The Apparently Impossible
Have you ever had tea on the top of a mountain? If so, you will agree that your cup of tea could by no means be termed excellent.
Now, why is it that a cup of tea made on a mountain-top is much inferior to one made at a lower level? If the fault lay in the tea, the defect could be easily remedied, but such is not the case, for it depends upon the fact that water on the top of a mountain boils at a lower temperature than water at the sea-level.
In order to make a good cup of tea, the water must boil at a temperature very near 100° C., and it is at this temperature that the water is generally boiled in your homes.
Why is it, then, that water boils at different temperatures at different altitudes? It is because, as the altitude is increased, so the atmospheric pressure becomes less.
At sea-level, atmospheric pressure is equal to about 15 lbs. to the square inch, but at the top of a mountain it is much less. The greater the atmospheric pressure the more heat is required before the bubbles of vapor formed within the water can break at the surface.
After this explanation, perhaps the subjoined experiment will be attempted with additional interest.
Take a flask, to which should be fitted a good cork or india-rubber stopper, and in it boil some water, taking care of course to remove the stopper beforehand.
After some minutes the steam from the boiling water will have expelled all the air from the flask. Now remove the source of heat, at the same time quickly inserting the stopper.
If the flask is allowed to stand for a minute or two, the temperature of the water will fall considerably below 100° C.
Next inform your friends that, without applying any extra heat, you will cause the water in the flask to boil vigorously again. This seems to them impossible, especially when you tell them that you are going to do it by means of cold water. Quickly turn the glass upside down, and squeeze a sponge soaked in cold water on its upturned under-surface. Immediately the liquid inside will begin to boil, as if extra heat had been applied ([Fig. 13]).
Fig. 13.—A curious boiling experiment.
But how are you to explain this apparently extraordinary phenomenon?
Well, directly the cold water comes in contact with the flask it causes the steam contained therein to condense, and, as no air can enter, thanks to the well-fitting cork, the pressure on the surface of the warm water is now considerably less than it was before.
Directly the pressure is lessened the vapor bubbles contained within the warm water are able to rise to the surface, and the water is seen to boil merrily.