What is Gas?

Gas is a substance, where the particles (molecules and atoms) can freely move. In thermodynamic equilibrium these particles are uniformly distributed in space, so that the pressure, partial pressures and gas composition are the same at all points of the container.

A solid is a substance, the particles are fixed in their positions, they can only vibrate and rotate. A liquid is a substance, where particles can freely slide on each other but cannot separate from each other, hindered by intermolecular forces.