Helium...

As we all know that balloon filled with helium gas floats on the air but the balloon filled with normal gas doesn't but do we know science behind it is that the density of hulium is much lower than the density of air around us and hence it seems like helium defy the principle of gravity.

It is a non flammable gas which makes it safe for science research and medical technology. It is also used as an inert shield for arc welding, pressuring liquid-fueled rocket fuel tanks, and in supersonic wind tunnels.