From proton donors to the pH scale, salt preparation to volumetric analysis — master the chemistry that runs from your stomach to your toothpaste.
Section 1 — Properties & Reactions of Acids
Common Acids — Quick Reference
| Name | Formula | Type | Strength | Basicity |
|---|---|---|---|---|
| Hydrochloric acid | HCl | Inorganic | Strong | Monobasic (1 H⁺) |
| Sulfuric acid | H₂SO₄ | Inorganic | Strong | Dibasic (2 H⁺) |
| Nitric acid | HNO₃ | Inorganic | Strong | Monobasic (1 H⁺) |
| Phosphoric acid | H₃PO₄ | Inorganic | Moderate | Tribasic (3 H⁺) |
| Ethanoic acid | CH₃COOH | Organic | Weak | Monobasic (1 H⁺) |
| Carbonic acid | H₂CO₃ | Inorganic | Weak | Dibasic (2 H⁺) |
| Citric acid | C₆H₈O₇ | Organic | Weak | Tribasic |
Chemical Reactions of Acids
Only metals above hydrogen in the reactivity series react. Nitric acid is the exception — it produces oxides of nitrogen instead of H₂.
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
Ionic: Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)
🧪 Test for H₂: Burns with a squeaky pop (burning splint test)
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)
K₂CO₃(aq) + 2HNO₃(aq) → 2KNO₃(aq) + CO₂(g) + H₂O(l)
Ionic: CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
🧪 Test for CO₂: Turns limewater milky white
Ca(HCO₃)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2CO₂(g) + 2H₂O(l)
Ionic: HCO₃⁻(aq) + H⁺(aq) → CO₂(g) + H₂O(l)
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Ionic: OH⁻(aq) + H⁺(aq) → H₂O(l) ← ALL neutralisations!
Section 2 — Properties & Reactions of Bases
Amphoteric Oxides — Reacting with Both Acids AND Alkalis
Some metal oxides and hydroxides can act as BOTH a base (reacting with acids) AND as an acid (reacting with strong alkalis). These are called amphoteric.
| Substance | With Acid (acts as base) | With NaOH (acts as acid) |
|---|---|---|
| ZnO / Zn(OH)₂ | ZnO + 2HCl → ZnCl₂ + H₂O | 2NaOH + ZnO → Na₂ZnO₂ + H₂O (zincate) |
| Al₂O₃ / Al(OH)₃ | Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O | NaOH + Al(OH)₃ → NaAlO₂ + 2H₂O (aluminate) |
| PbO / Pb(OH)₂ | PbO + 2HCl → PbCl₂ + H₂O | 2NaOH + PbO → Na₂PbO₂ + H₂O (plumbate) |
Classification of Oxides
| Type | What it is | Examples | Reactions |
|---|---|---|---|
| Acidic oxide | Oxide of a non-metal | CO₂, SO₂, SO₃, NO₂ | Reacts with alkalis; dissolves in water to form acid |
| Basic oxide | Oxide of a metal | MgO, Fe₂O₃, CuO | Reacts with acids; some react with water to form alkali |
| Amphoteric oxide | Certain metals | Al₂O₃, ZnO, PbO | Reacts with BOTH acids AND strong alkalis |
| Neutral oxide | Certain non-metals | CO, NO, N₂O | Does NOT react with acids or alkalis |
Section 3 — Strength of Acids & Alkalis: The pH Scale
Weak acid: Only PARTIALLY ionised. Only SOME molecules produce H⁺ ions. Low [H⁺]. Examples: CH₃COOH, H₂CO₃.
⚠️ Do NOT confuse strength with concentration! A dilute strong acid still has all its molecules ionised; a concentrated weak acid still has only some ionised.
← ACIDIC (pH < 7) NEUTRAL (pH = 7) ALKALINE (pH > 7) →
| Solution | Approx pH | Classification |
|---|---|---|
| Concentrated HCl / strong acids | 0–1 | Very strongly acidic |
| Lemon juice / vinegar | 2–3 | Acidic |
| Ethanoic acid (dilute) | 4–5 | Weakly acidic |
| Pure water / distilled water | 7 | Neutral |
| Sea water / baking soda | 8–9 | Weakly alkaline |
| Aqueous ammonia | 9–10 | Weakly alkaline |
| NaOH / oven cleaner | 13–14 | Very strongly alkaline |
Indicators — Colour Reference
| Indicator | Colour in Acid | Colour in Alkali | Notes |
|---|---|---|---|
| Litmus | Red | Blue | Cannot estimate exact pH |
| Methyl orange | Red | Yellow | Best for strong acid–weak base titrations |
| Phenolphthalein | Colourless | Pink | Best for weak acid–strong base titrations |
| Bromothymol blue | Yellow | Blue | Good for near-neutral solutions |
| Universal indicator | Red/orange/yellow | Blue/violet | Can estimate actual pH value |
Section 4 — Salts
Both H⁺ ions replaced → Na₂SO₄ is a normal salt (sodium sulfate)
Only one H⁺ replaced → NaHSO₄ is an acid salt (sodium hydrogensulfate)
CuSO₄·5H₂O = hydrated copper(II) sulfate (blue crystals) → heat → CuSO₄ (white powder)
MgSO₄·7H₂O = Epsom salt (magnesium sulfate) · CoCl₂·6H₂O = hydrated cobalt(II) chloride (pink/blue)
Preparing Salts — Choosing the Right Method
| Method | Salts Prepared | Reactants | Key Steps |
|---|---|---|---|
| Ionic Precipitation | Insoluble salts (BaSO₄, PbCl₂) | Two soluble salts (one with cation, one with anion) | Mix → precipitate forms → filter → wash → dry |
| Titration | K⁺, Na⁺, NH₄⁺ salts | Alkali/carbonate + acid | Titrate to find exact volume → repeat without indicator → evaporate |
| Insoluble base + acid | Soluble salts (not K/Na/NH₄) | Insoluble oxide/hydroxide + dilute acid | Add excess solid → filter off excess → evaporate/crystallise |
| Reactive metal + acid | Mg, Zn, Fe, Al salts | Metal + dilute acid | Add excess metal → filter → evaporate/crystallise |
| Direct combination | Anhydrous chlorides (AlCl₃, FeCl₃) | Metal + Cl₂ gas | Heat metal in chlorine stream (fume cupboard) |
Section 5 — Neutralisation & Everyday Applications
H⁺(aq) + OH⁻(aq) → H₂O(l)
💊 Antacids
Neutralise excess HCl in the stomach. Active ingredients: NaHCO₃, Mg(OH)₂, Al(OH)₃, MgCO₃, CaCO₃. Reaction with stomach acid is exothermic — that's why some fizz!
