Question

______is the second-largest component of air, constituting almost $21\%$ volume of air.

Answer 1

Constituents of air:

1. Nitrogen($\mathrm{N}$) and Oxygen($\mathrm{O}$) account for $99\%$ of the gases in dry air, with Argon($\mathrm{Ar}$), Carbon dioxide(${\mathrm{CO}}_2$), Helium($\mathrm{He}$), Neon($\mathrm{Ne}$), and other gases making up minute portions.
2. Free oxygen consists of Oxygen molecules That are not attached to another element. When Carbon is attached to Oxygen it forms Carbon dioxide or when attached to Hydrogen it forms water.
3. Carbon dioxide (${\mathrm{CO}}_2$ ) is considered to be a “standard” component of air.
4. Oxygen is the main component for breathing and circulation in humans. It is an element of life.

Therefore, Oxygen is the second-largest component of air, constituting almost $21\%$the volume of air.