Class 10 Science Notes — Chapter 1: Chemical Reactions and Equations
How chemical reactions are represented, balanced, and classified.
Detailed NCERT notes
- A chemical reaction converts reactants into products with rearrangement of atoms — mass is conserved.
- Observable signs: change in state, change in colour, evolution of gas, change in temperature, formation of precipitate.
- A chemical equation is balanced when the number of atoms of each element is equal on both sides (law of conservation of mass).
- Steps to balance: write skeletal equation → balance metal atoms → non-metals → hydrogen → oxygen → add physical states (s, l, g, aq) → indicate conditions (Δ for heat, catalyst).
- Types of chemical reactions:
- Combination: two or more reactants → single product (e.g., CaO + H₂O → Ca(OH)₂, exothermic).
- Decomposition: single reactant → two or more products; thermal (CaCO₃ → CaO + CO₂), electrolytic (2H₂O → 2H₂ + O₂), photochemical (2AgCl → 2Ag + Cl₂).
- Displacement: more reactive element displaces less reactive one (Fe + CuSO₄ → FeSO₄ + Cu).
- Double displacement: exchange of ions between two compounds; usually forms a precipitate (Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl).
- Precipitation: an insoluble solid forms when two solutions are mixed.
- Neutralisation: acid + base → salt + water.
- Redox reactions: oxidation = gain of oxygen / loss of hydrogen / loss of electrons; reduction = loss of oxygen / gain of hydrogen / gain of electrons.
- In a redox reaction, oxidising agent gets reduced; reducing agent gets oxidised.
- Corrosion: oxidation of metals when exposed to air/water (e.g., rusting of iron); prevented by painting, oiling, galvanising, alloying.
- Rancidity: oxidation of fats/oils in food producing bad smell/taste; prevented by antioxidants, airtight packing, nitrogen flushing.
Formulas & key results
- Balanced equation: atoms of each element equal on both sides
- Types: combination, decomposition, displacement, double displacement, redox
- Oxidation (gain O / lose H / lose e⁻) vs Reduction (opposite)
Mind map
- Reaction indicators → balancing → types → redox
- Corrosion (rusting) and rancidity
- Physical states and conditions on the arrow
Tricks & shortcuts
- Balance in order: metals → non-metals → H → O.
- OIL RIG mnemonic: Oxidation Is Loss, Reduction Is Gain (of electrons).
Common mistakes to avoid
- Changing subscripts to balance (only coefficients change).
- Ignoring physical states (s, l, g, aq).
- Reversing oxidation/reduction direction in redox.
Competency-based questions & answers
- Q. Why does silver chloride turn grey in sunlight?A. Photodecomposition: 2AgCl → 2Ag + Cl₂. Metallic silver deposition gives the grey colour (used in photography).
- Q. Explain why rusting is called an electrochemical process.A. Iron gets oxidised at anode, oxygen gets reduced at cathode; water/electrolyte acts as medium — an electrochemical cell forms on the surface.
- Q. Why do chips packets contain nitrogen gas?A. Nitrogen is inert; it displaces oxygen, preventing oxidation of fats — no rancidity.