Overview
What you'll learn
Define acids, bases and alkalis. Tell strong from weak, concentration from strength, and monobasic from di- and tribasic. Then recall the three reactions of an acid.
Use the pH scale. Read pH 0–14, pick the right indicator for a titration, and interpret pH curves.
Prepare salts. Apply the solubility rules, then pick the right method — titration, acid + excess insoluble reactant, or precipitation.
Describe the Haber process. Recall the raw materials, conditions and catalyst, and explain how yield is balanced against rate and cost.
Tutor's Insight
"Big topic. Big payoff."
Acids, bases and salts run through every paper — worth roughly 20% of Paper 2 in an average year, and all three subtopics turn up as MCQ, structured and practical questions. pH and indicators, neutralisation, salt preparation, the Haber process, and questions that mix them. There's no cheap shortcut here. The marks belong to people who've drilled the methods until they're automatic.
5.1 Acids and Bases
5.1 Acids and Bases
What this subtopic asks of you
- Define and recognise acids, bases and alkalis — strong vs weak, concentration vs strength, basicity, and the three reactions of an acid.
- Use the pH scale and pick the right indicator — read pH curves from titrations, then classify oxides as basic, acidic, amphoteric or neutral.
5.1 Acids and Bases
An acid releases H⁺ in water
Definition
Acid
A substance that ionises in water to give H⁺ ions and an anion.
e.g. HCl → H⁺ + Cl⁻
e.g. HCl → H⁺ + Cl⁻
Strong vs Weak
How much it ionises
Strong — fully ionised in water (HCl, HNO₃, H₂SO₄).
Weak — partly ionised (HF, ethanoic CH₃COOH, carbonic H₂CO₃).
Weak — partly ionised (HF, ethanoic CH₃COOH, carbonic H₂CO₃).
Strength ≠ Concentration
Two different ideas
Concentration — how many particles per dm³.
Strength — the degree of ionisation. You can have dilute strong acid or concentrated weak acid.
Strength — the degree of ionisation. You can have dilute strong acid or concentrated weak acid.
Basicity — count the H⁺ one molecule can release. Monobasic = 1 H⁺ (HCl, HNO₃); dibasic = 2 H⁺ (H₂SO₄, H₂CO₃); tribasic = 3 H⁺ (H₃PO₄).
Free Notes · O-Level Pure Chemistry
Read the full chapter
The three reactions of an acid, bases vs alkalis, the pH scale and indicators, oxide families, solubility rules, salt preparation, the Haber process and 4 worked exam questions.
5.1 Acids and Bases
Every acid reacts in three ways
Reaction 01
With a reactive metal
acid + metal → salt + H₂
e.g. 2HCl + Mg → MgCl₂ + H₂↑
Works for metals above hydrogen in the reactivity series.
e.g. 2HCl + Mg → MgCl₂ + H₂↑
Works for metals above hydrogen in the reactivity series.
Reaction 02
With a base / alkali
acid + base → salt + water
e.g. HCl + NaOH → NaCl + H₂O
This is neutralisation — H⁺ + OH⁻ → H₂O.
e.g. HCl + NaOH → NaCl + H₂O
This is neutralisation — H⁺ + OH⁻ → H₂O.
Reaction 03
With a carbonate
acid + carbonate → salt + water + CO₂
e.g. 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑
The gas turns limewater milky — the test for carbonates.
e.g. 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑
The gas turns limewater milky — the test for carbonates.
Learn the three product patterns first — salt + H₂, salt + water, salt + water + CO₂ — then the balancing falls into place.
5.1 Acids and Bases
Every alkali is a base — but not every base is an alkali
Base
A proton acceptor
A metal oxide or metal hydroxide that reacts with an acid to give salt + water.
e.g. CuO, Fe(OH)₃, MgO
e.g. CuO, Fe(OH)₃, MgO
Alkali
A soluble base
A base that dissolves in water to release OH⁻ ions.
Strong: NaOH, KOH (fully ionised).
Weak: NH₃ in water (partly ionised).
Strong: NaOH, KOH (fully ionised).
Weak: NH₃ in water (partly ionised).
Exam Habit · The Ammonium Test
Alkali + ammonium salt → salt + water + NH₃↑ e.g. 2NaOH + (NH₄)₂SO₄ → Na₂SO₄ + 2H₂O + 2NH₃↑. Heat the mixture; the ammonia gas turns moist red litmus blue. This is the standard test for an ammonium ion (NH₄⁺).
