. Read the thermometer. Does the water become appreciably hotter over the very hot fire than it does over the low fire, if it is boiling in both cases? But in which case is more steam given off? Will a very hot fire make the water boil away more rapidly than a low fire?
When you are cooking potatoes, are you trying to keep them very hot or are you trying to boil the water away from them? Which are you trying to do in making candy, to keep the sugar very hot or to boil the water away from it?
All the extra heat you put into boiling water goes toward changing the water into steam; it cannot raise the water's temperature, because at the moment when water gets above the boiling point it ceases to be water and becomes steam. This steam takes up much more room than the water did, so it passes off into the air. You can tell when a teakettle boils by watching the spout to see when the steam[3] pours forth from it in a strong, steady stream. If the steam took no more room than the water, it could stay in the kettle as easily as the water.
[Footnote 3]: What you see is really not the steam, but the vapor formed as the steam condenses in the cool room. The steam itself is invisible, as you can tell by looking at the mouth of the spout of a kettle of boiling water. You will see a clear space before the white vapor begins. The clear space is steam.
Distilling. When liquids are mixed together and dissolved in each other, it looks as if it would be impossible to take them apart. But it isn't. They can usually be separated almost perfectly by simply boiling them and collecting their vapor. For different substances boil at different temperatures just as they melt at different temperatures. Liquid air will boil on a cake of ice; it takes the intense heat of the electric furnace to boil melted iron. Alcohol boils at a lower temperature than water; gasoline boils at a lower temperature than kerosene. And people make a great deal of practical use of these facts when they wish to separate substances which have different boiling temperatures. They call this distilling. You can do some distilling yourself and separate a mixture of alcohol and water in the following manner:
Experiment 39. First, pour a little alcohol into a cup—a few drops is enough—and touch a lighted match to it. Will it burn? Now mix two teaspoonfuls of alcohol with about half a cup of water and enough blueing to color the mixture. Pour a few drops of this mixture into the cup and try to light it. Will it burn?
Fig. 55. By distillation clear alcohol can be separated from the water and red ink with which it was mixed.
Now pour this mixture into a flask. Pass the end of the long bent glass rod (the "worm") through a one-hole rubber stopper that will fit the flask (Fig. 55). Put the flask on a ring stand and, holding it steady, fasten the neck of the flask with a clamp that is attached to the stand. Put the stopper with the worm attached into the flask, and support the worm with another clamp. Put a dry cup or beaker under the lower end of the worm. Set a lighted burner under the flask. When the mixture in the flask begins to boil, turn the flame down so that the liquid will just barely boil; if it boils violently, part of the liquid splashes up into the lower end of the worm.
As the vapor rises from the mixture and goes into the worm, it cools and condenses. When several drops have gone down into the cup, try lighting them. What is it that has boiled and then condensed: the water, the alcohol, or the blueing? Or is it a mixture of them?
