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SUMMARY Matter is anything that has mass and occupies volume. The SI unit of mass and volume is kilogram (kg) and cubic metre (m°), respectively. Matter is classified into groups based on their physical and chemical properties, i.e. physical properties (solid, liquid and gas) and chemical properties (elements, compounds and mixtures). Every matter is made up of certain particles which differ in shape, size and nature. The particles of matter have a tendency to diffuse. Solids have definite shape, distinct boundaries and fixed volumes. Liquids do not have a definite shape. Gases have neither definite shape nor volume. The state of matter can be interchanged by changing temperature or pressure. At specific conditions of temperature and pressure; the conversion of a matter from its solid to its liquid state is called fusion. The conversion of a matter from its liquid state to vapour (gaseous state) is called boiling. It is a bulk phenomenon. The conversion of a matter from its liquid to solid state is called freezing. The conversion of a matter from its liquid to gaseous state is called vapourisation. The conversion of matter from its gaseous to liquid state is called condensation. The process of change of solid state directly into gaseous state without passing through the liquid state upon heating and vice-versa on cooling is called sublimation. The heat energy which has to be supplied to change the state of substance is called latent heat. Latent heat of vapourisation is the heat energy required to change 1kg of a liquid to gas at atmospheric pressure at its boiling point. Latent heat of fusion is the amount of heat energy required to change 1 kg of solid into liquid at its melting point. The process of conversion of a liquid into its vapour state at any temperature below its boiling point is called evaporation. 
