Question

how would you show that air has water vapor in it

Answer 1

Hi,
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Just take a cold glass of water and place it anywhere for a while. You will see drop of water outside the glass. As the water vapours​ in the air strike the cold glass, they loose their kinetic energy and condense on the outer surface. That's why you see the water outside glass.
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HOPE THIS HELPS YOU:-))

Answer 2

if we take out a bottle from refrigerator, we will see that some droplets appear on bottle that are water vapour.