The state of a substance is determined by the inter-molecular forces present in it. If the inter-molecular forces are so strong that the substance is rigid and has fixed shape and volume, then the state of the substance is solid. The state of a substance is liquid if the inter-molecular forces hold the particles in such a way that the substance has no fixed shape but fixed volume. The state of a substance is gas if the inter-molecular forces among the particles are weak. Gases do not have a fixed shape or volume.