If hydrogen is the lightest element, why don't we use it to inflate balloons

Its unstable and flammable... the Hindenberg used it... we saw how that went

The element is very flammable. 

I believe the above is an example of a physical change
Physical changes alter the physical form of substances without changing their chemical identities. A physical change may involve changing the state of a substance, such as melting ice, or changing the shape or the size of the pieces in a sample. Other physical changes involve the formation of mixtures in which all of the components retain their chemical identifies, such as dissolving sugar in water.