Overview
What you’ll learn
- Describe acids as proton donors in solution and recall the characteristic reactions of acids with metals, carbonates and bases.
- Write balanced equations (full and ionic) for the three acid reaction types, with state symbols.
- Distinguish strong acids from weak acids by degree of ionisation, and relate concentration to strength correctly.
- Use the pH scale and indicators to classify solutions as acidic, neutral or alkaline.
- Describe neutralisation as H⁺ reacting with OH⁻ to form water, and apply it to antacids, insect stings and soil treatment.
- Classify oxides as acidic, basic, amphoteric or neutral, with examples and the reactions of amphoteric oxides with both acid and alkali.
In 30 seconds
- 1Acids produce H⁺(aq) ions in water; alkalis produce OH⁻(aq). H⁺ + OH⁻ → H₂O is neutralisation.
- 2Three acid reactions to memorise: acid + metal → salt + hydrogen, acid + carbonate → salt + water + carbon dioxide, acid + base → salt + water only.
- 3Strong acids (HCl, H₂SO₄, HNO₃) ionise completely in water; weak acids (ethanoic, citric, carbonic) ionise only partially — strength ≠ concentration.
- 4The pH scale runs 0–14: pH < 7 acidic, pH 7 neutral, pH > 7 alkaline; one pH step is a ten-fold change in H⁺ concentration.
- 5pH 1 hydrochloric acid and pH 1 ethanoic acid have the same pH, but the ethanoic acid reacts far more slowly (fewer H⁺ ions actually free).
- 6A basic oxide neutralises acid (CuO), an acidic oxide dissolves to give acid (CO₂ → H₂CO₃), an amphoteric oxide does both (Al₂O₃, ZnO), and neutral oxides do neither (CO, H₂O, NO).
- 7Neutralisation in action: calcium hydroxide treats acidic soil; magnesium hydroxide antacid treats excess stomach HCl; a weak alkali (sodium hydrogencarbonate) soothes bee stings.
- 8Salt + water + carbon dioxide is the signature of a carbonate; salt + hydrogen is the signature of a reactive metal (never use reactive Na, K with acid).
Notes · 1 of 5
Acids and their three reactions
Acids in water release H⁺; the three families of reactions all deliver that proton.
An acid is a substance that produces hydrogen ions, H⁺(aq), when dissolved in water. Every "acidic" property — sour taste, turning litmus red, reacting with metals and carbonates — comes from that ion. A salt is the compound formed when the H⁺ of an acid is replaced by a metal ion (or ammonium); hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.
| Reactant | Products | Example | Test for the gas |
|---|---|---|---|
| Reactive metal | Salt + hydrogen | Zn + 2HCl → ZnCl₂ + H₂ | Lighted splint pops |
| Carbonate | Salt + water + carbon dioxide | CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ | Limewater turns milky |
| Base (incl. alkali) | Salt + water only | CuO + 2HCl → CuCl₂ + H₂O | — (no gas) |
Notes · 2 of 5
Strong vs weak acids, and the pH scale
Strength is about complete ionisation; concentration is about how much is dissolved.
A strong acid ionises completely in water: every HCl molecule releases its H⁺. A weak acid, like ethanoic acid in vinegar or citric acid in lemon juice, ionises only partially — most molecules stay whole, so fewer H⁺ ions swim free. Strength describes the degree of ionisation, never the amount dissolved.
| Pair | Difference |
|---|---|
| 1.0 mol/dm³ HCl (strong) vs 1.0 mol/dm³ ethanoic acid (weak) | Same concentration, different strength: the HCl has many more free H⁺ and reacts much faster |
| 0.1 mol/dm³ HCl vs 1.0 mol/dm³ HCl | Same strength (both fully ionised), different concentration |
| pH 1 HCl vs pH 1 ethanoic acid | Same pH (same free H⁺ concentration), but the weak acid holds far more un-ionised acid in reserve |
The pH scale measures the concentration of free H⁺(aq): pH 0–6 acidic, 7 neutral, 8–14 alkaline. Each step of pH is a factor of ten: pH 2 has ten times the H⁺ concentration of pH 3. Indicators translate pH into colour — litmus (red in acid, blue in alkali), universal indicator (a full rainbow from red to purple), methyl orange and phenolphthalein for titrations.
Notes · 3 of 5
Bases, alkalis and neutralisation
Bases neutralise acids; soluble bases are alkalis; the net ionic story is one proton meeting one hydroxide.
A base is any substance that neutralises an acid — metal oxides and hydroxides are the usual suspects (CuO, MgO, NaOH). A base that dissolves in water is an alkali, releasing hydroxide ions: NaOH(aq) → Na⁺(aq) + OH⁻(aq). All alkalis are bases, but not all bases dissolve.
