Acids, bases, salts and tests
The properties of acids and bases, how salts are made, and the tests for gases, cations and anions.
📌 Key points
- Acids are proton (H+) donors and produce H+ ions in water; bases are proton acceptors; alkalis are soluble bases that produce OH− ions. A strong acid (e.g. HCl) is completely dissociated in water; a weak acid (e.g. ethanoic acid) is only partially dissociated.
- pH scale: below 7 acidic, 7 neutral, above 7 alkaline. Litmus is red in acid and blue in alkali; methyl orange is red in acid and yellow in alkali; thymolphthalein is colourless in acid and blue in alkali.
- Acid + metal → salt + hydrogen; acid + base → salt + water; acid + carbonate → salt + water + carbon dioxide. Neutralisation: H+ + OH− → H2O.
- Metal oxides are basic, non-metal oxides are acidic; amphoteric oxides such as aluminium oxide and zinc oxide react with both acids and bases.
- Solubility: all sodium, potassium and ammonium salts and all nitrates are soluble; chlorides are soluble except silver and lead; sulfates are soluble except barium, calcium and lead; carbonates are insoluble except sodium, potassium and ammonium.
- Soluble salt from an insoluble base, metal or carbonate: add excess solid to the acid, filter, then crystallise the filtrate. From an alkali: use titration. Insoluble salt: mix two solutions (precipitation), filter, wash and dry.
- Gas tests: hydrogen gives a "pop" with a lighted splint; oxygen relights a glowing splint; carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper; ammonia turns damp red litmus blue; sulfur dioxide turns acidified potassium manganate(VII) from purple to colourless.
- With aqueous sodium hydroxide: Cu2+ light blue precipitate; Fe2+ green; Fe3+ red-brown; Zn2+ and Al3+ white, soluble in excess; Ca2+ white, insoluble in excess. With aqueous ammonia, Zn2+ white precipitate dissolves in excess but Al3+ does not.
- Anion tests: with acidified aqueous silver nitrate, chloride gives a white, bromide a cream and iodide a yellow precipitate; sulfate gives a white precipitate with acidified aqueous barium nitrate; carbonate fizzes with dilute acid, giving carbon dioxide.
- Flame tests: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green.
📖 Key terms
- Acid
- A proton (H+ ion) donor.
- Base
- A proton acceptor; bases neutralise acids.
- Alkali
- A soluble base that produces OH− ions in water.
- Neutralisation
- The reaction of an acid with a base to form a salt and water: H+ + OH− → H2O.
- Salt
- A compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion.
- Amphoteric oxide
- An oxide that reacts with both acids and bases.
- Weak acid
- An acid that is only partially dissociated in water.
✏️ Worked example
Describe how to make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
- Warm the dilute sulfuric acid in a beaker.
- Add copper(II) oxide a little at a time, stirring, until some remains undissolved (excess), so all the acid has reacted.
- Filter to remove the excess copper(II) oxide.
- Heat the filtrate to the crystallisation point, then leave it to cool so crystals form.
- Filter off the crystals and dry them between filter papers. Equation: CuO + H2SO4 → CuSO4 + H2O.
In salt-preparation questions, marks are given for "add excess solid", "filter to remove the excess" and "heat to the crystallisation point, then cool". Do not just "boil to dryness".
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