Chemical Reaction  The transformation of a chemical substance into another chemical substance. In a chemical reaction chemical change takes place. For ex-  2Mg + O2 → 2MgO

Common observations in a chemical reaction – • Change of state • Change in colour • Evolution of gas • Change in temperature • Formation of a precipitate

Chemical Equation Representation of chemical reaction using symbols of substances to represent the chemical reaction in concise and informative way. For convenience, symbol of substance is used.

Reactant: Substances which take part in a chemical reaction. Product: New substance formed after a chemical reaction. For ex- 2H2(g) + O2(g) → 2H2O(l)

Balanced Chemical Equation– The no. of atoms of each element equal on both sides. To obey Law of Conservation of Mass, the initial no. of atoms of an element should be equal to final no. of atoms.

Unbalanced Chemical Equation– The number of atoms of each element in reactants is not equal to the number of atoms of each element present in product.

Making Chemical Equation More Informative– 1. By writing the physical states of substances in a chemical equation 2. Writing the condition in which the reaction takes place

Exothermic Reaction– Reactions which produce energy. Eg- Respiration Endothermic Reaction– Reactions which absorb/require energy. Eg- Photosynthesis