Chapter 1: Chemical Reactions and Equations

Science · CBSE Class 10 · NCERT Solutions

📖 Chapter 1 — Chemical Reactions and Equations (CBSE Class 10 Science): Detailed notes — definitions, balancing, all reaction types with equations, oxidation effects. No need to open the book.

Chapter 1 — Chemical Reactions and Equations

🧪 1. Chemical Reaction & its Signs

A chemical reaction is a process in which one or more substances (reactants) change into new substances (products) with different properties.

Signs of a chemical reaction: ① change in state ② change in colour ③ evolution of a gas ④ change in temperature (heat given out/absorbed) ⑤ formation of a precipitate.

⚖️ 2. Chemical Equation & Balancing

A chemical equation represents a reaction using symbols and formulae. Reactants → Products. Example: Mg + O₂ → MgO (skeletal); balanced: 2Mg + O₂ → 2MgO.

Balanced equation: one having an equal number of atoms of each element on both sides. Equations must be balanced to obey the Law of Conservation of Mass (mass is neither created nor destroyed in a reaction).

Symbols: (s)=solid, (l)=liquid, (g)=gas, (aq)=aqueous; ↑=gas evolved, ↓=precipitate; Δ=heat.

🔀 3. Types of Chemical Reactions

Type	Description & Example
Combination	Two+ substances combine to form one. CaO + H₂O → Ca(OH)₂ (exothermic)
Decomposition	One substance breaks into two+. By heat (thermal): CaCO₃ →Δ→ CaO + CO₂ ; light: 2AgCl →light→ 2Ag + Cl₂ ; electricity: 2H₂O →elec→ 2H₂ + O₂
Displacement	More reactive element displaces less reactive. Fe + CuSO₄ → FeSO₄ + Cu
Double Displacement	Exchange of ions. Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl (precipitation)

⚡ 4. Oxidation & Reduction (Redox)

Oxidation = gain of oxygen / loss of hydrogen. Reduction = loss of oxygen / gain of hydrogen. A reaction where both occur together is a redox reaction. Example: CuO + H₂ → Cu + H₂O (CuO reduced, H₂ oxidised).

🔩 5. Effects of Oxidation in Daily Life

Corrosion: slow eating up of metals by air/moisture (iron → rust Fe₂O₃·xH₂O; silver → black; copper → green). Prevented by painting, oiling, galvanisation.
Rancidity: oxidation of fats/oils in food → bad smell & taste. Prevented by antioxidants, air-tight packing, flushing with nitrogen, refrigeration.

🧠 Remember: Balance = Law of Conservation of Mass. OIL RIG: Oxidation Is Loss, Reduction Is Gain (of oxygen). Decomposition = opposite of combination. Nitrogen flushing prevents rancidity.

⭐ Exam Tips: ① Balance equations (both sides equal atoms). ② 4 reaction types + examples. ③ Thermal/photolytic/electrolytic decomposition. ④ Redox (who is oxidised/reduced). ⑤ Corrosion & rancidity + prevention.

📘 Textbook Question Answers

Q12PbO + C → 2Pb + CO₂: which statements are incorrect?

(i) (a) and (b). Lead is getting reduced (correct), carbon dioxide is getting oxidised (incorrect — CO₂ is a product of oxidation of C), carbon is oxidised (correct), lead oxide is reduced (correct). So incorrect ones are (a) and (b)? Actually correct answer: (i) (a) and (b) are the incorrect statements.

Q2Fe₂O₃ + 2Al → Al₂O₃ + 2Fe is an example of —

(d) displacement reaction (Al displaces Fe). It is also called the thermite reaction.

Q3Dilute HCl + iron filings →

(a) Hydrogen gas and iron chloride are produced. Fe + 2HCl → FeCl₂ + H₂↑

Q4What is a balanced chemical equation? Why balance?

An equation with equal numbers of atoms of each element on both sides. Equations must be balanced to satisfy the Law of Conservation of Mass — mass cannot be created or destroyed in a chemical reaction.

Q5Translate into balanced equations.

(a) 3H₂ + N₂ → 2NH₃
(b) 2H₂S + 3O₂ → 2H₂O + 2SO₂
(c) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄↓
(d) 2K + 2H₂O → 2KOH + H₂↑

Q6Balance the equations.

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl↓ + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄↓ + 2HCl

Q7Write balanced equations.

(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄↓ + 2KCl

Q8Balanced equation + type of reaction.

