CBSE · Class 10 Science

Important Chemical Reactions

Every chemical reaction that appears in CBSE Class 10 Science board exams — with balanced equations, observations, and exactly what examiners look for.

Types of Chemical Reactions

Combination Reaction

very high frequency

Balanced Equation

CaO + H₂O → Ca(OH)₂

Type

Combination

Observation: Heat is released (exothermic). Quicklime turns to slaked lime.

💡

Two or more substances combine to form a single product. Always exothermic — write heat release in answer.

Decomposition Reaction (Thermal)

very high frequency

Balanced Equation

CaCO₃ → CaO + CO₂ (heat)

Type

Decomposition

Observation: Calcium carbonate breaks into calcium oxide and carbon dioxide on heating.

💡

One reactant breaks into two or more products. Opposite of combination. Common: 2H₂O₂ → 2H₂O + O₂

Electrolytic Decomposition

very high frequency

Balanced Equation

2H₂O → 2H₂ + O₂ (electric current)

Type

Decomposition

Observation: Hydrogen collects at cathode (double volume), oxygen at anode.

💡

Ratio of H₂ : O₂ = 2:1 by volume. A very common 1-mark question.

Displacement Reaction

very high frequency

Balanced Equation

Fe + CuSO₄ → FeSO₄ + Cu

Type

Displacement

Observation: Blue colour of CuSO₄ fades. Iron nail turns reddish-brown (copper deposits).

💡

More reactive metal displaces less reactive metal. Zn + H₂SO₄ → ZnSO₄ + H₂ is another common example.

Double Displacement (Precipitation)

very high frequency

Balanced Equation

Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl

Type

Double Displacement

Observation: White precipitate of BaSO₄ forms. Insoluble product separates from solution.

💡

↓ symbol = precipitate. Write state symbols — (aq), (s), (l), (g) — for full marks.

Neutralization Reaction

high frequency

Balanced Equation

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Type

Double Displacement

Observation: Acid and base react to form salt and water. pH moves towards 7.

💡

Always acid + base → salt + water. The heat released is called heat of neutralization.

Oxidation, Reduction and Redox

Thermite Reaction

very high frequency

Balanced Equation

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

Type

Redox / Displacement

Observation: Large amount of heat released. Used in welding railway tracks.

💡

Al is oxidised (gains O), Fe₂O₃ is reduced (loses O). Al acts as reducing agent.

Oxidation of Copper

high frequency

Balanced Equation

2Cu + O₂ → 2CuO (heat)

Type

Oxidation

Observation: Copper surface turns black (copper oxide forms).

💡

When H₂ gas is passed over black CuO, it turns back to brown copper — reduction.

Hydrogen Reducing Metal Oxide

high frequency

Balanced Equation

CuO + H₂ → Cu + H₂O

Type

Redox

Observation: Black CuO turns reddish-brown. H₂ is oxidised, Cu²⁺ is reduced.

💡

CuO → reducing agent is H₂. CuO is oxidising agent. Classic oxidation-reduction pair.

Corrosion and Rancidity

Rusting of Iron

very high frequency

Balanced Equation

4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (hydrated iron oxide)

Type

Oxidation

Observation: Reddish-brown rust forms on iron in presence of oxygen and moisture.

💡

Both O₂ and H₂O are required — iron rusts faster in salt water (electrolyte speeds reaction).

Rancidity

high frequency

Balanced Equation

Fats/Oils + O₂ → Peroxides (bad smell/taste)

Type

Oxidation

Observation: Smell and taste of food changes — butter, chips go stale.

💡

Prevention: antioxidants (N₂ flushing, vacuum packing). Fridge slows it down.

Acids, Bases and Salts

Acid + Metal

very high frequency

Balanced Equation

Zn + H₂SO₄ → ZnSO₄ + H₂↑

Type

Displacement

Observation: Hydrogen gas is produced (bubbles). Metal dissolves.

💡

H₂ gas test — burning splint produces pop sound.

Acid + Metal Carbonate

very high frequency

Balanced Equation

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑

Type

Double Displacement

Observation: CO₂ gas produced — turns lime water milky.

💡

CO₂ test — lime water (Ca(OH)₂) turns milky due to CaCO₃ precipitate.

Acid + Metal Hydroxide

very high frequency

Balanced Equation

HCl + NaOH → NaCl + H₂O

Type

Neutralization

Observation: pH moves to 7. Heat is released.

💡

This is also called a neutralization reaction. Product is always salt + water.

Metals and Non-Metals

Metal + Cold Water (Highly Reactive)

very high frequency

Balanced Equation

2Na + 2H₂O → 2NaOH + H₂↑

Type

Displacement

Observation: Vigorous reaction. Hydrogen gas catches fire. Solution turns alkaline.

💡

Na, K react violently with cold water. Ca reacts less vigorously. Mg reacts only with hot water/steam.

Metal + Dilute Acid (Moderately Reactive)

very high frequency

Balanced Equation

Fe + 2HCl → FeCl₂ + H₂↑

Type

Displacement

Observation: Metal dissolves, hydrogen gas is produced.

💡

Metals above hydrogen in reactivity series displace H₂ from dilute acids. Cu, Ag (below H) do not react.

Metal Oxide + Water → Base

high frequency

Balanced Equation

CaO + H₂O → Ca(OH)₂

Type

Combination

Observation: Heat is released. Slaked lime (Ca(OH)₂) is formed.

💡

Metal oxides are basic in nature. Ca(OH)₂ is used in whitewashing. This is also used to test alkalinity.

Non-metal Oxide + Water → Acid

high frequency

Balanced Equation

CO₂ + H₂O → H₂CO₃

Type

Combination

Observation: Carbonic acid (H₂CO₃) forms. Solution turns litmus red (acidic).

💡

Non-metal oxides are acidic in nature. SO₂ + H₂O → H₂SO₃ is another common example.

Carbon Compounds

Combustion of Ethanol

very high frequency

Balanced Equation

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + Heat

Type

Combustion (Oxidation)

Observation: Blue flame produced. CO₂ and water vapour released.

💡

Complete combustion gives CO₂ + H₂O. Incomplete combustion gives CO (carbon monoxide) — a toxic gas.

Esterification

very high frequency

Balanced Equation

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Type

Esterification

Observation: Sweet-smelling ester (ethyl ethanoate) is formed. Reaction is reversible.

💡

Catalyst: Conc. H₂SO₄ (acts as dehydrating agent). Reversible reaction — write equilibrium arrows. Used to make soaps, perfumes.

Saponification (Soap Making)

very high frequency

Balanced Equation

CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

Type

Hydrolysis

Observation: Ester is hydrolysed by NaOH to give sodium salt of acid + alcohol.

💡

Soap = sodium/potassium salt of a long-chain fatty acid. Hot NaOH hydrolyses fats/oils → soap + glycerol.

Addition Reaction (Unsaturated → Saturated)

high frequency

Balanced Equation

CH₂=CH₂ + H₂ → CH₃-CH₃ (catalyst: Ni/Pt)

Type

Addition

Observation: Alkene becomes alkane. Double bond breaks.

💡

Test: Bromine water decolourises with unsaturated compounds (alkenes, alkynes) — not with alkanes.

Oxidation of Ethanol to Ethanoic Acid

high frequency

Balanced Equation

C₂H₅OH + [O] → CH₃COOH

Type

Oxidation

Observation: Ethanol turns to ethanoic acid (vinegar). Acidic smell.

💡

Oxidising agents: alkaline KMnO₄ or acidified K₂Cr₂O₇. This reaction converts alcohol → carboxylic acid.

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