5.1 Acids and Bases
A scale — and a way to read it
Acidic · pH 0–6
→
Neutral · pH 7
→
Alkaline · pH 8–14
| Indicator | Colour in acid | Colour in alkali | Transition pH |
|---|---|---|---|
| Litmus | red | blue | — |
| Methyl orange | red | yellow | 3.1 – 4.4 |
| Bromothymol blue | yellow | blue | 6.0 – 7.6 |
| Phenolphthalein | colourless | pink | 8.3 – 10.0 |
| Thymolphthalein | colourless | blue | 9.3 – 10.5 |
5.1 Acids and Bases
Pick the indicator from the curve
The Procedure
Titration — in five steps
1. Pipette a known volume of alkali into a conical flask; add 2–3 drops of indicator.
2. Fill a burette with acid; record the initial reading.
3. Add acid, swirling, until the indicator just changes colour (end-point).
4. Note the final burette reading; repeat for concordant titres.
5. Repeat without indicator using the average titre to make a clean salt.
2. Fill a burette with acid; record the initial reading.
3. Add acid, swirling, until the indicator just changes colour (end-point).
4. Note the final burette reading; repeat for concordant titres.
5. Repeat without indicator using the average titre to make a clean salt.
Equivalence pH
Match the indicator to the pair
Strong acid + strong alkali → pH ≈ 7 — any indicator works (methyl orange, BTB, phenolphthalein).
Strong acid + weak alkali → pH ≈ 5 — use methyl orange.
Weak acid + strong alkali → pH ≈ 9 — use phenolphthalein.
Weak acid + weak alkali → no sharp jump — titration not used.
Strong acid + weak alkali → pH ≈ 5 — use methyl orange.
Weak acid + strong alkali → pH ≈ 9 — use phenolphthalein.
Weak acid + weak alkali → no sharp jump — titration not used.
5.1 Acids and Bases
Four families of oxide
Basic Oxide
Metal + oxygen
Reacts with acid → salt + water.
e.g. Na₂O, CaO, CuO, MgO
e.g. Na₂O, CaO, CuO, MgO
Acidic Oxide
Non-metal + oxygen
Reacts with alkali → salt + water.
e.g. CO₂, SO₂, SO₃, P₄O₁₀
e.g. CO₂, SO₂, SO₃, P₄O₁₀
Amphoteric
Both worlds at once
Reacts with both acids and alkalis.
e.g. Al₂O₃, ZnO, PbO
e.g. Al₂O₃, ZnO, PbO
Neutral
Won't react with either
Insoluble in water; no acid- or alkali-reaction.
e.g. CO, NO, H₂O
e.g. CO, NO, H₂O
Quick check: basic vs acidic follows from metal vs non-metal. The amphoteric oxides are the named few — Al₂O₃, ZnO, PbO.
5.2 Salts
5.2 Salts
What this subtopic asks of you
- Predict whether a salt is soluble — apply solubility rules to common chlorides, sulfates, nitrates, carbonates and hydroxides.
- Choose and carry out the right preparation — titration, acid + excess insoluble reactant, or precipitation.
5.2 Salts
Five rules tell you if a salt dissolves
| Anion family | Rule | Example / note |
|---|---|---|
| Sodium · Potassium · Ammonium salts | all soluble | SPA |
| Nitrates (NO₃⁻) | all soluble | — |
| Chlorides (Cl⁻) | soluble — except Pb²⁺, Ag⁺ | PbCl₂, AgCl |
| Sulfates (SO₄²⁻) | soluble — except Ba²⁺, Ca²⁺, Pb²⁺ | BaSO₄, CaSO₄, PbSO₄ |
| Carbonates · Hydroxides | insoluble — except SPA salts | Na₂CO₃, NaOH soluble |
Memory hook: "SPA + nitrates always swim." Chlorides and sulfates are mostly soluble — learn the exceptions.
5.2 Salts
Start at the salt: soluble or not?
Method 01
Titration
When to use — soluble salt of Na, K or NH₄⁺ (or a nitrate).
Why — both reactants are soluble, so you must measure exactly how much to mix.
Reactants: acid + soluble alkali (e.g. HCl + NaOH → NaCl).
Why — both reactants are soluble, so you must measure exactly how much to mix.
