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Introduction to Acids

Acids: aqueous hydrogen ions, complete versus partial ionisation, and the distinction between acid strength and concentration.

  • SEC G3 Pure Chemistry 2027

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Hydrochloric acid and ethanoic acid can have the same concentration yet react at different rates. To explain this, distinguish the amount of acid added from the fraction that forms ions in water.

Revise this together with the Acids, Bases and Salts hub and pH Scale and Indicators so definitions and practical observations stay linked.

1. Definition

A. Acid (aqueous definition)

An acid produces hydrogen ions, H + (aq), when dissolved in water. These ions account for the acidic behaviour of the aqueous solutions studied here. A hydrogen ion is a proton; an acid donates protons in acid–base reactions.

2. Key Ideas

  • Aqueous acids show acidic behaviour because they contain H + (aq). Dry hydrogen chloride does not contain these aqueous ions; its solution does.
  • Universal Indicator is red, orange or yellow in acidic solutions, depending on their pH. Blue litmus turns red.
  • A substance containing hydrogen is not automatically an acid (example: ammonia, NH₃).
  • Strong/weak is about extent of ionisation. Concentrated/dilute is about amount per volume.
AcidFormulaBehaviour in water
hydrochloric acidHClHCl(aq) → H + (aq) + Cl⁻(aq)
nitric acidHNO₃HNO₃(aq) → H + (aq) + NO₃⁻(aq)
sulfuric acidH₂SO₄strong acid; can supply two protons per formula unit in complete neutralisation
ethanoic acidCH₃COOHCH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H + (aq)
Common strong and weak acids

Hydrochloric acid and nitric acid ionise essentially completely in dilute water solutions; ethanoic acid ionises partially. Sulfuric acid is also classed as strong. Its first proton is released essentially completely, but its second is not released to the same extent. You do not need its detailed ionisation equilibria here.

3. Detailed Explanations

Quick Recall (before Section 3)
  • Acids show acidic behaviour in water because they produce H + (aq).
  • Strong vs weak = degree of ionisation (complete vs partial). Concentrated vs dilute = amount per volume.
  • Strong acids use → in ionisation equations; weak acids use ⇌.

A. Strong vs weak acids (ionisation)

Strong acids ionise essentially completely in dilute aqueous solution. Weak acids ionise partially. At ordinary laboratory concentrations, ethanoic acid contains mainly un-ionised molecules, with smaller amounts of hydrogen and ethanoate ions.

Ionisation does not mean that a molecule simply disappears: its atoms become part of the ions. For an acid represented by HA, each ionised unit gives one H⁺ and one A⁻.

Equal acid amounts with different degrees of ionisationTwo equal-volume boxes contain the same total number of acid units. In the strong-acid box all eight units are ionised: eight positive hydrogen ions and eight negative A ions. In the weak-acid box one unit is ionised and seven remain as neutral HA molecules. Each box has equal positive and negative charge. The weak-acid fraction is illustrative, not experimental.Strong acidSame total acid amount and volumeH⁺A⁻H⁺A⁻H⁺A⁻H⁺A⁻H⁺A⁻H⁺A⁻H⁺A⁻H⁺A⁻All eight units ionisedH⁺ count equals A⁻ countWeak acidSame total acid amount and volumeH⁺A⁻HAHAHAHAHAHAHAOne ionised; seven as moleculesH⁺ count equals A⁻ count
Both boxes represent equal solution volumes containing eight acid units. HA is a neutral acid molecule; each ionised unit gives H⁺ and A⁻. The strong-acid box has eight of each ion; the weak-acid box has seven HA molecules and one pair of ions. Water is omitted. Counts illustrate complete versus partial ionisation, not a measured or fixed weak-acid fraction; particles and sizes are schematic.

The reversible arrow for ethanoic acid shows that ions and molecules can interconvert. It does not mean equal amounts, and “weak” does not mean a fixed percentage ionises. The fraction depends on the acid and the concentration.

Connect particles to observations: Compare equal volumes of hydrochloric acid and ethanoic acid at the same acid concentration and temperature. Use equal clean magnesium samples with the same exposed surface area, with each acid in excess. Collect the gas under the same conditions.

Hydrochloric acid initially has a higher hydrogen-ion concentration, so magnesium produces hydrogen faster. As ethanoic acid’s hydrogen ions are used, more molecules ionise. A weak acid can therefore continue reacting; the initially ionised fraction is not its total reaction capacity.

