1. Sodium Compounds: Baking Soda, Washing Soda & Caustic Soda
Essential sodium salts: (1) Baking Soda: Sodium hydrogen carbonate / Sodium bicarbonate (NaHCO3). White crystalline solid used in baking powder (mixed with mild edible acid like tartaric acid) and as mild antacid. (2) Washing Soda: Sodium carbonate decahydrate (Na2CO3·10H2O). Contains 10 water molecules of crystallization; when exposed to dry air, it loses 9 water molecules via efflorescence forming sodium carbonate monohydrate (Na2CO3·H2O). (3) Soda Ash: Anhydrous sodium carbonate (Na2CO3). (4) Caustic Soda: Sodium hydroxide (NaOH). Deliquescent white solid used in soap manufacturing and petroleum refining. (5) Common Salt: Sodium chloride (NaCl). (6) Chile Saltpeter: Sodium nitrate (NaNO3).
2. Calcium Compounds: Plaster of Paris, Gypsum, Quicklime & Bleaching Powder
High-frequency calcium minerals: (1) Gypsum: Calcium sulphate dihydrate (CaSO4·2H2O). When heated carefully at 373 K (100 °C), it loses three-fourths of its water of crystallization to form Plaster of Paris. (2) Plaster of Paris (POP): Calcium sulphate hemihydrate (CaSO4·1/2H2O or 2CaSO4·H2O). Sets into hard mass upon mixing with water. Overheating above 393 K produces anhydrous CaSO4 known as "dead burnt plaster", which loses its setting property. (3) Quicklime: Calcium oxide (CaO). Reacts vigorously with water (slaking of lime) releasing heat to form slaked lime. (4) Slaked Lime / Limewater: Calcium hydroxide (Ca(OH)2). Used in white-washing; turns milky when CO2 gas passes through due to insoluble CaCO3 formation. (5) Limestone / Marble / Chalk: Calcium carbonate (CaCO3). (6) Bleaching Powder: Calcium hypochlorite / Calcium oxychloride (CaOCl2). Reacts with dilute acids to liberate chlorine gas (active bleaching agent).
3. The Vitriols & Hydrated Sulphates
Historical vitriol salts widely tested in match-the-column questions: (1) Blue Vitriol: Copper(II) sulphate pentahydrate (CuSO4·5H2O). Four water molecules are coordinated to Cu2+ while the fifth is hydrogen-bonded to sulphate anion [Cu(H2O)4]SO4·H2O; turns white upon heating due to dehydration into anhydrous CuSO4. (2) Green Vitriol: Ferrous sulphate heptahydrate (FeSO4·7H2O). Decomposes on heating yielding Fe2O3, SO2, and SO3 gases. (3) White Vitriol: Zinc sulphate heptahydrate (ZnSO4·7H2O). (4) Oil of Vitriol: Concentrated Sulphuric Acid (H2SO4), so called historically because it was distilled from green vitriol. (5) Epsom Salt: Magnesium sulphate heptahydrate (MgSO4·7H2O).
4. Alums & Double Salts
True alums have general formula M_I M_III (SO4)2 · 12H2O or M_I2 SO4 · M_III2 (SO4)3 · 24H2O: (1) Potash Alum (Fitkari): Potassium aluminium sulphate 24-hydrate (K2SO4·Al2(SO4)3·24H2O). Used for water purification (coagulation of colloidal impurities via Hardy-Schulze rule) and as styptic to stop minor bleeding. (2) Chrome Alum: K2SO4·Cr2(SO4)3·24H2O. (3) Ammonium Iron(III) Alum: (NH4)2SO4·Fe2(SO4)3·24H2O. (4) Mohr's Salt: Ferrous ammonium sulphate hexahydrate ((NH4)2Fe(SO4)2·6H2O). A stable primary standard double salt used in redox titrations with KMnO4.
5. Industrial Gases, Oxides & Specialty Reagents
Everyday chemical names: (1) Laughing Gas: Nitrous oxide (N2O). (2) Tear Gas: Chloropicrin / Trichloronitromethane (CCl3NO2). (3) Phosgene: Carbonyl chloride (COCl2), highly toxic gas formed when chloroform (CHCl3) is exposed to air and light. (4) Heavy Water: Deuterium oxide (D2O), used as neutron moderator in nuclear reactors. (5) Dry Ice: Solid carbon dioxide (CO2). (6) Marsh Gas: Methane (CH4). (7) Water Gas: Equimolar mixture of Carbon monoxide and Hydrogen (CO + H2), also known as synthesis gas (syngas). (8) Producer Gas: Carbon monoxide and Nitrogen (CO + N2). (9) Aqua Regia: Mixture of concentrated Hydrochloric acid and concentrated Nitric acid in a 3:1 volumetric ratio (3 HCl : 1 HNO3), capable of dissolving noble metals gold and platinum.
6. Teacher's Exam Hall Hack: Water of Crystallization Quick Match
Competitive exams constantly quiz the exact water coefficient in hydrated salts. Memorize the descending countdown: Potash alum (24 H2O) -> Washing soda (10 H2O) -> Green/White vitriol & Epsom salt (7 H2O) -> Mohr's salt (6 H2O) -> Blue vitriol (5 H2O) -> Gypsum (2 H2O) -> Plaster of Paris (1/2 H2O or 1 H2O per 2 CaSO4 units). Keeping this series memorized saves time in match-the-column questions.