⚗️ CHEMICAL REACTIONS AND EQUATIONS
1. Chemical Reaction and Chemical Equation
Chemical reaction – a process in which new substance(s) are formed.
Example: hydrogen gas reacts with oxygen gas to form water under suitable conditions.
Example: hydrogen gas reacts with oxygen gas to form water under suitable conditions.
Chemical equation – a simple way of showing a chemical reaction using symbols and formulae.
H₂(g) + O₂(g) → H₂O(l)
Reactants (left) → Product (right)
Reactants (left) → Product (right)
State symbols: (s) solid • (l) liquid • (g) gas ↑ • (aq) dissolved in water (soluble) • (ppt ↓) insoluble solid formed (precipitate)
2. How to Write a Chemical Formula
Write the symbol of each ion with its charge (valency), then cross the charges to get the formula.
| Positive ion | Symbol |
|---|---|
| Sodium | Na⁺ |
| Potassium | K⁺ |
| Silver | Ag⁺ |
| Copper (I) | Cu⁺ |
| Magnesium | Mg²⁺ |
| Calcium | Ca²⁺ |
| Zinc | Zn²⁺ |
| Iron (II) – ferrous | Fe²⁺ |
| Copper (II) | Cu²⁺ |
| Lead (II) | Pb²⁺ |
| Barium | Ba²⁺ |
| Aluminium | Al³⁺ |
| Iron (III) – ferric | Fe³⁺ |
| Negative ion | Symbol |
|---|---|
| Hydride | H⁻ |
| Chloride | Cl⁻ |
| Bromide | Br⁻ |
| Iodide | I⁻ |
| Oxide | O²⁻ |
| Sulphide | S²⁻ |
| Nitride | N³⁻ |
| Hydroxide | OH⁻ |
| Nitrate | NO₃⁻ |
| Hydrogen carbonate | HCO₃⁻ |
| Carbonate | CO₃²⁻ |
| Sulphite | SO₃²⁻ |
| Sulphate | SO₄²⁻ |
| Phosphate | PO₄³⁻ |
| Ammonium (positive) | NH₄⁺ |
Examples:
Sodium chloride: Na⁺ + Cl⁻ → NaCl
Aluminium chloride: Al³⁺ + Cl⁻ → AlCl₃
Ferrous sulphate: Fe²⁺ + SO₄²⁻ → FeSO₄
Lead nitrate: Pb²⁺ + NO₃⁻ → Pb(NO₃)₂
Sodium chloride: Na⁺ + Cl⁻ → NaCl
Aluminium chloride: Al³⁺ + Cl⁻ → AlCl₃
Ferrous sulphate: Fe²⁺ + SO₄²⁻ → FeSO₄
Lead nitrate: Pb²⁺ + NO₃⁻ → Pb(NO₃)₂
3. Balanced Chemical Equation
In a balanced equation the number of atoms of each element is the same on the left (LHS) and right (RHS).
A₂(g) + B₂(g) → 2AB(g)
4 g + 6 g = 10 g
4 g + 6 g = 10 g
Why balance? Total mass of reactants must equal total mass of products. This is the Law of Conservation of Mass: mass can neither be created nor destroyed in a chemical reaction (under ordinary conditions).
Steps to balance (hit and trial)
- Balance metals first (K, Na, Ag, Cu, Mg, Zn, Ca, Fe, Al, Mn, Pb, Ba)
- Then non-metals (Cl, Br, S, N, C, P)
- Then oxygen (O)
- Last hydrogen (H)
- Repeat until all atoms balance. Never change the formula – only put numbers (coefficients) in front.
Practice (balanced)
3Fe + 4H₂O → Fe₃O₄ + 4H₂
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
2Cu₂O + Cu₂S → 6Cu + SO₂
3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O
2Cu₂O + Cu₂S → 6Cu + SO₂
3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
CBSE 2025: p Al + q H₂O → r Al₂O₃ + s H₂. Values of p, q, r, s?
2Al + 3H₂O → Al₂O₃ + 3H₂ ⇒ p=2, q=3, r=1, s=3. (Check: 2 Al on both sides; O = 3; H = 6.)
