Introduction

Everything around us undergoes changes. Some changes are temporary, while others produce completely new substances. When a substance changes into one or more new substances with different properties, the process is called a chemical reaction.

What is a Chemical Reaction?

A chemical reaction is a process in which reactants are transformed into products. During this process, the original substances lose their identity and new substances are formed.

Signs of a Chemical Reaction

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

Example:
When magnesium burns in air, it forms magnesium oxide, a white powder.

Chemical Equations

A chemical equation is a short and systematic way of representing a chemical reaction using symbols and chemical formulas.

Example:

Hydrogen + Oxygen → Water

Chemical form:

2H₂ + O₂ → 2H₂O

Chemical equations help us understand which substances take part in a reaction and what products are formed.

Balanced Chemical Equations

A chemical equation must have the same number of atoms of each element on both sides. Such an equation is called a balanced chemical equation.

Balancing is necessary because matter cannot be created or destroyed during a chemical reaction.

Example:

2H₂ + O₂ → 2H₂O

Here, the number of hydrogen and oxygen atoms is equal on both sides.

Types of Chemical Reactions

1. Combination Reaction

When two or more substances combine to form a single product, the reaction is called a combination reaction.

Example:

CaO + H₂O → Ca(OH)₂

In this reaction, calcium oxide and water combine to form calcium hydroxide.

2. Decomposition Reaction

When a single compound breaks down into two or more simpler substances, it is called a decomposition reaction.

Example:

CaCO₃ → CaO + CO₂

Here, calcium carbonate decomposes into calcium oxide and carbon dioxide.

3. Displacement Reaction

A more reactive element can replace a less reactive element from its compound. This is known as a displacement reaction.

Example:

Zn + CuSO₄ → ZnSO₄ + Cu

Zinc displaces copper from copper sulphate solution.

4. Double Displacement Reaction

When two compounds exchange their ions to form new compounds, the reaction is called a double displacement reaction.

Example:

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

A white precipitate of barium sulphate is formed.

5. Oxidation and Reduction

Oxidation

The addition of oxygen or the removal of hydrogen from a substance.

Reduction

The removal of oxygen or the addition of hydrogen to a substance.

When oxidation and reduction occur together, the reaction is called a redox reaction.

Effects of Oxidation in Daily Life

Corrosion

The gradual destruction of metals due to the action of air, moisture, or chemicals is called corrosion.

Example:
Rusting of iron.

Corrosion weakens metals and causes economic loss.

Rancidity

The spoilage of food containing fats and oils due to oxidation is called rancidity.

Example:
Old chips or snacks developing an unpleasant smell and taste.

Rancidity can be reduced by storing food in airtight containers and keeping it away from air and sunlight.

Conclusion

This chapter explains how chemical reactions occur and how they are represented through chemical equations. It also covers balanced equations, different types of reactions, and important everyday phenomena such as corrosion and rancidity. Understanding these concepts helps us connect chemistry with the changes that occur around us in daily life.