Types of Chemical Reactions for CBSE Class 10 Science

Chemical reactions govern every transformation of matter in the universe. While millions of distinct chemical changes occur in laboratories, nature, and industrial plants, almost all inorganic reactions can be classified into four fundamental categories based on how atoms and ions rearrange themselves: Combination, Decomposition, Displacement, and Double Displacement reactions.

In CBSE Class 10 Science, mastering these four types of reactions—along with their distinctive visual signs, colour changes, and energy exchanges—is essential for scoring full marks in board examinations.

What You Will Learn

• Detailed mechanisms of the four primary reaction types
• Combination Reactions: Synthesis of single products and whitewashing chemistry
• Decomposition Reactions: Thermal, electrolytic, and photolytic breakdown
• Displacement Reactions: The role of the metal reactivity series
• Double Displacement Reactions: Precipitation and acid-base neutralisation
• Key NCERT activities (ferrous sulphate, lead nitrate, silver chloride, electrolysis of water)
• Board examination comparison tables and common student traps

1. Combination Reactions (Synthesis)

Formal Definition

A reaction in which two or more reactants combine to form a single product is called a combination reaction.

$\text{General Form: } A + B \longrightarrow AB$

High-Yield Examples:

1. Burning of Coal: Carbon burns in atmospheric oxygen to produce carbon dioxide gas: $\text{C}(s) + \text{O}_2(g) \longrightarrow \text{CO}_2(g)$
2. Formation of Water: Hydrogen gas reacts with oxygen gas: $2\text{H}_2(g) + \text{O}_2(g) \longrightarrow 2\text{H}_2\text{O}(l)$
3. Slaking of Lime (NCERT Activity 1.4): Quicklime (calcium oxide) reacts vigorously with water to form slaked lime (calcium hydroxide), releasing immense heat: $\text{CaO}(s) \text{ [Quicklime]} + \text{H}_2\text{O}(l) \longrightarrow \text{Ca(OH)}_2(aq) \text{ [Slaked lime]} + \text{Heat}$

The Chemistry of Whitewashing (CBSE Classic Question)

• Slaked lime solution, $\text{Ca(OH)}_2$, is applied to building walls during whitewashing.
• It reacts slowly with carbon dioxide in the air to form a thin, hard, shiny layer of calcium carbonate ($\text{CaCO}_3$) after two to three days: $\text{Ca(OH)}_2(aq) + \text{CO}_2(g) \longrightarrow \text{CaCO}_3(s) \text{ [Shiny finish]} + \text{H}_2\text{O}(l)$
• Note that marble, limestone, and eggshells also share the chemical formula $\text{CaCO}_3$.

2. Decomposition Reactions

Formal Definition

A reaction in which a single reactant breaks down to give two or more simpler products is called a decomposition reaction.

$\text{General Form: } AB \longrightarrow A + B$

Decomposition reactions require energy to break the chemical bonds of the reactant. Depending on the form of energy supplied, they are subdivided into three types:

                      Decomposition Reactions
                                 |
       +-------------------------+-------------------------+
       |                         |                         |
Thermal Decomposition     Electrolytic Decomposition    Photolytic Decomposition
     (Uses Heat)              (Uses Electricity)             (Uses Light)

Type A: Thermal Decomposition (Energy Supplied as Heat)

1. Heating Ferrous Sulphate Crystals (NCERT Activity 1.5): Green ferrous sulphate crystals ($\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$) first lose water of crystallisation and turn white. On further strong heating, anhydrous $\text{FeSO}_4$ decomposes into reddish-brown ferric oxide and suffocating gases ($\text{SO}_2$ and $\text{SO}_3$): $2\text{FeSO}_4(s) \text{ [Green]} \xrightarrow{\quad \Delta \quad} \text{Fe}_2\text{O}_3(s) \text{ [Reddish-brown]} + \text{SO}_2(g) + \text{SO}_3(g)$ Observation: Smell of burning sulphur due to $\text{SO}_2$ and $\text{SO}_3$.
2. Decomposition of Calcium Carbonate (Limestone): Used extensively in the cement manufacturing industry: $\text{CaCO}_3(s) \xrightarrow{\quad \Delta \quad} \text{CaO}(s) \text{ [Quicklime]} + \text{CO}_2(g)$
3. Decomposition of Lead Nitrate (NCERT Activity 1.6): Heating white lead nitrate powder produces yellow lead(II) oxide and dense brown fumes of nitrogen dioxide ($\text{NO}_2$): $2\text{Pb(NO}_3)_2(s) \xrightarrow{\quad \Delta \quad} 2\text{PbO}(s) \text{ [Yellow]} + 4\text{NO}_2(g) \uparrow \text{ [Brown fumes]} + \text{O}_2(g) \uparrow$

