Important Compounds & Common Names

These alkali-metal salts dominate the "common name" questions because they sit in everyday life — the kitchen, the laundry, the soap factory.

• Washing soda is Na₂CO₃·10H₂O (two sodiums, carries 10 water molecules of crystallisation).
• Baking soda is mildly basic and edible, while washing soda is strongly alkaline and not edible.
• Caustic soda NaOH and caustic potash KOH are strong alkalis that burn the skin — hence "caustic".
• Sodium bicarbonate in fire extinguishers releases CO₂ to smother flames.

The calcium compounds form a single connected story — heat limestone to get quicklime, add water for slaked lime, and so on. The exam tests both the names and how one converts into another.

• Limestone, marble and chalk are all the same compound, CaCO₃ — only the physical form differs.
• Plaster of Paris is CaSO₄·½H₂O (half a water molecule) and is made by heating gypsum to about 120°C.
• Adding water to POP sets it back into gypsum CaSO₄·2H₂O — this hardening sets a fractured limb.
• Bleaching powder CaOCl₂ is made by passing chlorine over dry slaked lime.
• Lime water is a dilute solution of slaked lime; it turns milky when CO₂ is passed through it.

"Vitriol" is the old name for a hydrated metal sulphate. They are colour-coded, and the exam almost always asks which colour goes with which metal.

• Green vitriol is ferrous (iron) sulphate FeSO₄·7H₂O.
• White vitriol is zinc sulphate ZnSO₄·7H₂O; Epsom salt is magnesium sulphate MgSO₄·7H₂O.
• A quick memory peg: Blue–Copper, Green–Fe(iron), White–Zinc.
• Sulphuric acid H₂SO₄ is called the "King of Chemicals"; concentrated H₂SO₄ is a powerful dehydrating agent.

A short but very frequently asked group — the household organics whose common names hide simple molecules.

• Ordinary alcohol is ethanol C₂H₅OH; methanol CH₃OH ("wood spirit") is highly poisonous and causes blindness.
• Methane CH₄ is "marsh gas", produced when vegetation rots in marshes.
• Glucose C₆H₁₂O₆ and fructose share the same formula but differ in structure (isomers).

A grab-bag of named compounds that appear regularly — the gases, the photographic and complex salts.

• Laughing gas is nitrous oxide N₂O, used as a mild anaesthetic.
• Hypo (sodium thiosulphate Na₂S₂O₃) is the "fixer" in photography.
• Mohr's salt is ferrous ammonium sulphate, a double salt used to prepare standard solutions in the lab.

The paper loves single-fact superlatives. Memorise this table cold — each row is a likely one-mark question.

• Oxygen is the most abundant element in the Earth's crust (by mass); aluminium is the most abundant metal in the crust.
• Diamond, an allotrope of carbon, is the hardest natural substance.
• Silver is the best conductor of electricity (copper is the best *practical/cheaper* conductor).
• Caesium and francium are the most reactive metals; fluorine is the most reactive non-metal.
• Mercury is the only metal liquid at room temperature, and bromine is the only non-metal liquid at room temperature.

A small supporting table — the common acids and bases, often paired with the compounds above.

• Muriatic acid is hydrochloric acid HCl; stomach acid is also dilute HCl.
• Milk of magnesia Mg(OH)₂ is an antacid that neutralises excess stomach acid.