Chemical Reactions and Equations Notes Class 10 Science
Chemical Reactions and Equations
Why this chapter matters
Iron rusts. Milk curdles. Bread rises. All are reactions.
You will write, balance, and classify reactions. Then predict corrosion and rancidity.
Chapter Learning Map
Follow the spine top to bottom. Three groups, all equally important.
1 · Chemical Equation & Balancing
Chemical equation uses symbols and formulae. Balanced equation has equal atoms on both sides. Mass is conserved.
Why balance: Law of conservation of mass — atoms cannot be created or destroyed.
Unbalanced: Mg + O2 → MgO.
Balanced: 2Mg + O2 → 2MgO. Count — Mg 2 both sides, O 2 both sides.
Balance by hit-and-trial. Start with most atoms, end with H and O. Never change formula.
Do not add extra numbers inside formula. H2O not H4O2 — coefficient changes, not subscript.
2 · Types of Reactions
Type depends on atoms joining, breaking, or swapping. Five main types in this chapter.
| Type | General form | Board example |
|---|---|---|
| Combination | A + B → AB | 2Mg + O2 → 2MgO |
| Decomposition | AB → A + B (heat/light/electric) | 2Pb(NO3)2 → 2PbO + 4NO2 + O2 |
| Displacement | A + BC → AC + B | Fe + CuSO4 → FeSO4 + Cu |
| Double displacement | AB + CD → AD + CB | Na2SO4 + BaCl2 → BaSO4 + 2NaCl |
| Precipitation | soluble → insoluble solid | Double disp. producing BaSO4 ↓ white precipitate |
C D D D P → “Cat Did Double Precipitate”.
Thermal: CaCO3 → CaO + CO2. Photo: 2AgCl → 2Ag + Cl2 (grey in light).
3 · Redox, Corrosion & Rancidity
Redox = oxidation and reduction together. Oxidation gain of O or loss of H. Reduction loss of O or gain of H.
CuO + H2 → Cu + H2O. CuO reduced to Cu, H2 oxidised to water. One agent reduces, other oxidises.
Chips bags filled with nitrogen. It prevents rancidity — avoids oil oxidation. No nitrogen = stale smell.
Do not confuse displacement redox with simple displacement. Check oxygen/hydrogen shift.
| Problem | What happens | Prevention |
|---|---|---|
| Corrosion | Fe + air + moisture → Fe2O3·xH2O rust (reddish) | Paint, oil, galvanise, alloy |
| Rancidity | Fats oxidise → foul smell/taste | Antioxidants, air-tight, nitrogen flush, fridge |
Apparatus Board — Whitelisted Only
Simple glassware. No complex drawings — circles and rects only. Label below, not on line.
Label apparatus below. Show precipitate by ↓, gas by ↑. Keep symbols upright.
5 · NCERT Activities & Balancing — 5-Step Method
| Activity | Observation | Inference |
|---|---|---|
| Mg + O₂ burn | White ash, bright flame | Combination → MgO |
| FeSO₄ heating | Brown solid, pungent gas | Thermal decomposition → Fe₂O₃ + SO₂ + SO₃ |
| Fe + CuSO₄ | Blue → green, brown coat | Displacement → FeSO₄ + Cu |
1) Count atoms both sides 2) Balance metals first 3) Balance non-metals next 4) Balance H → O last 5) Verify. Example: Fe + H₂O → Fe₃O₄ + H₂ deals with Fe first, then O, then H → 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Glucose oxidises, heat released. Hence exothermic — like burning fuel.
6 · Topper Secrets & Extra Important Examples
Mark gas ↑ and precipitate ↓ in equation — board gives 0.5M for correct state symbols.
Photolysis vs Electrolysis: AgCl needs light (photo), H₂O needs electricity (electro) — condition decides type.
Pb(NO₃)₂ + 2KI → PbI₂↓ yellow + 2KNO₃. Yellow precipitate confirms reaction — classic 2M question.
Rust: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (reddish). Prevention: painting, oiling, galvanising (Zn coating), alloying. Galvanising sacrifices Zn, protects Fe.
Do not confuse Ag₂S (silver tarnish black) with Fe₂O₃ (iron rust brown). Metal decides colour.
⚡ Formula Vault
Revise these before every test. Each carries marks.
Important Question Bank
Tap “Show answer”.
A · Multiple Choice (1 mark each)
(a) BaO (b) BaSO4 (c) NaCl (d) BaCl
Show answer
Answer: (b) BaSO4. Insoluble white solid settles.
(a) combination (b) thermal decomposition (c) displacement (d) redox only
Show answer
Answer: (b) thermal decomposition. Heat breaks lead nitrate.
(a) less reactive (b) more reactive (c) inert (d) heavier
Show answer
Answer: (b) more reactive. Reactivity series decides.
(a) adding oxygen (b) nitrogen flush (c) heating (d) adding water
Show answer
Answer: (b) nitrogen flush. Removes oxygen, slows oxidation.
(a) oxidised (b) reduced (c) both (d) none
Show answer
Answer: (b) reduced. Loses oxygen → reduced; H2 oxidised.
B · Assertion–Reason (1 mark each)
(a) Both true, R explains A. (b) Both true, R doesn’t explain A. (c) A true, R false. (d) A false, R true.
Assertion: White wash with CaO gives shine after 2–3 days.
Reason: Ca(OH)2 + CO2 → CaCO3 forms shiny layer.
Show answer
Answer: (a). CO2forms calcite, explains shine.
Assertion: 2AgCl → 2Ag + Cl2 needs sunlight.
Reason: Photodecomposition needs light.
Show answer
Answer: (a). Light provides energy.
Assertion: Corrosion is reduction.
Reason: Fe gains oxygen.
Show answer
Answer: (d). A false (oxidation), R false (gain O is oxidation, not reduction).
C · Case-Based (4 marks)
(i) Type? (1) (ii) Balanced eq. (1) (iii) Observation reason? (1) (iv) Name prevention for iron rusting similar process? (1)
Show answer
(i) Displacement — Fe more reactive displaces Cu.
(ii) Fe + CuSO4 → FeSO4 + Cu (blue → green).
(iii) FeSO4 green forms; Cu deposits.
(iv) Painting/galvanising prevents corrosion (like coating).
D · Short & Long Answer
Show answer
Given: Fe + H2O → Fe3O4 + H2
Step 1: Balance Fe, then O, then H — 3Fe + 4H2O → Fe3O4 + 4H2.
Final: Fe 3, O 4, H 8 both sides ✓
Show answer
Thermal: CaCO3 → CaO + CO2 (heat).
Photo: 2AgCl → 2Ag + Cl2 (sunlight).
Electrolytic: 2H2O → 2H2 + O2 (electric).
Rancidity: Antioxidants, air-tight, nitrogen, refrigeration.
Final: All balanced, conditions noted.
E · Previous-Year Style
Show answer
Glucose oxidises: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy.
Heat released → exothermic.
Common Mistakes Table
| Mistake students make | Correct idea |
|---|---|
| Change subscript to balance | Change coefficients only; subscripts define substance. |
| Call BaSO4 formation combination | It is double displacement + precipitation (white solid). |
| Think corrosion = rancidity | Corrosion rusts metals; rancidity oxidises fats — different. |
Active Recall & Revision
Cover answers. Say aloud. Then check.
Q: Signs a reaction occurred?
Change in state, colour, gas evolved, temp change, precipitate.
Q: Balance Fe + H2O equation?
3Fe+4H2O→Fe3O4+4H2.
Revise after 2 days, then 3 days, then 5 days, then 7 days. This locks memory.
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