Type B: Electrolytic Decomposition (Electrolysis - Energy as Electricity)

Electrolysis of Water (NCERT Activity 1.7): When an electric current passes through acidified water: $2\text{H}_2\text{O}(l) \xrightarrow{\quad \text{Electric Current} \quad} 2\text{H}_2(g) \text{ [Cathode]} + \text{O}_2(g) \text{ [Anode]}$

Important: <u>The volume of hydrogen gas collected at the cathode is exactly twice the volume of oxygen gas collected at the anode because water contains hydrogen and oxygen in a 2

ratio by volume ($H_2O$).</u>

Type C: Photolytic Decomposition (Photolysis - Energy as Sunlight)

1. Silver Chloride in Sunlight (NCERT Activity 1.8): White silver chloride turns grey when exposed to sunlight due to the formation of metallic silver: $2\text{AgCl}(s) \text{ [White]} \xrightarrow{\quad \text{Sunlight} \quad} 2\text{Ag}(s) \text{ [Grey]} + \text{Cl}_2(g)$
2. Silver Bromide in Sunlight: $2\text{AgBr}(s) \text{ [Pale Yellow]} \xrightarrow{\quad \text{Sunlight} \quad} 2\text{Ag}(s) \text{ [Grey]} + \text{Br}_2(g)$ Application: These photolytic reactions were historically used in black-and-white photography.

3. Displacement Reactions (Single Replacement)

Formal Definition

A chemical reaction in which a more reactive element displaces a less reactive element from its compound (or aqueous salt solution) is called a displacement reaction.

$\text{General Form: } A + BC \longrightarrow AC + B \quad (\text{where } A \text{ is more reactive than } B)$

The Role of the Reactivity Series:

A metal can displace another metal only if it lies higher in the electrochemical reactivity series: $\text{K} > \text{Na} > \text{Ca} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Pb} > [\text{H}] > \text{Cu} > \text{Hg} > \text{Ag} > \text{Au}$

Key Examples:

1. Iron with Copper Sulphate: $\text{Fe}(s) + \text{CuSO}_4(aq) \text{ [Blue]} \longrightarrow \text{FeSO}_4(aq) \text{ [Pale Green]} + \text{Cu}(s) \text{ [Reddish brown]}$ Iron is more reactive than copper; hence it displaces copper from $\text{CuSO}_4$.
2. Zinc with Copper Sulphate: $\text{Zn}(s) + \text{CuSO}_4(aq) \text{ [Blue]} \longrightarrow \text{ZnSO}_4(aq) \text{ [Colourless]} + \text{Cu}(s)$ Zinc is more reactive than copper.
3. Lead with Copper Chloride: $\text{Pb}(s) + \text{CuCl}_2(aq) \text{ [Green]} \longrightarrow \text{PbCl}_2(aq) \text{ [Colourless]} + \text{Cu}(s)$

Remember: If copper is placed in a solution of iron sulphate ($\text{Cu} + \text{FeSO}_4$), no reaction occurs because copper is less reactive than iron!

4. Double Displacement Reactions (Metathesis)

Formal Definition

A reaction in which two compounds react by exchanging their ions to form two new compounds is called a double displacement reaction.

$\text{General Form: } AB + CD \longrightarrow AD + CB$

Subtypes:

1. Precipitation Reactions: A double displacement reaction in which an insoluble solid (precipitate) is formed: $\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \longrightarrow \text{BaSO}_4(s) \downarrow \text{ [White ppt]} + 2\text{NaCl}(aq)$
2. Neutralisation Reactions: An acid reacts with a base to form a salt and water: $\text{HCl}(aq) + \text{NaOH}(aq) \longrightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$

5. Comprehensive Summary Matrix

Exam Tip: Whenever you identify a reaction as "Decomposition", always specify whether it is Thermal, Electrolytic, or Photolytic decomposition for full marks!

Common Mistake: Confusing single displacement with double displacement. Single displacement involves a free element reacting with a compound ($A + BC$), whereas double displacement involves two ionic compounds reacting together ($AB + CD$).