Chapter XXI: Section 19: Boiling and condensing

What makes a geyser spout?

How does a steam engine go?

Once more let us imagine we are looking at molecules of water through our magical microscope. But this time suppose that the water has been made very hot. If we could watch the molecules smash into each other and bound about more and more madly, suddenly we should see large numbers of them go shooting off from the rest like rifle bullets, and they would fly out through the seemingly great spaces between the slower molecules of air. This would mean that the water was boiling and turning to steam.

Here are a couple of experiments that will show you how much more room water takes when it turns to steam than while it remains just water:

EXPERIMENT 36. Pour a half inch of water into the bottom of a
test tube. Put a cork in the test tube so tightly that it will
not let any steam pass it, but not too tightly. Hold the test
tube with a test-tube clamp at arm's length over a flame,
pointing the cork away from you. Wait for results.

The reason the cork flew out of the test tube is this: Steam takes a great deal more room than water does,--many times as much room; so when the water in the test tube turned to steam, the steam had to get out and pushed the cork out ahead of it.

EXPERIMENT 37. Pour about half an inch of water into the
bottom of a flask. Bring it to a vigorous boil over the burner
and let it boil half a minute. Now take the flask off the
flame and quickly slip the mouth of a toy balloon over the
mouth of the flask. Watch what happens. If things go too
slowly, you can speed them up by stroking the outside of the
flask with a cold, wet cloth.

When the balloon has been drawn into the flask as far as it
will go, you can put the flask back on the burner and heat the
water till it boils. When the balloon has been forced out of
the flask again and begins to grow large, take the flask off
the burner. Do this before the balloon explodes.

The reason the balloon was drawn into the flask was that the steam in the flask turned back to water as it cooled, and took very much less space. This left a vacuum or empty space in the flask. What pushed the balloon into the empty space?

HOW STEAM MAKES AN ENGINE GO. The force of steam is entirely due to the fact that steam takes so much more room than the water from which it is made. A locomotive pulls trains across continents by using this force, and by the same force a ship carries thousands of tons of freight across the ocean. The engines of the locomotive and the ship are worked by the push of steam. A fire is built under a boiler. The water is boiled; the steam is shut in; the only way the steam can get out is by pushing the piston ahead of it; the piston is attached to machinery that makes the locomotive or ship move.

ONE THEORY ABOUT THE CAUSE OF VOLCANOES. The water that sinks deep down into some of the hot parts of the earth turns to steam, takes up more room, and forces the water above it out as a geyser. It is thought by some scientists that volcanoes may be started by the water in the ocean seeping down through cracks to hot interior parts of the world where even the stone is melted; then the water, turning to steam, pushes its way up to the surface, forcing dust and stone ahead of it, and making a passage up for the melted stone, or lava. The persons who hold this view call attention to the fact that volcanoes are always in or near the sea. If this is the true explanation of volcanoes, then we should have no volcanoes if steam did not take more room than does the water from which it comes.

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:

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?

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?

Alcohol is really made in this way, only it is already mixed in the water in which the grains fermented and from which people then distil it. Gasoline and kerosene are distilled from petroleum; there is a whole series of substances that come from the crude oil, one after the other, according to their boiling points, and what is left is the foundation for a number of products, including paraffine and vaseline.

EXPERIMENT 40. Put some dry, fused calcium chlorid on a saucer
and set it on the plate of the air pump. This is to absorb the
moisture when you do the experiment. (This calcium chlorid
is _not_ the same as the chlorid of lime which you buy for
bleaching or disinfecting.) Fill a flask or beaker half full
of water and bring it to a boil over a Bunsen burner. Quickly
set the flask on the plate of the air pump. The water will
stop boiling, of course. Cover the flask and the saucer of
calcium chlorid with the bell jar immediately, and pump the
air out of the jar. Watch the water.

The water begins to boil again because water will boil at a lower temperature when there is less air pressure on its surface. So although the water is too cool to boil in the open air, it is still hot enough to boil when the air pressure is partially removed. It is because of this that milk is evaporated in a vacuum for canning; it is not necessary to make it so hot that it will be greatly changed by the heat, if the boiling is done in a vacuum. On a high mountain the slight air pressure lets the water boil at so low a temperature that it never becomes hot enough to cook food.

_APPLICATION 30._ Two college students were short of money and
had to economize greatly. They got an alcohol lamp to use in
cooking their own breakfasts. They planned to boil their eggs.

"Let's boil the water gently, using a low flame," one said;
"we'll save alcohol."

"It would be better to boil the eggs fast and get them done
quickly, so that we could put the stove out altogether," the
other replied.

Which was right?

_APPLICATION 31._ Two girls were making candy. They put a
little too much water into it.

"Let us boil the candy hard so that it will candy more
quickly," said one.

"Why, you wasteful girl," said the other. "It cannot get any
hotter than the boiling point anyhow, so you can't cook it any
faster. Why waste gas?"

Which girl was right?

INFERENCE EXERCISE

Explain the following:

161. Warm air rises.

162. The lid of a teakettle rattles.

163. Heating water makes a steam engine go.

164. When an automobile with good springs and without shock
absorbers goes over a rut, the passengers do not get a jolt,
but immediately afterward bounce up into the air.

165. Comets swing around close to the sun, then off again into
space; how do they get away from the sun?

166. When you wish to pour canned milk out, you need two holes
in the can to make it flow evenly.

167. Liquid air changes to ordinary air when it becomes even
as warm as a cake of ice.

168. Skid chains tend to keep automobiles from skidding on wet
pavement.

169. A warm iron and a blotter will take candle grease out of
your clothes.

170. Candies like fudge and nougat become hard and dry when
left standing several days open to the air.