2Al + 3H₂O → Al₂O₃ + 3H₂ ⇒ p=2, q=3, r=1, s=3. (Check: 2 Al on both sides; O = 3; H = 6.)
CBSE 2023: MnO₂ + x HCl → MnCl₂ + y H₂O + z Cl₂
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ ⇒ x=4, y=2, z=1
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ ⇒ x=4, y=2, z=1
CBSE 2025: Why balance an equation? Name the law. Balance Zn + H₃PO₄ → Zn₃(PO₄)₂ + H₂
So that total mass of reactants = total mass of products. Law of Conservation of Mass.
3Zn + 2H₃PO₄ → Zn₃(PO₄)₂ + 3H₂
So that total mass of reactants = total mass of products. Law of Conservation of Mass.
3Zn + 2H₃PO₄ → Zn₃(PO₄)₂ + 3H₂
4. Characteristics of a Chemical Reaction – “F E C T S”
F – Formation of precipitate (e.g. BaSO₄, insoluble)
E – Evolution of a gas
C – Colour change
T – Temperature change
S – State change
5. Types of Chemical Reactions
① Combination ② Decomposition ③ Displacement ④ Double displacement (and also oxidation–reduction).
6. Combination Reaction
Two or more reactants combine to form a single product. X + Y + Z → S
(a) Burning of magnesium ribbon (Activity 1.1)
2Mg(s) + O₂(g) → 2MgO(s) + Heat
(from air) (white powder)
(from air) (white powder)
- Mg burns with a dazzling white flame (very bright light).
- White MgO powder is left in the watch glass.
- Heat is released, so temperature rises: Combination + Exothermic reaction.
Why rub Mg with sandpaper? To remove the layer of magnesium oxide already present on the ribbon.
Why keep Mg away from eyes? The reaction is exothermic and gives such bright, dazzling white light that you cannot see for a short time after looking at it.
Why keep Mg away from eyes? The reaction is exothermic and gives such bright, dazzling white light that you cannot see for a short time after looking at it.
CBSE 2025: Which is NOT true about burning Mg in air?
✅ (C) It is an endothermic reaction (it is exothermic).
✅ (C) It is an endothermic reaction (it is exothermic).
(b) Quicklime reacts with water (Activity 1.4)
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
Quicklime Slaked lime
Quicklime Slaked lime
- Quicklime = CaO; Slaked lime = Ca(OH)₂.
- Temperature rises (exothermic); quicklime reacts vigorously with water, so add water slowly.
- Combination + exothermic reaction. Quicklime is used in making cement.
CBSE 2023: Balanced equation of quicklime and water?
✅ (C) CaO + H₂O → Ca(OH)₂ + Heat
✅ (C) CaO + H₂O → Ca(OH)₂ + Heat
(c) Whitewashing walls
- Ca(OH)₂ solution is used to whitewash walls.
- It reacts slowly with CO₂ from air. In 2–3 days a layer of CaCO₃ forms, which gives a shiny finish.
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(d) Other examples
Burning of coal: C(s) + O₂(g) → CO₂(g)
Water formation: 2H₂(g) + O₂(g) → 2H₂O(l)
Water formation: 2H₂(g) + O₂(g) → 2H₂O(l)
CBSE 2023: A process involving a combination reaction: ✅ (B) Burning of coal
7. Decomposition Reaction
Opposite of combination: one reactant breaks into two or more products. AB → A + B
CaCO₃(s) —Heat→ CaO(s) + CO₂(g)
Limestone Quicklime
Limestone Quicklime
CBSE 2024: CaCO₃ —Δ→ X + CO₂. Find X.
✅ (A) Quicklime (CaO)
✅ (A) Quicklime (CaO)
Heat
Thermal decomposition (thermolysis)
Thermal decomposition (thermolysis)
Electricity
Electrolytic decomposition (electrolysis)
Electrolytic decomposition (electrolysis)
Light
Photochemical decomposition (photolysis)
Photochemical decomposition (photolysis)
(a) Thermal decomposition of ferrous sulphate
FeSO₄·7H₂O —Heat→ FeSO₄ + 7H₂O
(green crystals) (white, anhydrous)
2FeSO₄(s) —Heat→ Fe₂O₃(s) + SO₂(g) + SO₃(g)
(white) (reddish-brown)
(green crystals) (white, anhydrous)
2FeSO₄(s) —Heat→ Fe₂O₃(s) + SO₂(g) + SO₃(g)
(white) (reddish-brown)
- Green crystals first lose water and turn white; on more heating reddish-brown Fe₂O₃ forms.
