Answers

2015-08-18T17:59:49+05:30
The thing is the molecules in hot air have more energy as compared to cold air.And this energy diminishes the inter moleculer force of attraction,making the air light....e.g,a hot air balloon.
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2015-08-18T18:09:50+05:30
As the air becomes hotter its weigth becomes low and low and it rises up and to fill the space leaved by the hotter air and the colder air moves down or sinks.

Hope this helps:)
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