Hydrogen production: hydrochloric acid and ethanoic acid

The hydrochloric-acid model rises more steeply and reaches 40 cubic centimetres sooner. The ethanoic-acid model rises more slowly to the same final volume. At 40 seconds the model volumes are 30 and 16 cubic centimetres respectively.

Scroll across the graph to read all labels.

The hydrochloric-acid model rises more steeply and reaches 40 cubic centimetres sooner. The ethanoic-acid model rises more slowly to the same final volume. At 40 seconds the model volumes are 30 and 16 cubic centimetres respectively.The hydrochloric-acid model rises more steeply and reaches 40 cubic centimetres sooner. The ethanoic-acid model rises more slowly to the same final volume. At 40 seconds the model volumes are 30 and 16 cubic centimetres respectively.
Illustrative model, not measured data: equal volumes of hydrochloric acid and ethanoic acid at the same acid concentration and temperature react with identical clean magnesium samples. Both acids are in excess; magnesium limits the final hydrogen amount. Gas volumes are compared at the same temperature and pressure.
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Values and uncertainty for Hydrogen production: hydrochloric acid and ethanoic acid
SeriesTime (s)Time uncertaintyVolume of hydrogen (cm³)Volume of hydrogen uncertainty
Hydrochloric acid (strong)00
Hydrochloric acid (strong)2018
Hydrochloric acid (strong)4030
Hydrochloric acid (strong)6038
Hydrochloric acid (strong)8040
Hydrochloric acid (strong)12040
Hydrochloric acid (strong)14040
Ethanoic acid (weak)00
Ethanoic acid (weak)208
Ethanoic acid (weak)4016
Ethanoic acid (weak)6024
Ethanoic acid (weak)8030
Ethanoic acid (weak)10035
Ethanoic acid (weak)12040
Ethanoic acid (weak)14040

In this illustrative model, the steeper initial curve indicates a faster reaction. Both reach 40 cm³: the same amount of magnesium limits the final hydrogen amount. The graph does not show that a strong acid must produce more gas.

Read the graph: How much hydrogen has each model reaction produced after 40 s? Which reaction has finished first?

Show answer

At 40 s, the strong-acid model has produced 30 cm³ and the weak-acid model 16 cm³. The strong-acid curve reaches its plateau first. These values describe the supplied model, not universal reaction times.

B. Strong vs concentrated (different ideas)

  • Acid concentration is the total amount of acid added per final solution volume, commonly in mol dm⁻³. It includes acid present as molecules and acid that has ionised; it is not automatically the hydrogen-ion concentration.
  • Strength is “what fraction of acid molecules ionise”.

So you can have:

  • a concentrated weak acid (lots of acid molecules per volume, but few ionise), and
  • a dilute strong acid (not many acid molecules per volume, but almost all ionise).
Sentence to memorise

“Strong/weak refers to degree of ionisation. Concentrated/dilute refers to concentration.”

C. Why aqueous acids conduct electricity

Aqueous hydrochloric acid contains mobile H⁺ and Cl⁻ ions. Both carry charge through the solution. Ethanoic acid solution also conducts because partial ionisation produces mobile ions; un-ionised acid molecules do not carry charge.

At the same acid concentration and temperature, dilute hydrochloric acid conducts more strongly than ethanoic acid because it has more ions. Conductivity alone is not a fair strength comparison when concentrations or temperatures differ.

For dry hydrogen chloride, there are molecular particles rather than aqueous ions. Avoid extending this example into the false claim that every pure acid has no ions or that all acid–base reactions require water.

4. Common Mistakes

  • Saying “strong acid = concentrated acid” (wrong comparison).
  • Writing “equilibrium means equal amounts” (irrelevant here): weak acid ionisation is partial, not “half and half”.
  • Writing formulas without state symbols when the question expects them (use (aq) for acids in water).
  • Thinking any compound with hydrogen is an acid (e.g. NH₃ is not an acid in water).

After this lesson, test recall with the Acids quiz and then broaden revision through Exam Skills.

5. Exam Tips

Keywords that score marks
  • “produces H + aq ions in water”
  • strong acid: “ionises completely”
  • weak acid: “ionises partially”
    - strength vs concentration: “degree of ionisation” vs “amount per volume”
  • If asked to list particles in dilute hydrochloric acid, name H + (aq) and Cl⁻(aq) as the solute particles. If asked for all main particles, include water molecules too.
  • If asked to compare strong vs weak acids fairly, keep concentration the same and compare ionisation.

