Common Science Part 16

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 s.p.a.ces 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 pa.s.s 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.

[Ill.u.s.tration: FIG. 51. In a minute the cork will fly out.]

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 s.p.a.ce. This left a vacuum or empty s.p.a.ce in the flask. What pushed the balloon into the empty s.p.a.ce?

[Ill.u.s.tration: FIG. 52. A toy balloon has been slipped over the mouth of a flask that is filled with steam.]

[Ill.u.s.tration: FIG. 53. As the steam condenses and leaves a vacuum, the air pressure forces the balloon into the flask.]

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 pa.s.sage 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:

[Ill.u.s.tration: 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 pa.s.ses 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 s.p.a.ce before the white vapor begins. The clear s.p.a.ce 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. Pa.s.s the end of the long bent gla.s.s 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?

[Ill.u.s.tration: FIG. 55. By distillation clear alcohol can be separated from the water and red ink with which it was mixed.]

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 pa.s.sengers 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 s.p.a.ce; 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.