Chemical Reactions and Equations Class 10 Notes (2026-27) — CBSE
Class 10 Science Chapter 1 revision notes: balancing equations, all 5 reaction types with examples, corrosion, rancidity, plus flashcards and a mind map.
Chemical Reactions and Equations — Class 10 Science Notes
Chapter Snapshot
A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different properties. This chapter covers how to represent reactions as balanced chemical equations, the five major types of reactions you must identify in the exam, and two everyday effects of oxidation — corrosion and rancidity.
Board relevance: questions from this chapter appear almost every year — balancing an equation, identifying the reaction type from a given equation, and activity-based questions (iron nail in copper sulphate, burning magnesium ribbon) are the most repeated patterns. Expect 4–6 marks.
Key Concepts & Definitions
Chemical reaction — a process involving the breaking and making of bonds between atoms to produce new substances. Observable signs: change in state, change in colour, evolution of a gas, change in temperature.
Chemical equation — the symbolic representation of a reaction. Reactants are written on the left, products on the right, separated by an arrow:
Mg + O₂ → MgO (skeletal/unbalanced)
2Mg + O₂ → 2MgO (balanced)
Skeletal equation — an equation in which the number of atoms of each element is not equal on both sides.
Balanced equation — an equation with equal numbers of atoms of every element on both sides. Equations must be balanced to satisfy the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction.
State symbols make equations more informative:
Symbol Meaning
(s) solid
(l) liquid
(g) gas
(aq) aqueous solution (dissolved in water)
↑ gas evolved
↓ precipitate formed
Δ over arrow heat supplied
Balancing method (hit-and-trial): count atoms of each element on both sides → balance the element appearing in the fewest formulas first → balance others one at a time → never change a chemical formula, only the coefficients in front.
Types of Chemical Reactions
1. Combination reaction
Two or more reactants combine to form a single product: A + B → AB
- CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat — quicklime + water → slaked lime. Used in whitewashing: Ca(OH)₂ slowly reacts with CO₂ in air to form a shiny CaCO₃ layer over 2–3 days.
- C(s) + O₂(g) → CO₂(g) — burning of coal
- 2H₂(g) + O₂(g) → 2H₂O(l)
2. Decomposition reaction
A single reactant breaks into two or more products: AB → A + B. Energy (heat, light, or electricity) is required, so these are usually endothermic.
Type Example Observation
Thermal (heat) 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) Green crystals turn reddish-brown; smell of burning sulphur
Thermal CaCO₃(s) → CaO(s) + CO₂(g) Limestone → quicklime (used in cement industry)
Thermal 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g) Brown fumes of nitrogen dioxide
Electrolytic (electricity) 2H₂O(l) → 2H₂(g) + O₂(g) Hydrogen collected is double the volume of oxygen
Photolytic (sunlight) 2AgCl(s) → 2Ag(s) + Cl₂(g) White silver chloride turns grey; used in black-and-white photography
Photolytic 2AgBr(s) → 2Ag(s) + Br₂(g) Light-yellow silver bromide turns grey
3. Displacement reaction
A more reactive element displaces a less reactive element from its compound: A + BC → AC + B
- Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) — iron nail in copper sulphate: blue colour fades to light green, brown copper deposits on the nail
- Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
- Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)
Reactivity order to remember for these examples: Zn Fe Pb Cu Ag.
4. Double displacement reaction
Two compounds exchange ions to form two new compounds: AB + CD → AD + CB
- Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq) — a white precipitate of barium sulphate forms. A reaction producing an insoluble precipitate is also called a precipitation reaction.
5. Oxidation and reduction (redox)
- Oxidation = gain of oxygen or loss of hydrogen.
- Reduction = loss of oxygen or gain of hydrogen.
- Both always occur together in a redox reaction. The substance that gives oxygen is the oxidising agent; the substance that gains oxygen is the reducing agent.
CuO + H₂ → Cu + H₂O
CuO is reduced to Cu (loses oxygen) — CuO is the oxidising agent.
H₂ is oxidised to H₂O (gains oxygen) — H₂ is the reducing agent.
