The transition of a substance directly from the solid to the gas state, without passing through the liquid state, is known as sublimation. This process occurs when the substance absorbs enough energy to overcome the intermolecular forces holding it in a solid state, but at a pressure and temperature where the liquid phase is unstable or does not exist. - Fusion (Melting) is the transition from solid to liquid. - Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. - Emulsion is a mixture of two or more liquids that are normally immiscible.