🌱 Soil Treatment
Lime (CaO or Ca(OH)₂) neutralises acidic soil. Warning: lime and ammonium fertiliser cannot be added together — they react to release ammonia gas (pungent smell + loss of nutrients).
🦷 Toothpaste
NaHCO₃ neutralises acid from bacteria. Fluoride ions replace OH⁻ in tooth enamel (hydroxyapatite → fluorapatite), making it harder and more acid-resistant.
Section 6 — Volumetric Analysis (Titration)
Titration Procedure — Step by Step
- 1Rinse the burette with the acid solution. Fill and record the initial reading.
- 2Rinse the pipette with the alkali. Pipette a fixed volume (e.g. 25.0 cm³) into a conical flask.
- 3Add 2–3 drops of indicator (phenolphthalein or methyl orange).
- 4Add acid dropwise from the burette, swirling constantly. Stop at the endpoint — indicator changes colour on one drop.
- 5Record final burette reading. Calculate volume used.
- 6Repeat until THREE concordant results (within 0.1 cm³ of each other). Average the concordant results.
Worked Example 1 — Find [HCl]
Worked Example 2 — Find Mass Conc
Section 7 — Resources & Simulations
Section 8 — CSEC Practice Questions
(A) A strong acid has a higher concentration than a weak acid
(B) A weak acid partially ionises in water while a strong acid fully ionises
(C) Vinegar is a strong acid because it has a sour taste
(D) Concentration and strength always mean the same thing
1 (A) is wrong — strength refers to degree of ionisation, not concentration. You can have a dilute strong acid.
2 (C) is wrong — vinegar (ethanoic acid) is a weak acid; taste doesn't determine strength.
3 (D) is wrong — strength and concentration are completely different properties. Concentration = amount dissolved. Strength = degree of ionisation.
(a) Magnesium ribbon reacting with dilute sulfuric acid
(b) Calcium carbonate reacting with dilute hydrochloric acid
(c) Copper(II) oxide reacting with dilute nitric acid
a Metal + acid → salt + hydrogen (Mg is above H in reactivity series):
Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)
b Metal carbonate + acid → salt + CO₂ + water:
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)
c Base (metal oxide) + acid → salt + water (neutralisation):
CuO(s) + 2HNO₃(aq) → Cu(NO₃)₂(aq) + H₂O(l)
1 A substance that reacts with BOTH acids and strong alkalis is called amphoteric.
2 With HCl (acting as a base): ZnO + 2HCl → ZnCl₂ + H₂O
3 With NaOH (acting as an acid): 2NaOH + ZnO → Na₂ZnO₂ + H₂O (forming sodium zincate)
1 Balanced equation: 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) [ratio 2:1]
2 n(NaOH) = 0.4 × (30.0/1000) = 0.012 mol
3 Ratio NaOH:H₂SO₄ = 2:1, so n(H₂SO₄) = 0.012/2 = 0.006 mol
a [H₂SO₄] = 0.006 / (20.0/1000) = 0.3 mol dm⁻³
b M(H₂SO₄) = 2+32+(4×16) = 98 g/mol. Mass conc = 0.3 × 98 = 29.4 g dm⁻³
1 Warm dilute sulfuric acid in a beaker. Slowly add excess copper(II) oxide (CuO) powder to the acid, stirring constantly. The black CuO dissolves to form a blue solution. Excess CuO ensures ALL the acid is used up.
2 To confirm all acid is used up, dip a piece of blue litmus paper into the solution — it should remain blue (no acid left).
3 Filter the hot mixture to remove excess undissolved CuO (the residue). Collect the blue filtrate (copper(II) sulfate solution).
4 Evaporate the filtrate gently until crystals begin to form (do not evaporate to complete dryness — this would destroy crystals).
5 Allow to cool slowly — crystals of CuSO₄·5H₂O form. Filter off the crystals and dry between filter papers.