Why are liquids and gases categorised as fluids?

The ratio of rates of diffusion of gases $X$ and $Y$ is $1:5$ and that of $Y$ and $Z$ is $1:6$. The ratio of rates of diffusion of $Z$ and $X$ is

At $T \ K$,hydrogen and oxygen gases are mixed in the ratio of $1:2$ by mass in a closed vessel of volume '$V$' litres. If the total pressure of the gaseous mixture is '$p$' atm,the partial pressure of oxygen (in atm) is

$A$ mixture of $0.5 \text{ mol}$ $N_2$ gas, $1.0 \text{ mol}$ $O_2$ gas and $1.5 \text{ mol}$ $H_2$ gas exerts a total pressure of $18 \text{ bar}$. Find the partial pressure of $O_2(g)$. (in $\text{ bar}$)

During the evaporation of a liquid:

$5$ moles of a gas is allowed to pass through a series of changes as shown in the graph,in a cyclic process. The processes $C \rightarrow A$,$B \rightarrow C$ and $A \rightarrow B$ respectively are