Chemistry · Class 9-10 · CBSE
Chemical Reactions, Balancing and Types of Changes
Learn how to balance chemical equations using the hit-and-trial method, explore the 5 major types of chemical reactions (combination, decomposition, displacement, double displacement, redox), and understand corrosion & rancidity.
1Chemical Changes and Law of Conservation of Mass
AChemical Reactionoccurs when old chemical bonds break and new chemical bonds form, creating substances with entirely new chemical properties. According to theLaw of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction; hence total number of atoms of each element must remain equal on both sides.
Observable Signs Indicating a Chemical Reaction
| Observable Sign | Characteristic Description | Classic Laboratory Example |
|---|---|---|
| Change in State | Solid/liquid transforms into gas or precipitate | Burning of wax candle; Electrolysis of water producing gases |
| Change in Colour | Distinct change in chemical absorption spectrum | Rusting of iron (grey to reddish brown); CuSO₄ + Fe turning green |
| Evolution of Gas | Vigorous effervescence or bubbling | Zinc granules reacting with dilute H₂SO₄ releasing H₂ gas |
| Change in Temperature | Exothermic (releases heat) or Endothermic (absorbs heat) | Quicklime (CaO) reacting with water boiling vigorously releasing heat |
Change in State
- Characteristic Description
- Solid/liquid transforms into gas or precipitate
- Classic Laboratory Example
- Burning of wax candle; Electrolysis of water producing gases
Change in Colour
- Characteristic Description
- Distinct change in chemical absorption spectrum
- Classic Laboratory Example
- Rusting of iron (grey to reddish brown); CuSO₄ + Fe turning green
Evolution of Gas
- Characteristic Description
- Vigorous effervescence or bubbling
- Classic Laboratory Example
- Zinc granules reacting with dilute H₂SO₄ releasing H₂ gas
Change in Temperature
- Characteristic Description
- Exothermic (releases heat) or Endothermic (absorbs heat)
- Classic Laboratory Example
- Quicklime (CaO) reacting with water boiling vigorously releasing heat
2Five Primary Types of Chemical Reactions
Five Major Types of Chemical Reactions
| Reaction Type | General Equation | Characteristic Mechanism | Real Chemical Example |
|---|---|---|---|
| Combination Reaction | A + B → AB | Two or more reactants join to form a single product | CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Slaking of lime) |
| Decomposition Reaction | AB → A + B | Single reactant breaks down into simpler products via heat/light/electricity | 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) (Thermal) |
| Displacement Reaction | A + BC → AC + B | More reactive element displaces less reactive element from its salt solution | Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) |
| Double Displacement | AB + CD → AD + CB | Exchange of ions between reactants; often forms an insoluble precipitate | Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ (white ppt) + 2NaCl(aq) |
| Redox Reaction | Oxidation + Reduction | Simultaneous loss of electrons (oxidation) and gain of electrons (reduction) | CuO + H₂ → Cu + H₂O (CuO is reduced, H₂ is oxidized) |
Combination Reaction
- General Equation
- A + B → AB
- Characteristic Mechanism
- Two or more reactants join to form a single product
- Real Chemical Example
- CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Slaking of lime)
Decomposition Reaction
- General Equation
- AB → A + B
- Characteristic Mechanism
- Single reactant breaks down into simpler products via heat/light/electricity
- Real Chemical Example
- 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) (Thermal)
Displacement Reaction
- General Equation
- A + BC → AC + B
- Characteristic Mechanism
- More reactive element displaces less reactive element from its salt solution
- Real Chemical Example
- Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Double Displacement
- General Equation
- AB + CD → AD + CB
- Characteristic Mechanism
- Exchange of ions between reactants; often forms an insoluble precipitate
- Real Chemical Example
- Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ (white ppt) + 2NaCl(aq)
Redox Reaction
- General Equation
- Oxidation + Reduction
- Characteristic Mechanism
- Simultaneous loss of electrons (oxidation) and gain of electrons (reduction)
- Real Chemical Example
- CuO + H₂ → Cu + H₂O (CuO is reduced, H₂ is oxidized)
Decomposition Reaction Variants (Energy Driven)
- 1
1. Thermal Decomposition: Driven by heat energy (e.g. 2Pb(NO₃)₂ → 2PbO + 4NO₂↑ (brown fumes) + O₂↑).
- 2
2. Electrolytic Decomposition: Driven by electrical energy (e.g. 2H₂O(l) → 2H₂(g) + O₂(g) at 2:1 volume ratio).
- 3
3. Photolytic Decomposition: Driven by sunlight energy (e.g. 2AgCl(s) → 2Ag(s) + Cl₂(g) used in black & white photography).
3Redox Reactions, Corrosion and Rancidity
Oxidationis addition of oxygen or loss of hydrogen/electrons.Reductionis gain of hydrogen/electrons or loss of oxygen. Substances that provide oxygen areOxidising Agents; substances that remove oxygen areReducing Agents.
Everyday Oxidation: Corrosion vs Rancidity
| Phenomenon | Target Material | Chemical Mechanism | Prevention Methods |
|---|---|---|---|
| Corrosion | Metals (Iron, Copper, Silver) | Atmospheric moisture, O₂ and CO₂ attack surface (Rust = Fe₂O₃·xH₂O) | Painting, greasing, galvanization (zinc coating), electroplating, alloying |
| Rancidity | Fats and cooking oils | Slow atmospheric oxidation of unsaturated fats producing foul smell and taste | Flushing with Nitrogen gas (e.g. potato chips packets), airtight containers, BHA/BHT antioxidants |
Corrosion
- Target Material
- Metals (Iron, Copper, Silver)
- Chemical Mechanism
- Atmospheric moisture, O₂ and CO₂ attack surface (Rust = Fe₂O₃·xH₂O)
- Prevention Methods
- Painting, greasing, galvanization (zinc coating), electroplating, alloying
Rancidity
- Target Material
- Fats and cooking oils
- Chemical Mechanism
- Slow atmospheric oxidation of unsaturated fats producing foul smell and taste
- Prevention Methods
- Flushing with Nitrogen gas (e.g. potato chips packets), airtight containers, BHA/BHT antioxidants
4Exam Corner: Tips, Mistakes & Board Questions
Critical Board Exam Balancing Rules
- Always balance atoms of polyatomic ions or metals first, then non-metals, and leave Hydrogen and Oxygen for the end.
- In electrolysis of water, the volume of hydrogen gas collected at the cathode is double the volume of oxygen collected at the anode (H₂O has 2:1 H:O atom ratio).
- Respiration is an exothermic reaction because glucose oxidation in cells releases 38 ATP of energy.
Exam question · CBSE Class 10 Board
- A.No gas is released
- B.Brown fumes of nitrogen dioxide (NO₂) are evolved
- C.Dense white fumes of ammonia are evolved
- D.Green chlorine gas is released
- Thermal decomposition of lead nitrate: 2Pb(NO₃)₂(s) → 2PbO(s) [yellow residue] + 4NO₂(g) [pungent brown fumes] + O₂(g).
- The brown fumes are characteristic of nitrogen dioxide gas.
- Correct answer is option B.
Answer: B. Brown fumes of nitrogen dioxide (NO₂) are evolved
Remember
- Chemical equations must be balanced to satisfy theLaw of Conservation of Mass.
- Precipitation reactionsare double displacement reactions producing an insoluble solid.
- Redox reactions feature simultaneousoxidation(electron loss) andreduction(electron gain).
- Food packages are flushed withNitrogengas to prevent rancidity by blocking oxidation.
Check yourself
Question 1
In the reaction CuO + H₂ → Cu + H₂O, which substance acts as the reducing agent?