When people want to get the salt out of sea water, they put the sea water in shallow open tanks and let the water evaporate. The salt is left behind.
Experiment 35. Dissolve some salt in warm water until no more will dissolve. Pour the clear liquid off into an evaporating dish, being careful not to let any solid particles of the salt go over. Either set the dish aside uncovered, for several days, or heat it almost to boiling and let it evaporate to dryness. What is left in the dish?
Application 28. Some girls were heating water for tea, and were in a hurry. They had only an open stew pan to heat the water in.
"Cover the pan with something; you'll let all the heat out!" Helen said.
"No, you want as much heat to go through the water as possible. Leave the lid off so that the heat can flow through easily," said Rose.
"The water will evaporate too fast if the lid is off, and all the heat will be used up in making it evaporate; it will take it much longer to get hot without the lid," Louise argued.
"That's not right," Rose answered. "Boiling water evaporates fastest of all. We want this to boil, so let it evaporate; leave the lid off."
What should they have done?
Application 29. Two men were about to cross a desert. They had their supply of water in canvas water bags that leaked just enough to keep the outside of the bags wet. Naturally they wanted to keep the water as cold as possible.
"I'm going to wrap my rubber poncho around my water bag and keep the hot desert air away from the water," said one.
"I'm not. I'm going to leave mine open to the air," the other said.
Which man was right? Why?
Inference Exercise
Explain the following:
151. When you go up high in an elevator, you feel the pressure of the air in your ears.
152. Water is always flowing into Great Salt Lake; it has no outlet; yet it is getting more nearly empty all the time.
153. A nail sinks while a cork floats in water.
154. Steep hillsides are paved with cobblestones instead of asphalt.
155. If you place one wet glass tumbler inside another you can pull them apart only with difficulty, and frequently you break the outer one in the attempt.
156. Sausages often break their skins when they are being cooked.
157. A drop of water splashed against a hot lamp chimney cracks it.
158. When you shoot an air gun, the air is compressed at first; then when it is released it springs out to its original volume and throws the bullet ahead of it.
159. Leather soles get wet through in rainy weather, while rubbers remain perfectly dry on the inside.
160. When you want to clean a wooden floor, you scrub it with a brush.
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.