What acids and bases actually are
An acid releases hydrogen ions, H⁺, when dissolved in water — that single ion is responsible for all acidic behaviour. A base is the opposite: it accepts H⁺, and a base that dissolves in water (releasing OH⁻) is called an alkali.
Strong acids and alkalis (hydrochloric acid, sodium hydroxide) ionise completely in water; weak ones (ethanoic acid, ammonia) only partly ionise. Strong versus weak is about the degree of ionisation, not how concentrated the solution is — a dilute strong acid is still a strong acid.
The pH scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is: below 7 is acidic, 7 is neutral, above 7 is alkaline. It tracks the concentration of H⁺ ions — more H⁺ means lower pH.
It is a logarithmic scale, so each step of 1 is a tenfold change in H⁺ concentration: pH 3 is ten times more acidic than pH 4. You measure pH with an indicator (litmus for a simple acid/alkali test, universal indicator for a full colour range) or a pH meter.
Neutralisation
Mix an acid with a base and they cancel out — neutralisation — producing a salt and water. Strip away the spectator ions and every neutralisation is really the same reaction:
Neutralisation is everywhere
Indigestion tablets are a base neutralising excess stomach acid; lime is spread on acidic soil to raise its pH; toothpaste neutralises the acid bacteria make on your teeth. Same reaction, three everyday uses.
The reactions of acids
Acids follow three reliable patterns, each giving a salt:
| Acid reacts with | Products | Test / sign |
|---|---|---|
| a metal | salt + hydrogen | bubbles; a lit splint gives a squeaky pop |
| a base or alkali | salt + water | neutralisation (no gas) |
| a carbonate | salt + water + carbon dioxide | fizzing; limewater turns milky |
A worked example
Write what forms when (a) magnesium reacts with hydrochloric acid, and (b) calcium carbonate reacts with hydrochloric acid.
- (a) Metal + acid → salt + hydrogen. The acid is hydrochloric, so the salt is a chloride: magnesium chloride + hydrogen.Acids give hydrogen with reactive metals; hydrochloric acid always makes a chloride.
- Mg + 2HCl → MgCl₂ + H₂.Balanced: two HCl supply the two chlorides and the hydrogen gas.
- (b) Carbonate + acid → salt + water + carbon dioxide: calcium chloride + water + CO₂.Carbonates always release CO₂ (the fizzing), plus a salt and water.
- CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.The CO₂ turns limewater milky — the standard test.
(a) magnesium chloride + hydrogen (Mg + 2HCl → MgCl₂ + H₂); (b) calcium chloride + water + carbon dioxide (CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂).
Indicators and titration
An indicator changes colour depending on pH, letting you tell an acid from an alkali or find the exact end point of a reaction. Litmus is red in acid and blue in alkali; universal indicator gives a full colour range and an approximate pH. For accurate work, a titration adds one solution from a burette to a fixed volume of the other until an indicator just changes colour — methyl orange (red→yellow) and phenolphthalein (colourless→pink) give sharp end points.
From the volume and concentration of the solution you add, you can calculate the concentration of the unknown.
Preparing salts
How you make a salt depends on whether it is soluble. For a soluble salt from an insoluble base, metal or carbonate: add the solid in excess to warm acid, filter off the excess, then evaporate and crystallise. For a soluble salt from an alkali, use a titration (no excess to filter) then crystallise the neutral solution.
An insoluble salt is made by precipitation — mixing two soluble solutions so the insoluble salt drops out, which is then filtered, washed and dried.
| Salt type | Method | Key steps |
|---|---|---|
| Soluble (from base/metal/carbonate) | excess solid + acid | react, filter off excess, evaporate, crystallise |
| Soluble (from alkali) | titration | find the exact volume, remake without indicator, crystallise |
| Insoluble | precipitation | mix two solutions, filter, wash, dry |
Brønsted–Lowry acids and Ka (A Level)
A more general definition: a Brønsted–Lowry acid is a proton (H⁺) donor and a base is a proton acceptor. When an acid donates a proton it becomes its conjugate base, forming acid–base conjugate pairs. This explains why water can act as both.
For a weak acid, only a fraction of molecules ionise, and the position of that equilibrium is measured by the acid dissociation constant, Ka (or pKa = −log Ka). A larger Ka means a stronger acid. The pH itself comes from the hydrogen-ion concentration.
Another worked example
In a titration, 25.0 cm³ of sodium hydroxide is exactly neutralised by 20.0 cm³ of 0.100 mol/dm³ hydrochloric acid. NaOH + HCl → NaCl + H₂O. Find the concentration of the sodium hydroxide.
- Moles HCl = c × V = 0.100 × (20.0/1000) = 2.0 × 10⁻³ mol.moles = concentration × volume in dm³.
- Mole ratio NaOH : HCl = 1 : 1, so moles NaOH = 2.0 × 10⁻³ mol.The equation shows one mole of each reacts.
- Concentration NaOH = n ÷ V = 2.0 × 10⁻³ ÷ (25.0/1000).c = n ÷ V with V in dm³.
- = 0.080 mol/dm³.2.0 × 10⁻³ ÷ 0.025 = 0.080.
0.080 mol/dm³.