CBSE Class 10 Science Chapter 1: Chemical Reactions and Equations — Revision Notes
What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances (called reactants) are converted into one or more different substances (called products). During a chemical reaction, the atoms of the reactants rearrange to form new substances with different properties.
Signs of a Chemical Reaction
How do we know a chemical reaction has occurred? Look for these signs:
- Change in colour — e.g., iron turns reddish-brown when it rusts
- Evolution of gas — e.g., bubbles form when acid reacts with a metal
- Change in temperature — reaction mixture becomes hot or cold
- Formation of precipitate — an insoluble solid forms in a solution
- Change in smell — food spoiling produces a bad smell
- Evolution of light or sound — e.g., burning magnesium produces bright light
Chemical Equations
A chemical equation is a symbolic representation of a chemical reaction. It shows the reactants on the left side and the products on the right side, separated by an arrow (→).
Word Equation
The simplest way to represent a reaction is using a word equation:
Magnesium + Oxygen → Magnesium oxide
Iron + Water + Oxygen → Iron oxide (Rust)
Skeletal Chemical Equation
A skeletal equation uses chemical symbols and formulas but is not balanced. For example, the burning of magnesium:
This equation is not balanced because the number of oxygen atoms is not equal on both sides.
Balanced Chemical Equation
A balanced chemical equation has equal numbers of atoms of each element on both sides. This follows the Law of Conservation of Mass.
How to Balance a Chemical Equation
Follow these steps to balance any chemical equation:
- Write the skeletal equation with correct formulas
- Count atoms of each element on both sides
- Add coefficients (numbers before formulas) to balance atoms
- Never change subscripts — only add coefficients
- Verify the final equation is balanced
Step 1: Count atoms — Fe: 1 vs 3, H: 2 vs 2, O: 1 vs 4
Step 2: Balance Fe — 3Fe + H₂O → Fe₃O₄ + H₂
Step 3: Balance O — 3Fe + 4H₂O → Fe₃O₄ + H₂
Step 4: Balance H — 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Final: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ ✓
State Symbols in Chemical Equations
State symbols are written in brackets after each substance to show its physical state:
| Symbol | State | Example |
|---|---|---|
| (s) | Solid | Zn(s) |
| (l) | Liquid | H₂O(l) |
| (g) | Gas | CO₂(g) |
| (aq) | Aqueous (dissolved in water) | NaCl(aq) |
The ↑ symbol shows gas is evolved. The ↓ symbol shows a precipitate is formed.
Types of Chemical Reactions
1. Combination Reaction
Two or more substances combine to form a single product.
C(s) + O₂(g) → CO₂(g) (burning of carbon)
CaO(s) + H₂O(l) → Ca(OH)₂(aq) (quicklime + water → slaked lime)
2. Decomposition Reaction
A single compound breaks down into two or more simpler substances.
Decomposition reactions can be caused by:
- Heat (Thermal decomposition) — e.g., CaCO₃ → CaO + CO₂
- Light (Photodecomposition) — e.g., 2AgCl → 2Ag + Cl₂
- Electricity (Electrolysis) — e.g., 2H₂O → 2H₂ + O₂
3. Displacement Reaction
A more reactive element displaces a less reactive element from its compound.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Iron displaces copper because iron is more reactive than copper. The blue colour of CuSO₄ solution fades and a reddish-brown copper deposit forms.
4. Double Displacement Reaction
Two compounds exchange their ions to form two new compounds. Usually results in formation of a precipitate, gas, or water.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s)↓ + 2NaCl(aq)
A white precipitate of barium sulphate (BaSO₄) is formed.
5. Oxidation and Reduction (Redox Reactions)
| Process | Oxygen | Hydrogen | Electrons |
|---|---|---|---|
| Oxidation | Gain of oxygen | Loss of hydrogen | Loss of electrons |
| Reduction | Loss of oxygen | Gain of hydrogen | Gain of electrons |
Reducing Agent: A substance that reduces others (and itself gets oxidised).
CuO loses oxygen → CuO is reduced → CuO is the oxidising agent
H₂ gains oxygen → H₂ is oxidised → H₂ is the reducing agent
Effects of Oxidation in Everyday Life
Corrosion
Corrosion is the slow deterioration of metals due to reaction with moisture, oxygen, or other substances in the environment.
- Rusting of iron: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (reddish-brown rust)
- Tarnishing of silver: Ag + H₂S → Ag₂S + H₂ (black tarnish)
- Green coating on copper: Cu + CO₂ + H₂O → CuCO₃·Cu(OH)₂
Rancidity
Rancidity is the oxidation of fats and oils in food, causing a bad smell and taste. It makes food unfit for consumption.
Methods to prevent rancidity:
- Adding antioxidants (e.g., BHA, BHT)
- Storing food in airtight containers
- Refrigeration (low temperature slows oxidation)
- Replacing air with nitrogen in chips packets
- Vacuum packaging
Key Formulas and Equations to Remember
Electrolysis of Water: 2H₂O → 2H₂ + O₂
Decomposition of Ferrous Sulphate: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
Thermite Reaction: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe + Heat
Rusting: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
Quick Revision Summary
| Type of Reaction | Pattern | Key Example |
|---|---|---|
| Combination | A + B → AB | C + O₂ → CO₂ |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ |
| Displacement | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu |
| Double Displacement | AB + CD → AD + CB | Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl |
| Oxidation-Reduction | Gain/loss of O, H, e⁻ | CuO + H₂ → Cu + H₂O |