Question

Why does a gas have no definite shape and no definite volume?

Answer 1

Because its shape and volume always change.

Explanation:

The molecules that make up gas are very energetic and are very far apart compared to the molecules in a solid or a liquid.

Answer 2

Because a gas is conceived to occupy WHATEVER volume is available to it....

Explanation:

Unlike solids, and liquids, the which have characteristic and more or less constant densities, i.e. `rho_"density"="mass"/"volume"`, a gas will fill WHATEVER volume is available to it, and in fact `"Dalton's Law of Partial Pressures"` EXPLICITLY states that the total pressure of a gaseous mixture, `P_"Total"-=P_1+P_2+...P_n`, and that the individual partial pressures, as they are called, are proportional to the mole fraction of each gas in solution....

And thus gases can have various pressures, and densities, the which are state functions of temperature...