Neutralisation, in ionic shorthand, is H⁺(aq) + OH⁻(aq) → H₂O(l) — the spectator ions (Na⁺ and Cl⁻ from hydrochloric acid and sodium hydroxide) sit out the reaction. With an insoluble base like copper(II) oxide the acid’s H⁺ is simply consumed until the base is in excess; the warmed solution then holds the salt.
| Problem | Treatment | Equation in words |
|---|---|---|
| Excess stomach acid (indigestion) | Magnesium hydroxide antacid | Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O |
| Bee sting (acidic) | Weak alkali, e.g. sodium hydrogencarbonate paste | NaHCO₃ neutralises the acid gently |
| Wasp sting (alkaline) | Weak acid, e.g. dilute vinegar | Acetic acid neutralises the alkali |
| Acidic farmland | Calcium hydroxide (slaked lime) | Ca(OH)₂ neutralises soil acids |
| Acidic gas emissions | Flue-gas scrubbing with calcium oxide | CaO + SO₂ → CaSO₃ |
Notes · 4 of 5
Strong and weak alkalis
The same complete-vs-partial ionisation story runs on the hydroxide side of the fence.
Alkalis split the same way acids do. A strong alkali ionises completely in water — aqueous sodium hydroxide and potassium hydroxide release all their OH⁻. A weak alkali such as aqueous ammonia ionises only partially: most of the dissolved ammonia stays as NH₃ molecules, releasing OH⁻ a little at a time. Calcium hydroxide is a special case — a strong alkali (whatever dissolves, ionises fully) but only slightly soluble, so its solutions are dilute.
| Alkali | Strength | Why |
|---|---|---|
| Aqueous sodium hydroxide, NaOH | Strong | Fully ionised in water |
| Aqueous potassium hydroxide, KOH | Strong | Fully ionised in water |
| Aqueous ammonia, NH₃ | Weak | Only a small fraction reacts with water to give OH⁻ |
| Calcium hydroxide, Ca(OH)₂ | Strong but sparingly soluble | What dissolves ionises fully; little dissolves |
Notes · 5 of 5
The four personalities of oxides
Acidic, basic, amphoteric, neutral — metals and non-metals behave differently on burning.
| Type | Behaviour | Examples |
|---|---|---|
| Acidic oxide | Non-metal oxide; dissolves in water to give an acid; reacts with bases | CO₂, SO₂, P₄O₁₀ |
| Basic oxide | Metal oxide; neutralises acid; some dissolve to give alkalis | CuO, MgO, Na₂O, CaO |
| Amphoteric oxide | Reacts with both acids and bases to give salt and water | Al₂O₃, ZnO, PbO |
| Neutral oxide | Neither acidic nor basic; no reaction with acids or bases | CO, H₂O, NO |
Dissolved carbon dioxide is why rain is naturally slightly acidic: CO₂ + H₂O → H₂CO₃ (carbonic acid, weak). Sulfur dioxide does the same far more aggressively — the seed of acid rain, fully developed in the air-quality chapter.
The test that sorts them: add the oxide to dilute hydrochloric acid (basic and amphoteric dissolve) and to dilute sodium hydroxide (acidic and amphoteric dissolve). Whatever dissolves in both is amphoteric; whatever dissolves in neither is neutral.
Revise
Key terms
The words examiners expect you to use precisely.
- Acid
- A substance that produces hydrogen ions, H⁺(aq), in water.
- Base
- A substance that neutralises an acid, typically a metal oxide or hydroxide.
- Alkali
- A soluble base that produces hydroxide ions, OH⁻(aq), in water.
- Salt
- The ionic compound formed when the H⁺ of an acid is replaced by a metal or ammonium ion.
- Strong acid
- An acid that ionises completely in water, e.g. HCl, H₂SO₄, HNO₃.
- Weak acid
- An acid that ionises only partially in water, e.g. ethanoic, citric and carbonic acids.
Show 6 more key termsShow fewer key terms
- pH scale
- A 0–14 scale of H⁺(aq) concentration: 7 neutral, below 7 acidic, above 7 alkaline.
- Neutralisation
- The reaction between an acid and a base: H⁺ + OH⁻ → H₂O, giving salt (and water).
- Acidic oxide
- A non-metal oxide that dissolves in water to form an acid or reacts with bases.
- Amphoteric oxide
- A metallic oxide that reacts with both acids and bases to form salt and water, e.g. Al₂O₃ and ZnO.
- Neutral oxide
- An oxide that reacts with neither acids nor bases, e.g. CO, NO, H₂O.
- Indicator
- A dye whose colour changes with pH, used to identify acids and alkalis or find end-points.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying all bases are alkalis.
