Which gas is the lightest in weight?

Give reasons for the following:
$(i)$ Metals conduct electricity.
$(ii)$ Non-metals in general do not displace hydrogen from dilute acids.
$(iii)$ Aluminium easily combines with oxygen but still it can be used for making kitchen utensils.

Why does a magnesium ribbon start floating when it is placed in hot water?

An element $A$ reacts with water to form a compound $B$ which is used in white washing. The compound $B$ on heating forms an oxide $C$ which on treatment with water gives back $B$. Identify $A$,$B$ and $C$ and give the reactions involved.

What is the ratio of dihydrogen gas to dinitrogen gas used in the production of ammonia gas by Haber's process?

What happens when carbon dioxide is dissolved in water at high pressure?