Acids and Bases

Summary: Acids are proton (H⁺) donors; bases are proton acceptors. Strong acids fully dissociate in water; weak acids partially dissociate. Common laboratory acids: HCl, H₂SO₄, HNO₃. Tags: igcse chemistry acids-bases Created: 2026-07-14 Last Updated: 2026-07-16

Content

What is an Acid?

An acid is a substance that donates hydrogen ions (H⁺) when dissolved in water. In water, H⁺ ions attach to water molecules to form hydronium ions (H₃O⁺), but for IGCSE purposes we refer to them as H⁺ ions.

Common acids in the IGCSE syllabus:

What is a Base?

A base is a substance that accepts hydrogen ions (H⁺). An alkali is a soluble base that releases hydroxide ions (OH⁻) in water.

Common bases and alkalis:

Strong vs Weak Acids

This is a very common exam question (appears in most papers):

• Strong acids — fully dissociate (ionise completely) in water. Every molecule releases its H⁺.

  • HCl → H⁺ + Cl⁻ (all HCl molecules split)
  • H₂SO₄ → 2H⁺ + SO₄²⁻
  • HNO₃ → H⁺ + NO₃⁻

• Weak acids — partially dissociate in water. Only some molecules release H⁺; an equilibrium is established.

  • CH₃COOH ⇌ H⁺ + CH₃COO⁻
  • H₂CO₃ ⇌ H⁺ + HCO₃⁻

Key exam point: At the same concentration, a strong acid has a lower pH (more H⁺ ions) than a weak acid. The strong acid also reacts faster with metals/carbonates and has higher electrical conductivity.

Basicity of Acids

Basicity = the number of H⁺ ions one molecule of acid can donate.

• HCl → monobasic (1 H⁺)
• H₂SO₄ → dibasic (2 H⁺)
• H₃PO₄ → tribasic (3 H⁺)

Concentrated vs Dilute

• Concentrated — a lot of solute per volume of solvent
• Dilute — a small amount of solute per volume
• Concentration is about how much acid; strength is about how much it dissociates
• You can have a concentrated weak acid, or a dilute strong acid

Reactions of Acids

Acids react in predictable patterns. These 4 reaction types are the core of IGCSE Acids & Bases:

1. Acid + Metal → Salt + Hydrogen

• Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
• Test for H₂: lighted splint → squeaky pop

2. Acid + Base/Metal Oxide → Salt + Water

• CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
• This is neutralisation when the base is soluble (an alkali)

3. Acid + Carbonate → Salt + Water + Carbon Dioxide

• CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
• Test for CO₂: bubble through limewater → turns milky/cloudy

4. Acid + Alkali → Salt + Water (Neutralisation)

• HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
• Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)

The pH Scale

• pH 0–6: acidic (lower = stronger/more concentrated acid)
• pH 7: neutral (pure water)
• pH 8–14: alkaline (higher = stronger/more concentrated alkali)
• pH is a logarithmic scale: pH 3 is 10× more acidic than pH 4
• Measured with universal indicator, pH meter, or pH paper

Key Concepts from Past Papers

Definitions You MUST Know

• Acid: a substance that donates H⁺ ions / a proton donor
• Base: a substance that accepts H⁺ ions / a proton acceptor
• Alkali: a soluble base that releases OH⁻ ions in water
• Strong acid: an acid that completely/full dissociates (ionises) in water
• Weak acid: an acid that partially dissociates (ionises) in water
• Neutralisation: the reaction of H⁺ ions with OH⁻ ions to form water

Recurring Mark Scheme Answers

From real 0620/0971 mark schemes — the exact wording that scores marks:

• “Strong acid completely ionises/dissociates; weak acid partially ionises/dissociates”
• “At the same concentration, the strong acid has a lower pH / more H⁺ ions”
• “The strong acid reacts faster with magnesium / produces the same volume of hydrogen more quickly”
• “Add universal indicator and compare the colour to a colour chart”
• “Ethanoic acid is a weak acid; hydrochloric acid is a strong acid”

Common Mistakes

• Confusing strong/concentrated: “Strong” refers to dissociation; “concentrated” refers to amount. They are different concepts.
• Saying acid ‘donates electrons’: Acids donate H⁺ ions, not electrons (that’s redox).
• Forgetting state symbols: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
• Writing H⁺ instead of H₃O⁺: IGCSE accepts H⁺(aq) — it’s simpler and what examiners expect.

Common Question Types

Type 1: Define / Explain Strong vs Weak Acids

• Frequency: Appears in ~70% of papers
• Marks: Usually 2 marks
• Model answer: “[Strong/Weak] acid [completely/partially] dissociates/ionises in water”
• Second mark: “So there are [more/fewer] H⁺ ions in solution”

Type 2: Write the Ionic Equation for Neutralisation

• Frequency: ~50% of papers
• Answer: H⁺(aq) + OH⁻(aq) → H₂O(l)
• Exam tip: This is ALWAYS the same. Memorise it.

Type 3: Describe a Reaction of an Acid

• Frequency: ~60% of papers
• Marks: 2–4 marks
• Expect: Write balanced equation, name the salt, describe observations (fizzing/bubbles, solid dissolving, temperature change)

Type 4: Identify the Salt Formed

• Frequency: ~40% of papers
• Method: Salt name = metal from the base + acid ending:
  • Hydrochloric acid → …chloride
  • Sulfuric acid → …sulfate
  • Nitric acid → …nitrate
  • Ethanoic acid → …ethanoate

Key Facts to Memorize

• H⁺(aq) + OH⁻(aq) → H₂O(l) — the ionic equation for ALL neutralisation reactions
• HCl, H₂SO₄, HNO₃ are strong; CH₃COOH (ethanoic) and H₂CO₃ (carbonic) are weak
• pH < 7 = acidic, pH 7 = neutral, pH > 7 = alkaline
• Acid + Metal → Salt + H₂ (squeaky pop test)
• Acid + Carbonate → Salt + H₂O + CO₂ (limewater turns milky)
• Metals below hydrogen in the reactivity series do NOT react with acids
• Nitric acid produces nitrates, NOT hydrogen gas when reacting with metals (it’s an oxidising agent)

Related Notes

• Indicators and pH — pH scale, universal indicator, litmus, phenolphthalein, methyl orange
• Making Salts — Preparing soluble and insoluble salts
• Neutralization — Applications and titration method
• Qualitative Analysis — Testing for cations, anions, and gases
• Solubility Rules — Which salts are soluble/insoluble
• Redox Reactions — Distinguishing acid-base from redox reactions
• IGCSE-Chem-Index — Full IGCSE Chemistry index

Sources

• OpenStax Chemistry 2e — [Chapter 14: Acid-Base Equilibria], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Acids and Alkalis], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 7: Acids, Bases and Salts, Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 21: Acids and Bases], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

This topic appears in 49 papers with 126 Q+A entries. The most representative questions:

• 0620/32 Feb/March 2018 Q8(b)(ii): Making ethanol — acid catalyst required (3 marks)
• 0620/32 May/June 2018 Q3(b): Strong vs weak acid definition (2 marks)
• 0620/33 May/June 2016 Q8(c)(ii): Tick-box identifying acid-metal oxide reaction (1 mark)
• 0620/33 October/November 2015 Q5(a)(ii): Strong vs weak acid explanation (1 mark)
• 0971/32 October/November 2023 Q6(c)(ii): Identifying neutralisation reaction (1 mark)
• 0620/32 Feb/March 2022 Q8(d)(ii): Halide test after acidification (2 marks)