6. Worked Examples

Modelled example 1

Choose the correct arrow

Core

Problem

Which equation should use → and which should use ⇌: (i) HCl(aq), (ii) CH₃COOH(aq)?
Study the worked solution

  1. Classify hydrochloric acid

    Method

    Use a one-way arrow for aqueous HCl.

    Reason

    HCl is a strong acid and ionises completely in water.

    Working

    HCl(aq) → H + (aq) + Cl⁻(aq).

  2. Classify ethanoic acid

    Method

    Use a reversible arrow for aqueous ethanoic acid.

    Reason

    Ethanoic acid is weak and ionises only partially.

    Working

    CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H + (aq).

  3. State what the arrows encode

    Method

    Connect arrow choice to degree of ionisation.

    Reason

    The distinction is strength, not the concentration of either solution.

    Working

    →: complete; ⇌: partial.

Guided practice 2

Strong vs concentrated (spot the trap)

About 6 min

Problem

Acid A is 0.10 mol dm⁻³ HCl and Acid B is 1.0 mol dm⁻³ CH₃COOH. Which is strong, and which is more concentrated?

Classify two independent properties

Strong acid
More concentrated solution

Hints

Hint 1: strength criterion

Strength describes degree of ionisation; HCl is complete and ethanoic acid is partial.

Hint 2: concentration criterion

Concentration compares the supplied mol dm⁻³ values.

View solution step by step

  1. Judge strength

    Method

    Classify HCl as strong and ethanoic acid as weak.

    Reason

    Strength depends on complete versus partial ionisation.

    Working

    Acid A: strong HCl; Acid B: weak CH₃COOH.

  2. Judge concentration

    Method

    Compare 0.10 with 1.0 mol dm⁻³.

    Reason

    Concentration is amount per volume and is independent of strength.

    Working

    Acid B is more concentrated; Acid A is relatively dilute.

Common misconception 3

“Contains hydrogen” is not enough

Find and correct the mistake

Learner claim

A student says NH₃ must be an acid because its formula contains hydrogen. Explain why this test is invalid and classify ammonia in water.

Use the aqueous-ion definition

Ammonia in water
Valid acid criterion

View solution step by step

  1. Reject the formula shortcut

    Method

    Do not classify a substance by merely spotting H in its formula.

    Reason

    Acid behaviour depends on producing H + (aq) in water.

    Working

    NH₃ does not ionise to release H + (aq).

  2. Classify ammonia

    Method

    Describe ammonia as a base.

    Reason

    It accepts a proton to form the ammonium ion.

    Working

    NH₃ + H⁺ → NH₄ +.

Challenge 4

List the main particles in solution

Minimal support

Particle-level transfer

What are the main solute particles in (i) 0.10 mol dm⁻³ HNO₃(aq) and (ii) 0.10 mol dm⁻³ CH₃COOH(aq)?

Translate strength into particle populations

Main nitric-acid particles
Main ethanoic-acid particles

Hints

Hint 1: connect strength to extent

Nitric acid is strong; ethanoic acid is weak.

Hint 2: picture complete versus partial

Complete ionisation leaves mainly ions; partial ionisation leaves many un-ionised molecules.

View solution step by step

  1. Represent strong nitric acid

    Method

    List mainly H + (aq) and NO₃⁻(aq).

    Reason

    Strong nitric acid ionises completely in dilute aqueous solution.

    Working

    HNO₃(aq) → H + (aq) + NO₃⁻(aq).

  2. Represent weak ethanoic acid

    Method

    List mainly CH₃COOH molecules plus small amounts of both ions.

    Reason

    Partial ionisation leaves most weak-acid molecules intact.

    Working

    Mainly CH₃COOH(aq); some H + (aq) and CH₃COO⁻(aq).

7. Mind Stretchers

Mind stretcher 1: Fix the student’s sentenceExtension

Question: A student writes: “A strong acid has a high concentration of acid.” Rewrite this correctly.

Show answer

“A strong acid ionises completely in water (high degree of ionisation). Concentration is a separate idea (moles per unit volume).”

Mind stretcher 2: Conductivity logicExtension

Question: Why does ethanoic acid solution conduct electricity, even though it is a weak acid?

Show answer

Even weak acids ionise partially, so ethanoic acid solution still contains mobile ions (H + (aq) and CH₃COO⁻(aq)) that carry charge. Compared with hydrochloric acid at the same acid concentration and temperature, ethanoic acid has fewer ions and lower conductivity, but its conductivity is not zero.

8. Quiz

Syllabus and review details

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