MINDMAPS FOR UPSCPET 2026 · VISUAL REVISION
Common salts: formula and transformation
Keep calcium and sodium compounds straight by formula and water of crystallisation
Household and industrial compounds
The lime transformations
- Quicklime is calcium oxide (CaO), made by heating limestone (CaCO₃ → CaO + CO₂).
- Adding water slakes CaO exothermically to slaked lime: CaO + H₂O → Ca(OH)₂.
- Whitewash Ca(OH)₂ reacts slowly with air's CO₂ to form a shiny CaCO₃ layer.
Baking soda and washing soda
- Baking soda is sodium hydrogen carbonate (NaHCO₃), a mild non-corrosive base.
- Heating NaHCO₃ releases CO₂ gas (2NaHCO₃ → Na₂CO₃ + H₂O + CO₂), leavening dough.
- Washing soda is hydrated sodium carbonate (Na₂CO₃·10H₂O), used to soften hard water.
Gypsum and plaster of Paris
- Gypsum is calcium sulphate dihydrate (CaSO₄·2H₂O).
- Controlled heating near 373 K converts gypsum to plaster of Paris by removing 1½ H₂O.
- Plaster of Paris is calcium sulphate hemihydrate (CaSO₄·½H₂O).
Water of crystallisation
- Water of crystallisation is the fixed number of H₂O molecules in one formula unit.
- Adding water to plaster of Paris reforms gypsum and produces a hard set mass.
- Two CaSO₄ units share one H₂O molecule in CaSO₄·½H₂O; store it in moisture-proof bags.
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