What this chapter is about
This chapter introduces the language chemists use to describe chemical changes. When substances interact and form new substances with different properties, a chemical reaction has occurred. You learn to represent these changes using chemical equations, where symbols and formulas replace long descriptions.
A Class 10 student meets this chapter first because it builds the foundation for all chemistry that follows — acids and bases, metals, carbon compounds all depend on understanding reactions. By the end, you should recognise different types of reactions, write balanced chemical equations, and identify signs that a chemical change has taken place.
You will also learn why equations must be balanced (atoms are neither created nor destroyed), how to balance them systematically, and how to classify reactions into useful categories like combination, decomposition, displacement and redox reactions.
Key ideas
- A chemical reaction involves breaking of bonds in reactants and forming new bonds to produce products with different properties.
- A chemical equation uses symbols and formulas to show reactants (left side) and products (right side), separated by an arrow (→).
- Equations must be balanced: the number of atoms of each element must be equal on both sides, following the law of conservation of mass.
- Physical states are shown as (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water).
- Combination reaction: two or more substances combine to form a single product. Example: A + B → AB.
- Decomposition reaction: a single compound breaks into two or more simpler substances. Example: AB → A + B.
- Displacement reaction: a more reactive element replaces a less reactive one from its compound. Example: A + BC → AC + B.
- Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen. Reactions involving both are called redox reactions.
Formulas and facts to remember
- Law of conservation of mass: In a chemical reaction, total mass of reactants equals total mass of products.
- Combination reaction example: 2Mg + O₂ → 2MgO (magnesium burns in air to form magnesium oxide).
- Decomposition by heat (thermal decomposition): CaCO₃ → CaO + CO₂ (limestone breaks down on heating).
- Decomposition by electricity (electrolysis): 2H₂O → 2H₂ + O₂ (water splits into hydrogen and oxygen).
- Displacement reaction example: Zn + CuSO₄ → ZnSO₄ + Cu (zinc displaces copper from copper sulphate solution).
- Double displacement reaction: exchange of ions between two compounds. Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.
- Oxidation-reduction (redox): In CuO + H₂ → Cu + H₂O, CuO is reduced (loses oxygen) and H₂ is oxidised (gains oxygen).
- Signs of a chemical reaction: change in colour, evolution of gas, formation of precipitate, change in temperature, change in smell.
Worked examples
Example 1: Balancing a chemical equation
Unbalanced equation: Fe + H₂O → Fe₃O₄ + H₂
Step 1: Count atoms on each side. Left: Fe = 1, H = 2, O = 1 Right: Fe = 3, H = 2, O = 4
Step 2: Balance iron by placing 3 before Fe on left. 3Fe + H₂O → Fe₃O₄ + H₂
Step 3: Balance oxygen by placing 4 before H₂O. 3Fe + 4H₂O → Fe₃O₄ + H₂
Step 4: Now left has H = 8. Place 4 before H₂ on right. 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Final check: Fe = 3, H = 8, O = 4 on both sides. Balanced.
Example 2: Identifying reaction type
When a strip of zinc is placed in blue copper sulphate solution, the solution becomes colourless and a reddish-brown deposit forms on the zinc.
Equation: Zn + CuSO₄ → ZnSO₄ + Cu
Analysis: Zinc (more reactive) has displaced copper (less reactive) from its compound. This is a displacement reaction. It is also a redox reaction: zinc is oxidised (loses electrons) and copper ions are reduced (gain electrons).
Example 3: Decomposition reaction in daily life
When silver jewellery turns black over time, silver reacts with hydrogen sulphide in air. 2Ag + H₂S → Ag₂S + H₂
This is a combination reaction forming silver sulphide (the black coating). When we clean the jewellery using certain chemicals, Ag₂S decomposes — that is a decomposition reaction.
Common mistakes
- Writing unbalanced equations and assuming they are correct → always count atoms of each element on both sides before finishing.
- Changing subscripts in formulas to balance an equation → only change coefficients (numbers before formulas), never subscripts.
- Confusing physical change with chemical change → physical changes (melting ice) do not produce new substances; chemical changes do.
- Thinking all reactions that release gas are decomposition → gas can also form in displacement or double displacement reactions.
- Forgetting to write physical states → states (s), (l), (g), (aq) give important information about the reaction conditions.
Quick revision
- Chemical equation: reactants → products, using symbols and formulas.
- Balance equations by adjusting coefficients, not subscripts.
- Combination: many → one; Decomposition: one → many.
- Displacement: reactive element kicks out less reactive one.
- Oxidation = gain of oxygen or loss of hydrogen; Reduction = opposite.
- Redox reactions have both oxidation and reduction happening together.