2.7 Acids, Bases & Salt PreparationsEdexcel IGCSE Chemistry: Revision notes

General solubility rules for ionic compounds in water:

• Most nitrates are soluble.
• Most chlorides are soluble, EXCEPT silver chloride and lead chloride, which are insoluble.
• Most sulfates are soluble, EXCEPT barium sulfate, calcium sulfate and lead sulfate, which are insoluble.
• Most carbonates are insoluble, EXCEPT sodium, potassium and ammonium carbonates.
• Most hydroxides are insoluble, EXCEPT sodium, potassium, ammonium and (partially) calcium hydroxide.
• All sodium, potassium and ammonium salts are soluble.

These rules determine which salt preparation method to use.

In terms of protons:

• An acid is a proton donor — it gives away H+ ions.
• A base is a proton acceptor — it takes H+ ions.

Metal oxides, metal hydroxides and ammonia can all act as bases. An alkali is simply a base that is soluble in water, releasing OH- ions in solution.

Acids react with metals, bases and metal carbonates to form salts:

The acid used determines the salt's name: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.

Example: copper(II) oxide + sulfuric acid → copper(II) sulfate + water (CuO + H2SO4 → CuSO4 + H2O)

1. Soluble salt from an insoluble reactant (e.g. metal or insoluble base) + acid:

1. Add excess insoluble reactant (e.g. metal oxide) to warm dilute acid until no more dissolves/reacts (excess remains).
2. Filter off the unreacted excess solid.
3. Gently heat/evaporate the filtrate to reduce the volume, forming a saturated solution.
4. Leave to cool and crystallise; filter and dry the crystals.

2. Soluble salt from an acid + alkali (titration method, needed as both are soluble/colourless so no excess can be filtered):

1. Titrate the alkali with the acid using an indicator to find the exact neutralising volume.
2. Repeat without indicator using the volumes found, mixing the exact amounts.
3. Evaporate to crystallisation point, cool, filter and dry the crystals.

3. Insoluble salt from two soluble reactants (precipitation):

1. Mix two soluble solutions (e.g. a soluble salt of the desired metal + a soluble salt providing the desired anion) so that an insoluble salt precipitates.
2. Filter to collect the precipitate.
3. Wash with distilled water to remove soluble impurities.
4. Dry, e.g. between filter paper.

Preparing hydrated copper(II) sulfate crystals from copper(II) oxide: react excess black copper(II) oxide with warm dilute sulfuric acid to form blue copper(II) sulfate solution, filter off excess copper(II) oxide, evaporate the filtrate to a saturated solution, cool to crystallise, then filter and dry the blue crystals.