- Smell of burning sulphur (suffocating, pungent smell) – this is SO₂.
- SO₂ and SO₃ are air pollutants and acidic.
- Thermal decomposition = Endothermic (heat absorbed). Do not point the boiling tube mouth at anyone; waft the gas gently to your nose.
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CBSE 2025: Sources of energy for decomposition? Example.
Heat, light and electricity. Heat example: 2FeSO₄(s) —Δ→ Fe₂O₃(s) + SO₂(g) + SO₃(g).
Heat, light and electricity. Heat example: 2FeSO₄(s) —Δ→ Fe₂O₃(s) + SO₂(g) + SO₃(g).
CBSE 2025: What is seen when hydrated ferrous sulphate is heated in a dry boiling tube?
Green crystals → lose water → white powder (FeSO₄·7H₂O → FeSO₄ + 7H₂O); on further heating → reddish-brown solid with smell of burning sulphur (2FeSO₄ → Fe₂O₃ + SO₂ + SO₃).
Green crystals → lose water → white powder (FeSO₄·7H₂O → FeSO₄ + 7H₂O); on further heating → reddish-brown solid with smell of burning sulphur (2FeSO₄ → Fe₂O₃ + SO₂ + SO₃).
(b) Thermal decomposition of lead nitrate
2Pb(NO₃)₂(s) —Heat→ 2PbO(s) + 4NO₂(g) + O₂(g)
(white) (yellow residue) (brown fumes)
(white) (yellow residue) (brown fumes)
- Cracking sound is heard.
- Brown fumes of NO₂ with irritating smell; NO₂ is acidic and turns moist blue litmus red.
- Yellow solid residue = PbO.
- Thermal decomposition + endothermic.
CBSE 2024: Products when lead nitrate is heated: ✅ (D) NO₂, PbO and O₂
(c) Electrolytic decomposition (electrolysis of water)
2H₂O(l) —Electric current→ 2H₂(g) + O₂(g)
- H₂ gas collects at the cathode (–); O₂ gas collects at the anode (+). (Trick: P A O – Positive = Anode = Oxygen)
- Volume ratio H₂ : O₂ = 2 : 1
- Mass ratio H₂ : O₂ = 1 : 8
- Test at cathode: burning candle → pop sound; H₂ is combustible but does not support combustion.
- Test at anode: candle burns more brightly / rekindles a glowing splinter; O₂ supports combustion but is not combustible.
- Pure water is a poor conductor, so a few drops of dilute acid (e.g. H₂SO₄) are added to make it conduct.
CBSE 2023: Why add acid? Gases at cathode/anode? Volume relation?
Pure water is a poor conductor; dilute acid makes it a good conductor. Cathode – hydrogen; anode – oxygen. Volume of gas at anode : cathode = 1 : 2.
Pure water is a poor conductor; dilute acid makes it a good conductor. Cathode – hydrogen; anode – oxygen. Volume of gas at anode : cathode = 1 : 2.
(d) Photochemical decomposition (photolysis)
2AgCl(s) —Sunlight→ 2Ag(s) + Cl₂(g)
(white → grey)
2AgBr(s) —Sunlight→ 2Ag(s) + Br₂(g)
(white → grey)
2AgBr(s) —Sunlight→ 2Ag(s) + Br₂(g)
- AgCl and AgBr are stored in dark (black) coloured bottles to protect them from sunlight.
- Used in black and white photography.
CBSE 2023: Silver chloride in a china dish turns grey in sunlight.
A. AgCl was white. B. Photochemical decomposition: 2AgCl(s) → 2Ag(s) + Cl₂(g) (sunlight). C. Used in black & white photography; silver bromide (AgBr) can also be used.
A. AgCl was white. B. Photochemical decomposition: 2AgCl(s) → 2Ag(s) + Cl₂(g) (sunlight). C. Used in black & white photography; silver bromide (AgBr) can also be used.
