1. CHEMICAL REACTIONS AND EQUATIONS

2. Chemical reaction Chemical reactions are the process in which new substances with new properties are formed. When a magnesium ribbon is heated, it burns in air with a dazzling white flame to form a white powder called magnesium oxide. The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction. Before burning in air, the magnesium ribbon is cleaned by rubbing with a sand paper. This is done to remove the protective layer of basic magnesium carbonate from the surface of magnesium ribbon.

3. Evolution of gas Formation of a precipitate Change In Colour Change In Temperature Change In State

4. Evolution of gas Some chemical reactions cherecterised by the evolution of gas. For example – when zinc granules react with the dilute sulphuric acid then the bubbles of hydrogen gas are formed. Formation of a precipitate Some chemical reactions are charecterised by the formation of a precipitate. For example- when potassium iodine solution is added to a solution of a lead nitrate’ then a yellow precipitate of lead iodine is formed.

5. Change in colour Some chemical reactions are charecterised by the change in colour. For example- when citric acid reacts with the potassium permanganate.solution, then the purple colour of the potassium permanganate. solution disappears. Change in temperature. Some chemical reactions are charecterised by the change in temperature. For example- when quicklime reacts with water, then slaked lime is formed and a lot of heat energy is produced. This heat raises the temperature due to which the reaction mixture becomes hot.

6. Change in states Some chemical reactions are charecterised by a change in state. For example- when wax is burned then water and carbon dioxide is formed. Now, wax is a solid, water is a liquid whereas carbon dioxide is a gas. This means that during the combustion reaction of wax, the physical state changes from solid to liquid and gas.

7. Chemical equations The method of representing a chemical reaction with the help if symbols and formula of the substances involved in it is known as chemical equations. Balanced and unbalanced chemical equations  A balanced chemical equation has an equal number of atoms of different elements in the reactants and products  An unbalanced chemical equation has an unequal number atoms of one or more elements in the reactants and products.

8. Types of chemical reactions Combination reaction. Decomposition reaction Displacement reaction Double displacement reaction. Oxidation Reaction Reduction reaction

9. Combination reaction Those reactions in which two or more substances combine to form a single substance, are called combination reaction. Decomposition reaction Those reactions in which a compound splits up into two or more simpler substances are known as decomposition reaction. Thermal decomposition reaction.

10. Decomposition reaction by using electricity Decomposition reaction with light energy Displacement reaction Those reactions in which one element takes the place of another element in a compound are known as displacement reaction.

11. Double displacement reaction Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reaction. Oxidation reaction The addition of oxygen to a substance is called oxidation reaction. Reduction Reaction The addition of hydrogen to a substance is called reduction.

12. CORROSION Corrosion is a process in which metals are eaten up gradually by the action of air, moisture or a chemical on their surface. Corrosion is caused by mainly by the oxidation of metals by the oxygen in air. Rusting of iron metal is most common form of corrosion. Corrosion weakens iron and steel objects and structures such as railings, car bodies, bridges and ships, etc.. And cuts short their life.

13. Rancidity The conditions produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity. The development of rancidity can be prevented or retarded in the following ways-

14. 1. Rancidity can be prevented by packaging fat and oil containing foods in nitrogen gas. 2. Rancidity can be retarded by keeping food in a refrigerator. 3. Rancidity can be retarded by storing food in air-tight containers. 4. Rancidity can be retarded by storing foods away from light.

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