INTRODUCTION

Whenever a chemical change occurs, we can say that a chemical reaction has taken place. A Chemical change occurs when –

• Change in state
• Change in colour
• Evolution of gas
• Change in temperature
• Formation of a precipitate

• For example -Reaction of Ammonia gas with hydrogen chloride gas: Ammonia gas reacts with Hydrogen chloride gas to produce solid Ammonium chloride

• Reaction of Copper sulphate with iron nail – When an iron nail is dipped in blue-coloured copper sulphate solution, the blue colour changes to green.

• Reaction of zinc with dilute hydrochloric acid – When a few granules of zinc are added to a test tube containing dilute hydrochloric acid, bubbles can be seen forming at the surface of zinc. These bubbles signify the formation of hydrogen gas.

Zn (s) + HCl (aq) —-> ZnCl₂ (aq) + H₂ (g)

here, evolution of gas takes places

• Reaction of quick lime with water – Some chemical reaction is characterized by a change in temperature. When water is added to a beaker containing quick lime (Calcium oxide), the beaker becomes hot and bubbles start to come out. The rise in temperature indicates a chemical reaction.

• Reaction of lead nitrate and potassium iodide – When potassium iodide is added to the lead nitrate solution, a yellow precipitate is obtained. This yellow precipitate is lead iodide which is insoluble in water.

CHEMICAL EQUATIONS

WRITING A CHEMICAL EQUATION

• Chemical equations can be made more concise and useful if chemical formulae are used instead of words. For example –

Mg + O₂ → MgO

• If the equation is unbalanced it means, the mass is not the same on both sides of the equation. Such a chemical equation is a skeletal chemical equation for a reaction. The above-mentioned equation is a skeletal chemical equation.

• Skeletal Equation- The number of atoms of each element on both sides is not equal.

BALANCED CHEMICAL EQUATIONS

• According to the law of conservation of mass – Mass can neither be created nor destroyed in a chemical reaction.

• In other words, the number of atoms of each element remains the same, before and after a chemical reaction.

• Hence, we need to balance a skeletal chemical equation. The chemical equation for the reaction of zinc with sulphuric acid may be represented as

Zn + H₂SO₄ → ZnSO₄ + H₂

In the above equation, the number of atoms of each element is already equal on both sides, and hence the reaction is balanced.

• Balancing the chemical equation of the reaction between iron and water vapor.

Fe + H₂O → Fe₃O₄ + H₂

• Step I: List the number of atoms in the reactants and products as follows

Element	Number of atoms in reactants (LHS)	Number of atoms in Products (RHS)
Fe	1	3
H	2	2
O	1	4

• Step II: We will use the hit-and-trial method to balance the skeletal equation . First we will balance Fe (metal) . There are three Iron atoms on the RHS and only one on the LHS. In order to balance Iron we will multiply by 3 beside Fe on L.H.S side.