Reactants: acid + soluble alkali (e.g. HCl + NaOH → NaCl).
Method 02
Acid + excess insoluble reactant
When to use — soluble salt of any other metal.
Why — the insoluble reactant simply stops dissolving when the acid is used up. No indicator needed.
Reactants: acid + insoluble base, carbonate, or reactive metal.
Why — the insoluble reactant simply stops dissolving when the acid is used up. No indicator needed.
Reactants: acid + insoluble base, carbonate, or reactive metal.
Method 03
Precipitation
When to use — insoluble salt.
Why — the salt drops out of solution as soon as the two ions meet.
Reactants: two soluble salts — one supplies the cation, one supplies the anion.
Why — the salt drops out of solution as soon as the two ions meet.
Reactants: two soluble salts — one supplies the cation, one supplies the anion.
5.2 Salts
Each method has a clean procedure
Titration · Steps
Find the volume, then make it clean
1. Pipette the alkali into a flask; add indicator.
2. Titrate with acid until the colour just changes.
3. Repeat — without indicator — using the average titre.
4. Evaporate to saturation, cool, crystallise, dry.
2. Titrate with acid until the colour just changes.
3. Repeat — without indicator — using the average titre.
4. Evaporate to saturation, cool, crystallise, dry.
Acid + Excess · Steps
Add solid until no more dissolves
1. Warm dilute acid in a beaker.
2. Add the insoluble reactant a little at a time, stirring, until excess.
3. Filter — the excess solid is the residue; salt solution is the filtrate.
4. Evaporate, crystallise, dry between filter papers.
2. Add the insoluble reactant a little at a time, stirring, until excess.
3. Filter — the excess solid is the residue; salt solution is the filtrate.
4. Evaporate, crystallise, dry between filter papers.
Precipitation · Steps
Mix, filter, wash, dry
1. Mix two soluble salt solutions in a beaker.
2. Filter — the precipitate is the residue.
3. Wash the residue with distilled water to remove ions.
4. Dry between filter papers.
2. Filter — the precipitate is the residue.
3. Wash the residue with distilled water to remove ions.
4. Dry between filter papers.
5.3 Ammonia
5.3 Ammonia
What this subtopic asks of you
- Describe the Haber process — raw materials, conditions, catalyst and equation. Explain how yield is balanced against rate and cost.
- Reason about reversible reactions — how pressure, temperature and concentration shift a dynamic equilibrium.
5.3 Ammonia · Haber Process
Pull nitrogen out of the air
The Reaction
N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
Reversible — at equilibrium the forward and backward rates are equal, but neither stops.
ΔH = −92 kJ/mol (exothermic)
ΔH = −92 kJ/mol (exothermic)
Raw Materials
Nitrogen and hydrogen
N₂ — from the fractional distillation of liquid air.
H₂ — from the cracking of crude oil fractions, or from natural gas.
H₂ — from the cracking of crude oil fractions, or from natural gas.
Conditions
450 °C · 200 atm · iron catalyst
A compromise temperature (higher T pushes equilibrium back, lower T is too slow), high pressure (favours NH₃), and a finely-divided iron catalyst to speed it up.
Yield Trade-offs
Pressure up, temperature down
Higher pressure → more NH₃ but a stronger (and more expensive) plant. Lower temperature → more NH₃ but a slower reaction. The named conditions are the chosen balance.
Why it matters
The ammonia from the Haber process is the raw material for nitrogen fertilisers, feeding a large share of the world's crops. It is the textbook example of how industry balances yield, rate and cost against the position of a dynamic equilibrium.
Practice
Exam-style questions
Question 01 · AMKSS 2024 Prelim
The Question
MCQ
Adding salt to acid
The pH of a sample of dilute hydrochloric acid is 2. What is the pH of the acid after the addition of 10 g of sodium chloride salt?
Worked Answer
B — pH 2
- pH depends on the concentration of H⁺ ions in solution.
- NaCl is the salt of a strong acid and strong alkali — it dissolves but does not react with HCl.
- No new H⁺ is added and none is removed → [H⁺] is unchanged.
- So the pH stays at 2 — answer B.
Question 02 · AMKSS 2024 Prelim
The Question
MCQ
Picking the right reactants
Which set of reactants is most appropriate to prepare a pure sample of copper(II) chloride crystals?
Worked Answer
B
- A — copper is below hydrogen, so it does not react with dilute HCl.
