Simply CBSE

Chemical Reactions and Equations — Class 10 Science

Quick revision notes for exam preparation.

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📌 Key Points

  • Signs of a chemical reaction: change in colour, gas evolution, precipitate formation, temperature change, change in state
  • Chemical equations must be balanced (Law of Conservation of Mass) using the hit-and-trial method
  • State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water)
  • Combination: A + B → AB. Example: CaO + H₂O → Ca(OH)₂ (exothermic)
  • Decomposition: AB → A + B. Three types: thermal (heat), electrolytic (electricity), photolytic (light)
  • FeSO₄ heating: green crystals → reddish-brown Fe₂O₃ + SO₂ + SO₃ (smell of burning sulphur)
  • Pb(NO₃)₂ heating: emits brown fumes of NO₂. CaCO₃ heating: gives CaO + CO₂
  • Electrolysis of water: 2H₂O → 2H₂ + O₂ (H₂:O₂ = 2:1 by volume)
  • AgCl and AgBr decompose in sunlight (photolytic) — used in black and white photography
  • Displacement: A + BC → AC + B. More reactive metal displaces less reactive (Fe + CuSO₄ → FeSO₄ + Cu)
  • Double displacement: AB + CD → AD + CB. Example: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
  • Oxidation = gain of oxygen or loss of hydrogen. Reduction = loss of oxygen or gain of hydrogen
  • Oxidising agent gets reduced itself. Reducing agent gets oxidised itself
  • Corrosion: rusting of iron needs moisture + oxygen. Prevention: galvanisation, painting, oiling, alloying
  • Rancidity: oxidation of fats/oils. Prevention: antioxidants, nitrogen flushing, airtight containers, refrigeration

📘 Important Definitions

Chemical Reaction
A process in which one or more substances (reactants) are transformed into new substances (products) with different chemical properties.
Balanced Chemical Equation
An equation with equal number of atoms of each element on both sides, following the Law of Conservation of Mass.
Combination Reaction
A reaction in which two or more substances combine to form a single product. Example: 2Mg + O₂ → 2MgO.
Decomposition Reaction
A reaction in which a single compound breaks down into two or more simpler substances. Example: CaCO₃ → CaO + CO₂.
Displacement Reaction
A reaction in which a more reactive element displaces a less reactive element from its compound. Example: Fe + CuSO₄ → FeSO₄ + Cu.
Double Displacement Reaction
A reaction in which two compounds exchange ions to form two new compounds. Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.
Redox Reaction
A reaction in which both oxidation (gain of oxygen / loss of hydrogen) and reduction (loss of oxygen / gain of hydrogen) occur simultaneously.
Corrosion
The process in which metals are slowly eaten away by the action of air, moisture, or chemicals on their surface. Example: rusting of iron.
Rancidity
The condition in which fats and oils in food get oxidised, producing a bad taste and smell. Prevented by antioxidants, nitrogen flushing.
Precipitation Reaction
A double displacement reaction in which an insoluble product (precipitate) is formed. Example: BaSO₄ precipitate from Na₂SO₄ + BaCl₂.

🔢 Formulas & Laws

Magnesium burning

2Mg(s) + O₂(g) → 2MgO(s)

Combination reaction — dazzling white flame, white ash

Slaking of lime

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

Exothermic combination — beaker becomes very hot

Iron + Copper sulphate

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Displacement — blue to green, brown copper deposits

Electrolysis of water

2H₂O(l) → 2H₂(g) + O₂(g)

Electrolytic decomposition — H₂:O₂ = 2:1

Redox: CuO + H₂

CuO(s) + H₂(g) → Cu(s) + H₂O(g)

CuO reduced (oxidising agent), H₂ oxidised (reducing agent)

⚠️ Common Mistakes

✗ Wrong: Changing subscripts instead of coefficients to balance an equation

✓ Correct: Never change the formula of a compound (subscripts). Only add coefficients before the formula to balance.

✗ Wrong: Writing Ca(OH) instead of Ca(OH)₂

✓ Correct: Calcium has valency 2, hydroxide (OH) has valency 1. So calcium hydroxide is Ca(OH)₂.

✗ Wrong: Confusing oxidising agent and reducing agent

✓ Correct: Oxidising agent itself gets REDUCED (gives oxygen). Reducing agent itself gets OXIDISED (removes oxygen).

✗ Wrong: Thinking iron rusts in dry air or oxygen-free water alone

✓ Correct: Rusting requires BOTH moisture and oxygen. Neither alone can cause rusting.

✗ Wrong: Confusing displacement with double displacement

✓ Correct: Displacement: one element replaces another (A + BC → AC + B). Double displacement: two compounds swap ions (AB + CD → AD + CB).

📝 Exam Focus

These questions are frequently asked in CBSE exams:

Identify and classify types of chemical reactions with equations
3m★★★
Balance chemical equations with state symbols
2m★★★
Observations from NCERT activities (FeSO₄ heating, Fe + CuSO₄, AgCl in sunlight)
3m★★★
Three types of decomposition reactions with examples
3m★★★
Identify oxidation, reduction, oxidising agent, reducing agent in a reaction
3m★★★
Explain corrosion and its prevention methods
3m★★
Explain rancidity and its prevention methods
2m★★
Distinguish exothermic and endothermic reactions with examples
2m★★

Diagram to practice: Practice drawing: electrolysis of water setup, activity of burning magnesium ribbon, iron nail in CuSO₄ solution, heating of FeSO₄ crystals

🎯 Last-Minute Recall

Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!