The gas particles move randomly with a distribution of speeds. The gas particles collide with one another. If a gas is heated from 50 C to 100 C at constant pressure, the volume will double. If the volume of a gas is doubled at constant temperature, the pressure will be cut in half. An ideal gas has zero volume at a temperature of absolute zero.
Real gases attract one another at some distances.
At 25 C, X will boil if the pressure is lowered enough. X is liquid at 120 C and 1 atm. The melting point of X varies little with pressure. The units which occupy the lattice points are atoms. They typically exhibit low melting points.