(a) 2KBr + BaI₂ → 2KI + BaBr₂ — double displacement
(b) ZnCO₃ → ZnO + CO₂ — decomposition
(c) H₂ + Cl₂ → 2HCl — combination
(d) Mg + 2HCl → MgCl₂ + H₂ — displacement

Q9-10Exothermic/endothermic reactions; why is respiration exothermic?

Exothermic: heat released (e.g., burning of coal, respiration). Endothermic: heat absorbed (e.g., decomposition of CaCO₃). Respiration is exothermic because glucose combines with oxygen to release energy: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.

Q11-13Decomposition vs combination; displacement vs double displacement.

Decomposition breaks one substance into many (opposite of combination which joins many into one). E.g., 2H₂O → 2H₂ + O₂ (decomp.) vs 2H₂ + O₂ → 2H₂O (comb.). Displacement: one element replaces another (Fe + CuSO₄ → FeSO₄ + Cu). Double displacement: two compounds exchange ions (Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl).

Q14-16Silver refining; precipitation reaction; oxidation/reduction examples.

Q14: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag. Q15: Precipitation = reaction forming an insoluble salt (precipitate), e.g., AgNO₃ + NaCl → AgCl↓ + NaNO₃. Q16: Oxidation (gain of O): 2Cu + O₂ → 2CuO ; Reduction (loss of O): CuO + H₂ → Cu + H₂O.

Q17-20Element X; paint on iron; nitrogen flushing; corrosion & rancidity.

Q17: X = copper (Cu); black compound = copper oxide (CuO). Q18: Paint prevents contact with air/moisture, so it prevents rusting (corrosion). Q19: Nitrogen is unreactive; it prevents oxidation of oils/fats, thus preventing rancidity. Q20: Corrosion — slow eating up of metal by air/moisture (rusting of iron). Rancidity — oxidation of oils/fats in food giving bad smell/taste.

Extra practice (4× the textbook) — MCQ, short answer, long answer & balancing. Great for exams.

🎯 Extra MCQ

1Which is NOT a sign of a chemical reaction?

Change of shape without new substance (physical change). (Signs: colour/state/gas/temperature/precipitate.)

2The reaction 2Mg + O₂ → 2MgO is —

a combination (and oxidation) reaction; also exothermic.

3Digestion of food is a — reaction.

Decomposition.

4Which gas is evolved when zinc reacts with dilute H₂SO₄?

Hydrogen (Zn + H₂SO₄ → ZnSO₄ + H₂↑).

📝 Short Answer

5Why is the amount of gas collected at one electrode in electrolysis of water double the other?

Water has twice as much hydrogen as oxygen (H₂O), so the volume of H₂ collected (at cathode) is double that of O₂ (at anode).

6Why does the colour of copper sulphate solution change when an iron nail is dipped?

Iron (more reactive) displaces copper: Fe + CuSO₄ → FeSO₄ + Cu. The blue colour of CuSO₄ fades to light green (FeSO₄).

7Define a redox reaction with an example.

A reaction in which oxidation and reduction occur simultaneously. E.g., ZnO + C → Zn + CO (ZnO reduced, C oxidised).

8State two ways to prevent rancidity.

Adding antioxidants; packing in air-tight containers / flushing with nitrogen; refrigeration.

9What is a precipitate? Give an example reaction.

An insoluble solid formed in a reaction. Pb(NO₃)₂ + 2KI → PbI₂↓ (yellow) + 2KNO₃.

⚖️ Balance these

10___ Fe + ___ H₂O → Fe₃O₄ + ___ H₂

3Fe + 4H₂O → Fe₃O₄ + 4H₂.

11___ Al + ___ O₂ → ___ Al₂O₃

4Al + 3O₂ → 2Al₂O₃.

12Pb(NO₃)₂ →Δ→ PbO + NO₂ + O₂ (balance)

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.

📖 Long Answer

13Explain the four types of chemical reactions with one example each.

① Combination: 2Mg+O₂→2MgO. ② Decomposition: CaCO₃→CaO+CO₂. ③ Displacement: Zn+CuSO₄→ZnSO₄+Cu. ④ Double displacement: AgNO₃+NaCl→AgCl↓+NaNO₃.

14What is corrosion? How can it be prevented?

Corrosion is the slow eating up of a metal surface by air, moisture or chemicals (e.g., rusting of iron). Prevention: painting, oiling/greasing, galvanisation (zinc coat), electroplating, alloying.

15Distinguish between exothermic and endothermic reactions with examples.

Exothermic reactions release heat (respiration, burning of fuels, CaO+H₂O). Endothermic reactions absorb heat (decomposition of CaCO₃, photosynthesis).