Define decomposition; give one equation each for heat, light, electricity:
Decomposition = one substance breaks into two or more simpler substances.
Heat: CaCO₃ → CaO + CO₂ • Light: 2AgCl → 2Ag + Cl₂ • Electricity: 2H₂O → 2H₂ + O₂
Decomposition = one substance breaks into two or more simpler substances.
Heat: CaCO₃ → CaO + CO₂ • Light: 2AgCl → 2Ag + Cl₂ • Electricity: 2H₂O → 2H₂ + O₂
8. Displacement Reaction
A more reactive element replaces a less reactive element from its compound. A + BC → AC + B (A is more reactive than B)
Reactivity series (most → least reactive)
K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au
Memory trick: Kudi Naal Car Mango Alto Zisko Fir Lekar Hum Chale Mathura Saath Ghumne
Memory trick: Kudi Naal Car Mango Alto Zisko Fir Lekar Hum Chale Mathura Saath Ghumne
Zn(s) + CuSO₄(aq) → ZnSO₄ + Cu
(blue) (colourless) (red-brown)
Zn(s) + H₂SO₄(aq) → ZnSO₄ + H₂↑
Fe(s) + CuSO₄(aq) → FeSO₄ + Cu
Cu(s) + PbSO₄(aq) → no reaction (Cu is less reactive than Pb)
(blue) (colourless) (red-brown)
Zn(s) + H₂SO₄(aq) → ZnSO₄ + H₂↑
Fe(s) + CuSO₄(aq) → FeSO₄ + Cu
Cu(s) + PbSO₄(aq) → no reaction (Cu is less reactive than Pb)
Activity 1.9 – Iron nail in copper sulphate solution
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Blue Pale/light green Reddish-brown
Blue Pale/light green Reddish-brown
- Solution colour changes from blue to pale green.
- Reddish-brown copper deposits on the iron nail.
CBSE 2025: Iron nails in aqueous CuSO₄ for 25 minutes – colour changes from:
✅ (B) blue to pale green
✅ (B) blue to pale green
Zinc + dilute sulphuric acid (important)
Zn(s) + H₂SO₄(aq, dil) → ZnSO₄ + H₂↑ + Heat
- H₂ is colourless and odourless, burns with a “pop” sound and puts out a burning candle.
- Temperature rises (exothermic). Dilute acid is used because concentrated acid makes the reaction much more exothermic and dangerous.
CBSE 2022: Gas formed (zinc + dilute acid)? ✅ (D) Hydrogen, which on burning produces a popping sound
9. Double Displacement Reaction
Exchange of ions between two reactants (not atoms). AB + CD → AD + CB
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
colourless colourless white ppt
colourless colourless white ppt
- White insoluble substance (precipitate) of BaSO₄ forms.
- It is a double displacement and precipitation reaction. Ions left in solution: Na⁺, Cl⁻.
Precipitation reaction – when two soluble salt solutions react to give an insoluble (or slightly soluble) salt, that salt is called a precipitate.
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s)↓ + 2KNO₃(aq)
colourless colourless yellow ppt
colourless colourless yellow ppt
- Yellow insoluble precipitate of PbI₂ forms. Double displacement + precipitation.
CBSE 2024: Which reaction is different from the other three?
✅ (B) CaO + H₂O → Ca(OH)₂ – it is a combination reaction; the others are double displacement.
✅ (B) CaO + H₂O → Ca(OH)₂ – it is a combination reaction; the others are double displacement.
CBSE 2025: Observation when sodium sulphate is added to barium chloride; equation; type?
A white insoluble precipitate of BaSO₄ forms.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Type: double displacement (precipitation) reaction.
A white insoluble precipitate of BaSO₄ forms.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Type: double displacement (precipitation) reaction.
