Question

A balloon with hydrogen in it rises up but a baloon with air come down. Why

Answer 1

density of hydrogen is less than density of air

or

hydrogen is lighter than air

Answer 2

Hydrogen is very less dense compared to air. ... (i)

So a balloon displaces a volume of air equal to the volume of balloon. Therefore by Archemedes' Principle, the bouyant force is equal go weight of air displaced. Now the weight of air > weight of helium balloon of same volume ( because of (i) ). Therefore, there is an upthrust and hydrogen balloon rises up. A balloon with air comes down as the weight of the balloon > weight of air displaced (note that balloon has rubber outer part, which is quite significantly denser than air)