Alkalis are the soluble minority; copper(II) oxide is a base that will not dissolve.
Instead: Every alkali is a base, but an insoluble base (CuO, Fe₂O₃) is not an alkali.
Mistake: Interchanging strength and concentration, e.g. calling dilute HCl a "weak acid".
Strength is the degree of ionisation; dilute HCl is still a strong acid.
Instead: Strong = fully ionised; concentrated = lots dissolved. Four combinations exist.
Show 10 more mistakesShow fewer mistakes
Mistake: Claiming ethanoic acid of pH 2 reacts slowly because it is "concentrated".
pH 2 fixes the free H⁺ concentration; the slow rate comes from partial ionisation in reserve.
Instead: Weak acids keep un-ionised molecules that release H⁺ only as the reaction proceeds.
Mistake: Writing acid + base → salt + hydrogen.
Hydrogen comes from metals, not bases.
Instead: Acid + base → salt + water only; the base consumes the H⁺.
Mistake: Using copper with dilute hydrochloric acid and expecting hydrogen.
Copper is below hydrogen in the reactivity series and does not displace it.
Instead: Only metals above hydrogen (Mg, Zn, Fe…) release hydrogen from dilute acids.
Mistake: Using sodium or potassium with acid in a planned experiment.
The reaction with the most reactive metals is explosively violent.
Instead: Choose a moderately reactive metal such as magnesium or zinc.
Mistake: Saying neutralisation always needs an alkali in solution.
Insoluble bases neutralise acids too — they just dissolve as they react.
Instead: CuO + 2HCl → CuCl₂ + H₂O is full neutralisation with a solid base.
Mistake: Treating carbon monoxide as an acidic oxide because it is a non-metal oxide.
CO does not react with bases or dissolve to give an acid.
Instead: CO (with NO and H₂O) is a neutral oxide; CO₂ and SO₂ are the acidic ones.
Mistake: Calling Al₂O₃ basic because it is a metal oxide.
Aluminium oxide reacts with acids and bases — that is amphoteric.
Instead: Show both reactions (with HCl and with NaOH) to classify it amphoteric.
Mistake: Writing the ionic equation of neutralisation as H₂ → H⁺ + H⁺.
Neutralisation pairs one H⁺ with one OH⁻.
Instead: H⁺(aq) + OH⁻(aq) → H₂O(l); charges balance at 0.
Mistake: Reading one pH unit as "one degree stronger".
The pH scale is logarithmic.
Instead: Each pH step is a ten-fold change in H⁺ concentration: pH 1 is 100× stronger than pH 3.
Mistake: Treating a bee sting with vinegar and a wasp sting with baking soda.
The treatments are swapped: bee stings are acidic, wasp stings alkaline.
Instead: Neutralise with the opposite type — weak alkali for bees, weak acid for wasps.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
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Define acids
An acid is defined in this course as a substance that
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
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12 questions worth 53 marks, each with a model answer, mark scheme and examiner tips.
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Chapter 6 Practice Paper
30 marks · 40 min · 4 pages
5 multiple-choice questions and structured questions with answer lines. Chapter 6 (acid reactions, strong vs weak acids, pH, neutralisation, oxides).
Mark scheme
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Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What does Sec 3 Chemistry Chapter 6 cover?
It covers sub-topic 3.1 of O-Level Pure Chemistry 6092: acids as H⁺ producers and their three reaction families (metals, carbonates, bases), strong vs weak acids, the pH scale and indicators, neutralisation and its everyday applications, and the classification of oxides as acidic, basic, amphoteric or neutral.
What is the fastest way to name a salt?
Take the cation from the other reactant (zinc, potassium, copper…) and the anion from the acid: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate. Zinc + hydrochloric acid gives zinc chloride; potassium hydroxide + nitric acid gives potassium nitrate.
Why does weak ethanoic acid still clean limescale?
Weak does not mean powerless — it means the acid ionises only partially, so H⁺ is released gradually. Given time (or warmth), the reserve of un-ionised acid keeps feeding the reaction until the carbonate deposit dissolves.
How do I remember the oxide classes?
Non-metal oxides are acidic (CO₂, SO₂), metal oxides basic (CuO, Na₂O), the three named metals’ oxides amphoteric (Al₂O₃, ZnO, PbO), and only CO, NO and H₂O are neutral. The test: shake with acid and with alkali — dissolves in both means amphoteric, in neither means neutral.
When is this chapter usually tested in school?
Acids and bases land at the end of Sec 3, so the EOY examination weighs them heavily — and titration calculations ride on this chapter all through Sec 4 and the O-Level papers. Scope varies by school, so check your school’s assessment outline.
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Chapter 7: Salts and Qualitative Analysis