Types of Chemical reactions: 1. Combination reaction: Reactions in which two or more reactants combine to form a single substance are called combination reactions. Example: (a) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide). The reaction is highly exothermic, a lot of heat is produced during the reaction. 2. Decomposition reaction: It is the reaction in which a single reactant breaks down to produce simpler products. This reaction is the opposite of a combination reaction. On the basis of the form of energy required for the reaction, these reactions are of three types: (a) Thermal decomposition reaction: It is a type of decomposition reaction, which uses energy in the form of heat. Examples: Ferrous sulphate on heating, lose water of crystallization and forms dehydrated FeSO4 which on decomposition gives ferric oxide, sulphur dioxide and sulphur trioxide. (b) Electrolysis (Electrolytic decomposition): This reaction involves the use of electricity for the decomposition of the reactant molecules. (c) Photochemical decomposition (photolysis): This is a type of decomposition reaction which involves the use of light energy for decomposition. Example: When silver chloride is exposed to sunlight, it decomposes to give silver metal and chlorine gas. In this reaction white colour of silver chloride changes to greyish-white due to the formation of silver metal. 3. Displacement reaction: It is the reaction in which one element displaces the other element in a compound. Generally, a more reactive element displaces the less reactive element from its compound. Examples: When an iron nail is dipped in copper sulphate solution, iron being more reactive than copper displaced copper from an aqueous solution of copper sulphate. 4. Double displacement reaction: It is a reaction in which two different ions or groups of atoms in the reactant molecules are displaced by each other. A double displacement reaction usually occurs in solution. It is also called a precipitation reaction as a precipitate is produced in such reactions. Example: When sodium sulphate solution is added to barium chloride solution, a white precipitate of barium sulphate is formed along with sodium chloride solution. The white precipitate of BaSO4 is formed, so it is called a precipitation reaction.