Which gas or pollutant reduces the density of air ?

A metal which forms a white deposit on its surface when exposed to moist air ?

In which form does oxygen is not in the earth's crust ?

I am a brown coloured gas and I'm a mixture of nitric oxide and oxygen , who am I ?

I am a metal who does not react chemically and I am called as noble metal , who am I ?

Put the appropriate product and balance it :-

4Fe + 3O2 -----> _________

Name a non-toxic metal ?