Other examples: ZnO + C → Zn + CO; MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O (HCl is oxidised to Cl₂).
Formulas & Reactions
Must-memorise equations for the board exam:
# Reaction Type
1 2Mg + O₂ → 2MgO Combination (Mg burns with dazzling white flame; white MgO powder)
2 CaO + H₂O → Ca(OH)₂ + heat Combination, exothermic
3 CaCO₃ →(Δ) CaO + CO₂ Thermal decomposition
4 2FeSO₄ →(Δ) Fe₂O₃ + SO₂ + SO₃ Thermal decomposition
5 2Pb(NO₃)₂ →(Δ) 2PbO + 4NO₂ + O₂ Thermal decomposition
6 2H₂O →(electricity) 2H₂ + O₂ Electrolytic decomposition
7 2AgCl →(sunlight) 2Ag + Cl₂ Photolytic decomposition
8 Fe + CuSO₄ → FeSO₄ + Cu Displacement
9 Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl Double displacement / precipitation
10 CuO + H₂ → Cu + H₂O Redox
11 C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy Respiration — exothermic
Energy classification:
- Exothermic — heat is released: all combustion reactions, respiration, CaO + H₂O, decomposition of vegetable matter into compost.
- Endothermic — heat/energy is absorbed: most decomposition reactions (thermal, electrolytic, photolytic).
Everyday Effects of Oxidation
Corrosion — the slow attack of metals by moisture, air, or acids around them.
Metal Corrosion product Appearance
Iron Rust, Fe₂O₃·xH₂O Reddish-brown flaky coating
Silver Ag₂S (with H₂S in air) Black tarnish
Copper Basic copper carbonate Green coating
Rusting needs both air (oxygen) and moisture. Damage from corrosion of bridges, railings, ship hulls, and car bodies is a huge economic cost, which is why prevention (painting, greasing, galvanising) matters.
Rancidity — oxidation of fats and oils in food, spoiling smell and taste.
Prevention: add antioxidants, store in airtight containers, refrigerate, and flush packets with nitrogen (why chips packets are puffed with gas).
Important Question Patterns
1. Balance the equation (1–2 marks). e.g. balance Fe + H₂O → Fe₃O₄ + H₂ → answer: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
2. Identify the reaction type (1–2 marks) from a given equation — check: one product (combination), one reactant (decomposition), element + compound (displacement), ion exchange/precipitate (double displacement).
3. Activity-based (2–3 marks): why is the magnesium ribbon cleaned with sandpaper before burning? (To remove the MgO layer so fresh Mg is exposed.) What is observed when an iron nail is dipped in CuSO₄?
4. Reasoning (2 marks): why is respiration exothermic? Why are decomposition reactions the opposite of combination reactions? Why is a balanced equation necessary?
5. Identify oxidised/reduced substances (2 marks) in a given redox equation, and name the oxidising and reducing agents.
6. Everyday chemistry (2–3 marks): corrosion examples and prevention; rancidity and its prevention; why food packets are flushed with nitrogen.
⚡ Quick Revision
- Balanced equation ⇐ law of conservation of mass; change coefficients, never formulas.
- 5 types: Combination (A+B→AB) · Decomposition (AB→A+B, needs energy) · Displacement (more reactive kicks out less reactive) · Double displacement (ion exchange, precipitate) · Redox (oxidation = +O/−H, reduction = −O/+H, always together).
- Decomposition energy sources: heat (FeSO₄, CaCO₃, Pb(NO₃)₂), electricity (water → 2 vol H₂ : 1 vol O₂), light (AgCl, AgBr → photography).
- Colour changes to remember: CuSO₄ blue → green (Fe nail) · FeSO₄ green → brown Fe₂O₃ · AgCl white → grey · Mg burns dazzling white.
- Rust = Fe₂O₃·xH₂O (needs air + moisture) · silver tarnish = black Ag₂S · copper coating = green.
- Rancidity = oxidation of fats/oils → prevent wit
Get Started Free | Features | Pricing | Blog