Leave an iron object in open for a month and we will get that iron is rusted which show that vapor if present in air rusting of iron occurs in presence of oxygen and water vapours
To demonstrate whether water vapours are present in air or not, just practice this experiment out. Take out a chilled water bottle from the refrigerator and fill a steel glass. After filling the glass you'll notice that some water droplets appears outside the glass. This only shows that water vapour is present in air.
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