Thermal contraction of air
By Jitender Singh
The air pressure decreases when its temperature is decreased. If pressure is kept constant, then air contracts (its volume decreases) on cooling.
Procedure
Boil a little water in the test tube. When the water has boiled for about 15 second, quickly invert the test tube and insert it into the water in the glass (with opening of the test tube downward). You will observe that after a while, water will fill it slowly.
Discussion
There was air in the test tube beside water. After the water started boiling, the steam displaced the air in the test tube.
When the test tube was inverted into the water, the trapped steam got cooled and was converted into water. This created a partial vacuum inside the test tube. Outside atmospheric pressure acting on the water in the glass forces the water up into the test tube.
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