s-Block Compounds — Chemistry Mnemonic
Target: mnemonic for s block compounds NaOH Na2CO3 CaO chemistry
Why is this hard to memorize?
The s-block compounds are industrially important chemicals that NEET and JEE test repeatedly. Key compounds: NaOH (caustic soda, made by Castner-Kellner process — electrolysis of brine), Na₂CO₃ (washing soda, Solvay process), NaHCO₃ (baking soda), CaO (quicklime), Ca(OH)₂ (slaked lime), CaCO₃ (limestone), CaSO₄·2H₂O (gypsum), CaSO₄·½H₂O (plaster of Paris). Know the preparation methods, properties, and uses of each compound for guaranteed marks.
Classic mnemonics you should know
"NaOH(caustic soda)=Castner-Kellner. Na₂CO₃(washing soda)=Solvay. NaHCO₃(baking soda)=intermediate in Solvay"
NaOH is made by electrolysis of NaCl solution (Hg cathode, Cl₂ at anode). Na₂CO₃ is made by Solvay process: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃↓ (filtered, heated to Na₂CO₃). Baking soda is the intermediate product.
"CaCO₃ →(heat)→ CaO + CO₂. CaO + H₂O → Ca(OH)₂ (exothermic!). Ca(OH)₂ + CO₂ → CaCO₃↓ (milky)"
Limestone cycle: CaCO₃ decomposes on heating to quicklime (CaO). CaO + water → slaked lime (very exothermic, can boil water). Slaked lime + CO₂ → limestone again (this is the lime water test for CO₂ — turns milky).
"CaSO₄·2H₂O →(120°C)→ CaSO₄·½H₂O + 1½H₂O. Plaster + water → gypsum again (sets hard). "Gypsum loses water at 120, gets it back when used""
Gypsum (CaSO₄·2H₂O) heated to 120°C loses 1.5 water molecules to form plaster of Paris (CaSO₄·½H₂O). When water is added, plaster sets by reforming gypsum — this expansion on setting is why it's used for casts and moulds.
The complete list
- NaOH: Castner-Kellner process (electrolysis)
- Na₂CO₃: Solvay process (NaCl + NH₃ + CO₂)
- NaHCO₃: baking soda (decomposes on heating)
- CaO: quicklime (from CaCO₃ decomposition)
- Ca(OH)₂: slaked lime (CaO + H₂O, exothermic)
- CaSO₄·½H₂O: plaster of Paris
- Lime water test: Ca(OH)₂ + CO₂ → milky CaCO₃
- Bleaching powder: Ca(OCl)Cl (CaOCl₂)
Frequently asked questions
What is the Solvay process?
Industrial manufacture of Na₂CO₃ (washing soda): (1) Saturate NaCl solution with NH₃. (2) Pass CO₂ through it: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃↓ + NH₄Cl. NaHCO₃ precipitates (less soluble). (3) Filter and heat: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. NH₃ is recovered by treating NH₄Cl with Ca(OH)₂. Efficient and cheap — the main industrial process.
Why does plaster of Paris set when water is added?
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum). The hemihydrate absorbs water to reform the dihydrate, creating interlocking needle-like crystals of gypsum that form a hard mass. The process is exothermic (you can feel the warmth). Slight expansion on setting makes it ideal for casts and moulds (fills every detail).
What is the lime water test for CO₂?
Pass the gas through lime water (Ca(OH)₂ solution). If CO₂ is present: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O (turns milky due to white CaCO₃ precipitate). On passing excess CO₂: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (soluble, milkiness disappears). This is a classic NEET practical question.
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