Question

How could you distinguish between the gases h2 and no2?

Answer 1

Explanation:

A molecule is defined as a substance which contains chemically bonded same type of elements.

For example, $H_{2}$ is a molecule and it exists as a gas which is colorless and odorless.  

Hydrogen gas is represented as $H_{2}$.

Whereas a compound is defined as a substance which contains chemically bonded different type of elements in a fixed ratio by mass.

For example, $NO_{2}$ is a compound and has a reddish brown color with toxic odor.

Nitrogen dioxide is represented as $NO_{2}$.

Answer 2

H2 is colorless and odorless
NO2 is brown, pungent, and toxic