Is it correct to say: Balancing by changing subscripts of a compound?

NEVER change subscripts (chemical formula). Only place coefficients in front of formulae to balance.

Is it correct to say: Confusing 'oxidation of X' with X gaining oxygen only?

Oxidation = loss of electrons OR gain of oxygen OR loss of hydrogen. All three definitions are equivalent.

Is it correct to say: Writing photochemical decomposition products of AgBr as Ag + Br?

Product is molecular halogen: 2AgBr → 2Ag + Br₂. Same rule for AgCl.

Is it correct to say: Forgetting physical states in an equation?

Always add (s), (l), (g), (aq) — often carries a 1-mark deduction.

Is it correct to say: Confusing corrosion of iron (rusting) with corrosion of copper (green Cu₂(OH)₂CO₃)?

Rusting is specific to iron (red-brown Fe₂O₃·xH₂O). Copper turns green with 'patina', silver turns black (Ag₂S).

Is it correct to say: Calling combination reactions always exothermic?

Most combinations ARE exothermic (CaO + H₂O), but not all. Some, like N₂ + O₂ → 2NO, are endothermic.

Chemical Reactions and Equations

Types of chemical reactions, balancing equations, oxidation and reduction, corrosion and rancidity — NCERT Class 10 Chemistry Ch 1

📐 8 formulas✏️ 3 examples🎯 6 practice⚖️ 5 marks🏫 CBSE📚 Class 10✓ 2025–26 syllabus

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Board Exam Tips

→Balancing an equation is worth 2-3 marks — count atoms of each element on both sides before you conclude.
→State the type of reaction whenever asked: combination, decomposition, displacement, double-displacement, redox.
→Physical states — (s), (l), (g), (aq) — MUST be written next to every formula. 1-mark deduction otherwise.
→Colour and state changes are favourite 1-mark VSAQs (e.g. white AgCl turns grey in sunlight).
→Corrosion prevention methods — a 3-mark question in almost every set.

📐 Formulas(8)

General Combination Reaction★ Board fav

A + B ⟶ A B

General Decomposition Reaction★ Board fav

A B ⟶ A + B

Thermal Decomposition of Calcium Carbonate

C a C O_{3} (s) Δ C a O (s) + C O_{2} (g)

Displacement Reaction

A + B C ⟶ A C + B

Double-Displacement (Precipitation)★ Board fav

A B + C D ⟶ A D + C B

Photochemical Decomposition of AgCl

2 A g C l (s) h ν 2 A g (s) + C l_{2} (g)

Oxidation and Reduction (Redox)

oxidation loss of e^{-} ∣∣ reduction gain of e^{-}

Formation of Rust

4 F e (s) + 3 O_{2} (g) + 2 x H_{2} O ⟶ 2 F e_{2} O_{3} \cdot x H_{2} O (s)

✏️ Solved Examples

1Solved Exampleeasy4 steps

Balance the equation: Fe + H₂O → Fe₃O₄ + H₂

Count atoms; Fe on right = 3, so multiply Fe on left by 3

3 F e + H_{2} O ⟶ F e_{3} O_{4} + H_{2}

2Solved Exampleboard4 steps

When zinc granules are added to copper sulphate solution, the blue colour fades and a reddish-brown deposit forms. (a) Write the balanced equation. (b) Identify the type of reaction. (c) Which species is oxidised and which reduced?

Write the balanced equation with states

Z n (s) + C u S O_{4} (a q) ⟶ Z n S O_{4} (a q) + C u (s)

3Solved ExampleHOTS4 steps

2.9 g of moist ferrous sulphate crystals on heating give 1.6 g of a residue. Identify the reaction, its type and write the balanced equation.

FeSO₄·7H₂O on heating loses water then decomposes

F e S O_{4} \cdot 7 H_{2} O Δ F e S O_{4} + 7 H_{2} O

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Balancing by changing subscripts of a compound
✓NEVER change subscripts (chemical formula). Only place coefficients in front of formulae to balance.
2
Confusing 'oxidation of X' with X gaining oxygen only
✓Oxidation = loss of electrons OR gain of oxygen OR loss of hydrogen. All three definitions are equivalent.
3
Writing photochemical decomposition products of AgBr as Ag + Br
✓Product is molecular halogen: 2AgBr → 2Ag + Br₂. Same rule for AgCl.
4
Forgetting physical states in an equation
✓Always add (s), (l), (g), (aq) — often carries a 1-mark deduction.
5
Confusing corrosion of iron (rusting) with corrosion of copper (green Cu₂(OH)₂CO₃)
✓Rusting is specific to iron (red-brown Fe₂O₃·xH₂O). Copper turns green with 'patina', silver turns black (Ag₂S).
6
Calling combination reactions always exothermic
✓Most combinations ARE exothermic (CaO + H₂O), but not all. Some, like N₂ + O₂ → 2NO, are endothermic.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
Balance: MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O
Q2
Give one example each of a combination, decomposition and displacement reaction.
Q3
Why does the colour of copper sulphate solution change when an iron nail is dipped into it?
Q4
Explain the term 'rancidity' and state one method to prevent it.
Q5
State three ways to prevent iron from rusting.
Q6
Identify the type of reaction: 2Pb(NO₃)₂ →(Δ) 2PbO + 4NO₂ + O₂

📝 Notes

Chemical Reactions and Equations

Chemistry begins with the language of balanced chemical equations — the shorthand for what atoms do when substances interact.

Rules to write a balanced equation

Write the correct chemical formulae of reactants and products.
Count atoms of each element on both sides.
Balance by placing coefficients — NEVER change subscripts.
Add physical states (s), (l), (g), (aq).
Add conditions (Δ, hν, catalyst) over the arrow.

Five main types of reactions

| Type | General form | Example | |---|---|---| | Combination | A + B → AB | CaO + H₂O → Ca(OH)₂ | | Decomposition | AB → A + B | 2H₂O → 2H₂ + O₂ (electrolysis) | | Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu | | Double displacement | AB + CD → AD + CB | Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl | | Redox | Oxidation + Reduction together | CuO + H₂ → Cu + H₂O |

Exothermic vs endothermic

Exothermic: heat released (respiration, burning of fuels, combination of CaO + H₂O).
Endothermic: heat absorbed (thermal decomposition, photosynthesis, dissolution of NH₄NO₃).

Corrosion and rancidity — everyday redox

Corrosion: slow oxidation of a metal surface — rusting of iron, tarnishing of silver (Ag₂S), green coat on copper.
Rancidity: oxidation of oils/fats causing bad taste and smell. Prevented by antioxidants (BHA, BHT), airtight packaging, or flushing with N₂.

Quick sanity checks

Number of atoms of every element must be equal on both sides of a balanced equation.
Sum of charges must also balance (for ionic equations).
Coefficients should be the smallest whole-number ratio.