10. Heat in Reactions
Exothermic
Heat is released (A + B → C + Heat).- Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (carbohydrate → glucose)
- Burning natural gas: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
- Burning of Mg ribbon: 2Mg + O₂ → 2MgO + Heat
- Decomposition of vegetable matter into compost
- Quicklime with water: CaO + H₂O → Ca(OH)₂ + Heat
Endothermic
Heat is absorbed (AB + Heat → A + B). Decomposition reactions are usually endothermic.- FeSO₄ crystals → FeSO₄ + H₂O
- CaCO₃ → CaO + CO₂
- Pb(NO₃)₂ → PbO + NO₂ + O₂
- AgCl, AgBr → Ag + Cl₂ / Br₂
- H₂O(l) → H₂ + O₂
CBSE 2022: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Heat is a/an: ✅ (C) Exothermic reaction (respiration)
CBSE 2023: Select the endothermic reaction: ✅ (B) Decomposition of calcium carbonate to form quicklime and CO₂ (compost, burning candle and respiration are exothermic).
11. Oxidation and Reduction
Oxidation of A: A gains oxygen or loses hydrogen. A is oxidised.
Reduction of B: B gains hydrogen or loses oxygen. B is reduced.
Redox reaction – a reaction in which oxidation and reduction happen together.
Example: ZnO + C → Zn + CO. ZnO loses O → reduced (ZnO is the oxidising agent); C gains O → oxidised (C is the reducing agent).
Example: ZnO + C → Zn + CO. ZnO loses O → reduced (ZnO is the oxidising agent); C gains O → oxidised (C is the reducing agent).
Trick: The substance that gets oxidised is the reducing agent. The substance that gets reduced is the oxidising agent.
CBSE 2024: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
MnO₂ loses oxygen → reduced; HCl loses hydrogen (to form Cl₂) → oxidised. ✅ (D) MnO₂ is reduced and HCl is oxidised
MnO₂ loses oxygen → reduced; HCl loses hydrogen (to form Cl₂) → oxidised. ✅ (D) MnO₂ is reduced and HCl is oxidised
Oxidation of copper
2Cu(s) + O₂(g) —Heat→ 2CuO(s)
reddish-brown black
reddish-brown black
- Reddish-brown copper turns black (CuO) – gain of oxygen = oxidation.
- If H₂ gas is passed over hot black CuO, it turns back to reddish-brown – loss of oxygen = reduction.
CuO(s) + H₂(g) —Heat→ Cu(s) + H₂O
CBSE 2025: Copper wire coated with black substance on burning. Equation? How to reverse?
2Cu + O₂ → 2CuO (black). Reverse it by passing hydrogen over the hot CuO: CuO + H₂ → Cu + H₂O.
2Cu + O₂ → 2CuO (black). Reverse it by passing hydrogen over the hot CuO: CuO + H₂ → Cu + H₂O.
12. Corrosion
When a metal is attacked by substances around it (moisture, oxygen, acids etc.) it corrodes. This process is called corrosion. Corrosion is an example of oxidation.
- Rusting of iron: iron + oxygen + water → hydrated iron oxide (rust, reddish-brown).
- Tarnishing of copper: reddish copper turns green (copper hydroxide + copper carbonate) in moist air with CO₂.
- Tarnishing of silver: shiny grey silver turns black (silver sulphide) due to H₂S in air.
13. Rancidity
When food containing fat or oil is left exposed to air for a long time, its taste and smell change. This is due to oxidation of fats/oils and is called rancidity.
Prevention: add antioxidants • store in air-tight containers • flush packets (e.g. chips) with nitrogen gas • refrigerate.
14. Chemical Change vs Physical Change
| Chemical change | Physical change |
|---|---|
| New substance(s) formed; a chemical reaction happens | No new substance; no chemical reaction |
| Shape, size, colour etc. may also change | Shape, size, colour etc. may change |
| Examples: burning paper, fermentation of grapes, souring of milk, curd formation, rusting of iron, food digestion, cooking food, respiration, battery use, baking a cake | Examples: boiling water, melting ice, melting wax, crushing a paper cup, crystallisation |
⚡ Quick revision: Balance in the order Metal → Non-metal → O → H • Combination = many → 1 • Decomposition = 1 → many • Displacement: reactive pushes out less reactive • Double displacement: ions swap (ppt) • Exothermic = heat out • Endothermic = heat in • Oxidation = gain O / loss H • Reduction = loss O / gain H • Rust, tarnish and rancidity = oxidation.