Acid and Bases

Litmus

red in acid, blue in alkali.

Phenolphthalein

colourless in acid, pink in alkali.

Methyl orange

red in acid, yellow in alkali.

pH scale

shows acidity or alkalinity: 0-3 strongly acidic, 4-6 weakly acidic, 7 neutral, 8-10 weakly alkaline, 11-14 strongly alkaline.

Universal indicator

shows a full range of colours across the pH scale, giving an approximate pH value.

Acids

produce hydrogen ions (H⁺) in aqueous solution.

Alkalis

produce hydroxide ions (OH⁻) in aqueous solution.

Neutralisation

When an acid reacts with an alkali, hydrogen ions combine with hydroxide ions to form water (H₂O).

Acid-alkali titration

Measure acid and alkali volumes using a burette and pipette with an appropriate indicator.

End-point

the point at which the indicator just changes colour during titration.

General solubility rules

Soluble: all sodium, potassium, ammonium compounds, all nitrates, and most chlorides and sulfates.

Insoluble compounds

silver and lead(II) chlorides; barium, calcium, and lead(II) sulfates; most carbonates and hydroxides except sodium, potassium, and calcium (slightly soluble).

Proton transfer

Acids and bases react by transferring protons (H⁺ ions).

Brønsted-Lowry definition

The acid donates a proton, and the base accepts it.

Acid as proton donor

An acid donates H⁺ ions to another substance.

Base as proton acceptor

A base accepts H⁺ ions.

Reactions of acids

Acids react with metals, bases, and carbonates to form salts.

Acid + metal

salt + hydrogen gas.

Acid + base

salt + water

Acid + carbonate

salt + water + carbon dioxide.

Bases

Metal oxides, hydroxides, and ammonia act as bases because they neutralise acids.

Preparation of pure dry soluble salt from insoluble reactant

Add excess insoluble base (e.g. copper(II) oxide) to warm acid until no more dissolves, filter out the excess, then evaporate and crystallise the solution to obtain pure crystals.

Preparation of soluble salt from acid and alkali

Use titration to find the exact neutralisation point between the acid and alkali, repeat the reaction without indicator, then evaporate the solution to crystallise the pure salt.

Preparation of insoluble salt from two soluble reactants

Mix two soluble salt solutions (e.g. lead nitrate and sodium sulfate). An insoluble salt precipitates, which is then filtered, washed, and dried. This is called a precipitation reaction.

Preparation of hydrated copper(II) sulfate crystals

React copper(II) oxide with sulfuric acid, filter to remove excess solid, then evaporate and cool to form blue hydrated CuSO₄ crystals.

Preparation of pure dry sample of lead(II) sulfate

Mix lead nitrate solution with sodium sulfate solution, producing a white precipitate of PbSO₄. Filter, wash with distilled water, and dry it.