QCE Chemistry - Unit 3 - Chemical equilibrium systems
Properties of Acids and Bases | QCE Chemistry
Revise the observable and particle-level properties of acids and bases for QCE Chemistry Unit 3.
Updated 2026-05-18 - 4 min read
QCAA official coverage - Chemistry 2025 v1.3
Exact syllabus points covered
- Identify that acids are substances that can act as proton (hydrogen ion) donors.
- Identify acids as monoprotic, diprotic or polyprotic.
- Identify hydrochloric, nitric and sulfuric acid as strong acids and group 1 hydroxides and barium hydroxide as strong bases.
- Identify carboxylic and carbonic acids as weak acids and ammonia and amines as weak bases.
- Discriminate between the terms strong, weak, concentrated and dilute for acids and bases.
- Discriminate between strong and weak acids and bases in terms of the extent of dissociation, rate of reaction, pH and electrical conductivity.
- Analyse data to determine the strength, concentration, pH and electrical conductivity of acids and bases.
Acids and bases can be recognised by their reactions, but QCE answers are stronger when you link the observation to particles in solution. Instead of only saying "acid reacts with magnesium", explain that acid particles provide $\mathrm{H^+}$ or $\mathrm{H_3O^+}$ that can be reduced to hydrogen gas.
Acids
Acids commonly:
- turn blue litmus red
- react with reactive metals to form a salt and hydrogen gas
- react with carbonates to form a salt, carbon dioxide and water
- neutralise bases
- conduct electricity in water because mobile ions are present
Examples:
$ 2\mathrm{HCl(aq)} + \mathrm{Mg(s)} \rightarrow \mathrm{MgCl_2(aq)} + \mathrm{H_2(g)} $
$ 2\mathrm{HCl(aq)} + \mathrm{Na_2CO_3(aq)} \rightarrow 2\mathrm{NaCl(aq)} + \mathrm{CO_2(g)} + \mathrm{H_2O(l)} $
The fizzing in an acid-carbonate reaction is $\mathrm{CO_2(g)}$.
Bases and alkalis
Bases commonly:
- turn red litmus blue
- neutralise acids
- include metal oxides, metal hydroxides, carbonates and ammonia
- may produce $\mathrm{OH^-}$ in water or accept protons from acids
An alkali is a base that dissolves in water. $\mathrm{NaOH}$ is an alkali. $\mathrm{CuO}$ is a base, but it is not an alkali because it is not very soluble.
Strong versus weak
Strength describes degree of ionisation.
- Strong acid: almost completely ionises in water.
- Weak acid: partially ionises and forms an equilibrium mixture.
- Strong base: almost completely dissociates or reacts to produce basic ions.
- Weak base: partially reacts with water to form ions.
Concentration describes how much solute is dissolved per litre. It is a separate idea.
A dilute strong acid can be strong because nearly every dissolved acid particle ionises. A concentrated weak acid can still be weak because only a fraction of its particles ionise at any moment.
QCE Chemistry expects you to know the common examples:
| Type | Examples | | --- | --- | | Strong acids | hydrochloric acid, $\mathrm{HCl}$; nitric acid, $\mathrm{HNO_3}$; sulfuric acid, $\mathrm{H_2SO_4}$ | | Weak acids | carboxylic acids such as ethanoic acid, $\mathrm{CH_3COOH}$; carbonic acid, $\mathrm{H_2CO_3}$ | | Strong bases | group 1 hydroxides such as $\mathrm{NaOH}$ and $\mathrm{KOH}$; barium hydroxide, $\mathrm{Ba(OH)_2}$ | | Weak bases | ammonia, $\mathrm{NH_3}$; amines |
Strong acids are usually written with a one-way arrow because the ionisation is treated as essentially complete:
$ \mathrm{HCl(aq)} + \mathrm{H_2O(l)} \rightarrow \mathrm{H_3O^+(aq)} + \mathrm{Cl^-(aq)} $
Weak acids are written as equilibria because only some acid molecules ionise at any one time:
$ \mathrm{CH_3COOH(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{H_3O^+(aq)} + \mathrm{CH_3COO^-(aq)} $
Weak bases also form an equilibrium with water:
$ \mathrm{NH_3(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{NH_4^+(aq)} + \mathrm{OH^-(aq)} $
Original Sylligence diagram for acid base strength conductivity.
Monoprotic and polyprotic acids
A monoprotic acid can donate one proton per acid molecule. $\mathrm{HCl}$ and $\mathrm{CH_3COOH}$ are common examples.
A polyprotic acid can donate more than one proton per molecule. $\mathrm{H_2SO_4}$ is diprotic because it can donate two protons in stages.
Conductivity
Acidic and basic solutions conduct electricity when they contain mobile ions. Strong acids usually conduct better than weak acids at the same concentration because strong acids produce more ions.
This is a useful data-analysis clue. If two acid samples have the same concentration but one has much higher conductivity, the higher-conductivity sample is likely to be the stronger acid because it has produced a greater concentration of mobile ions. If the concentrations are different, be careful: conductivity is affected by both acid/base strength and how much solute is present.
Rate of reaction can also help. A strong acid and a weak acid at the same concentration may both react with magnesium or carbonate, but the strong acid usually reacts faster because it supplies a higher concentration of reactive $\mathrm{H_3O^+}$ ions.
Worked example
Exam traps
Other traps:
- saying every base contains $\mathrm{OH^-}$
- saying all hydrogen-containing substances are acids
- calling insoluble bases alkalis
- forgetting that weak acids still react with bases
- comparing acid strength using conductivity data without checking that concentration is controlled