- B — CuCO₃ is insoluble. Add excess to warm HCl; filter off the excess; evaporate the filtrate; crystallise. Classic acid + insoluble carbonate.
- C — NH₄Cl is a salt, not an acid, so it would not give clean CuCl₂.
- D — both Cu(NO₃)₂ and KCl are soluble, and CuCl₂ is also soluble, so no precipitate forms. Answer: B.
Question 03 · AMKSS 2024 Prelim
The Question
MCQ
Tuning the Haber yield
In the Haber process, nitrogen and hydrogen react to form ammonia:
N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g), ΔH = −92 kJ/mol
Which change will increase the yield of ammonia?
Worked Answer
A
- Higher pressure favours the side with fewer gas moles — 2 mol NH₃ vs 4 mol on the left. Yield ↑.
- The forward reaction is exothermic, so a higher temperature shifts the equilibrium back. Yield ↓.
- The stoichiometric ratio is 1 : 3, not 1 : 1, so equal volumes do not help.
- A catalyst speeds the rate but does not shift the equilibrium position — yield unchanged. Answer A.
Question 04
The Question
Structured
Choosing an indicator
25.0 cm³ of dilute ethanoic acid (a weak acid) is titrated against aqueous sodium hydroxide (a strong alkali).
(a) State the approximate pH at the equivalence point. (b) Name a suitable indicator, and explain why methyl orange would not be suitable.
Worked Answer
pH ≈ 9 · phenolphthalein
- A weak acid with a strong alkali gives a salt (sodium ethanoate) that is slightly alkaline, so the equivalence pH is about 9.
- Phenolphthalein changes colour over pH 8.3–10.0, which straddles the equivalence point, so it marks the end-point sharply.
- Methyl orange changes over pH 3.1–4.4 — far below the equivalence pH — so it would change colour too early and give an inaccurate titre.
- Suitable indicator: phenolphthalein.
Frequently Asked Questions
Acid-Base Chemistry — FAQ
What is the difference between the strength and the concentration of an acid?
Strength is the degree of ionisation — a strong acid (HCl, HNO₃, H₂SO₄) ionises fully in water, while a weak acid (ethanoic acid CH₃COOH, carbonic acid H₂CO₃) only ionises partly. Concentration is how many acid particles there are per dm³. They are independent, so you can have a dilute strong acid or a concentrated weak acid.
What are the three reactions of an acid?
Acid + reactive metal → salt + hydrogen (e.g. 2HCl + Mg → MgCl₂ + H₂). Acid + base/alkali → salt + water (neutralisation, H⁺ + OH⁻ → H₂O). Acid + carbonate → salt + water + carbon dioxide (e.g. 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂), where the CO₂ turns limewater milky.
What is the difference between a base and an alkali?
A base is a metal oxide or metal hydroxide (a proton acceptor) that reacts with an acid to give salt + water, e.g. CuO, MgO, Fe(OH)₃. An alkali is a soluble base — a base that dissolves in water to release OH⁻ ions, e.g. NaOH, KOH and aqueous ammonia. So every alkali is a base, but not every base is an alkali.
How do you choose the right indicator for a titration?
Match the indicator to the equivalence pH of the pair. Strong acid + strong alkali gives pH ≈ 7, so any indicator works. Strong acid + weak alkali gives pH ≈ 5, so use methyl orange. Weak acid + strong alkali gives pH ≈ 9, so use phenolphthalein. A weak acid + weak alkali has no sharp pH jump, so titration is not used.
How do you decide which method to use to prepare a salt?
Start with the salt's solubility. For a soluble salt of Na, K or NH₄⁺ (or a nitrate) use titration. For a soluble salt of any other metal use acid + excess insoluble reactant (base, carbonate or reactive metal), then filter off the excess. For an insoluble salt use precipitation — mix two soluble salt solutions so the salt drops out, then filter, wash and dry.
What conditions are used in the Haber process and why?
N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ/mol. The chosen conditions are 450 °C, 200 atm and a finely-divided iron catalyst. High pressure favours ammonia (the side with fewer gas moles). Low temperature favours the exothermic forward reaction but is too slow, so 450 °C is a compromise between yield and rate. The catalyst speeds the reaction but does not change the equilibrium position.
O-Level Pure Chemistry · Syllabus 6092 · Topic 5 of 12 · © 2026 Overmugged. For personal study use only.