Q.Give suitable explanation for the following facts about gases.

The gaseous state is the simplest of the three states of matter, and the one we depend on most directly -- we can survive weeks without food and days without water, but only minutes without air. The atmosphere around us is roughly 78% nitrogen, 21% oxygen and 1% other gases by volume, and while different gases behave very differently chemically, they behave remarkably similarly physically -- which is precisely why a single set of gas laws can describe them all.

Only eleven elements exist as gases under ordinary atmospheric conditions: the diatomic molecules hydrogen (H$_2$), nitrogen (N$_2$), oxygen (O$_2$), fluorine (F$_2$) and chlorine (Cl$_2$); ozone (O$_3$), another elemental form of oxygen; and the six monatomic noble gases helium, neon, argon, krypton, xenon and radon. A handful of compounds -- CO, CO$_2$, NO$_2$, NO -- are also gases at room conditions.

Gas vs vapour. A gas is a substance normally in the gaseous state at room temperature and 1 atm; a vapour is the gaseous form of something that is normally a liquid or solid under those same conditions (water vapour, for instance, is the gaseous form of a substance -- water -- that is a liquid at room temperature). …