Acids, Bases and Salts: Revision Notes

Condensed for the final weeks. For the full explanation, use the Acids, Bases and Salts study guide.

Definitions

(0620 Extended, 5070 required) The proton-donor/acceptor definitions below, and strong vs weak acids, go beyond 0620 Core.

• Acid — proton (H⁺) donor. In water, produces H⁺ ions.
• Base — proton acceptor. An alkali is a base that dissolves in water, producing OH⁻ — all alkalis are bases, but not all bases are alkalis, since many metal oxides and hydroxides are insoluble.
• Strong acid — fully dissociated in water (HCl, HNO₃, H₂SO₄).
• Weak acid — partially dissociated (ethanoic, citric, carbonic).

Strong and weak refer to degree of dissociation, never to concentration — a dilute strong acid and a concentrated weak acid are entirely different comparisons, and both combinations are possible. See the Acids, Bases and Salts study guide for the full syllabus coverage and worked reasoning behind every table above.

The three reactions of acids

acid + metal            -> salt + hydrogen
acid + base/alkali      -> salt + water            (neutralisation)
acid + carbonate        -> salt + water + carbon dioxide

Neutralisation ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l) — this single equation is what every acid-base neutralisation reduces to once spectator ions are removed, whatever the full molecular equation looks like.

Naming the salt: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.

Bases also react with ammonium salts, releasing ammonia gas on warming — a fourth reaction worth knowing alongside the three above, and a common way ammonia is generated and tested for in the lab (damp red litmus turns blue).

Indicators and pH

Indicator	Acid	Neutral	Alkali
Litmus	Red	Purple	Blue
Methyl orange	Red	Yellow	Yellow
Thymolphthalein	Colourless	Colourless	Blue
Universal indicator	Red–orange	Green	Blue–purple

pH < 7 acidic · pH 7 neutral · pH > 7 alkaline. Lower pH means higher H⁺ concentration, and each whole-number drop represents a tenfold increase in that concentration.

Oxides — the four types

Type	Reacts with	Example
Acidic	Bases	CO₂, SO₂, NO₂ (non-metal oxides)
Basic	Acids	CaO, MgO, Na₂O (metal oxides)
Amphoteric	Both acids and bases	Al₂O₃, ZnO
Neutral	Neither	CO, NO, H₂O

The amphoteric two — aluminium and zinc — are worth memorising as a set. (Lead(II) oxide, PbO, is also chemically amphoteric, but neither 0620 nor 5070 names it at this level — stick to Al₂O₃ and ZnO for the syllabus.) Acidic oxides are typically formed by non-metals and are gases at room temperature that dissolve to give acidic solutions; basic oxides are typically formed by metals — the metal/non-metal split is the quickest way to predict which category an unfamiliar oxide belongs to.

Choosing a salt preparation method

Is the salt SOLUBLE?
├── YES, and a suitable REACTIVE METAL is available
│      -> acid + excess metal, filter off unreacted metal, crystallise
├── YES, and the base is an INSOLUBLE metal oxide/hydroxide
│      -> acid + excess insoluble base, filter, crystallise
├── YES, and an INSOLUBLE CARBONATE is available
│      -> acid + excess insoluble carbonate, filter, crystallise
├── YES, and the base is SOLUBLE (alkali)
│      -> TITRATION with indicator, repeat without indicator
└── NO (insoluble salt)
       -> PRECIPITATION: mix two soluble solutions, filter, wash, dry
          *(0620 Extended, 5070 required)*

All four routes to a soluble salt — titration (acid + alkali), excess metal, excess insoluble base, and excess insoluble carbonate — use an excess of the solid reactant (metal, base or carbonate) where possible, so all the acid is used up; the leftover solid is then filtered off. Titration is used only when both reactants are soluble (acid + alkali), since there’s no excess solid to filter.

Solubility rules to know: all sodium, potassium, ammonium and nitrate salts are soluble. Most chlorides are soluble except silver and lead. Most sulfates are soluble except barium, calcium and lead. Most carbonates are insoluble except sodium, potassium and ammonium. Most hydroxides are insoluble except sodium, potassium, ammonium and (partially) calcium.

Hydrated vs anhydrous salts. A hydrated salt has water chemically combined within its crystal structure — written directly into the formula, e.g. CuSO₄·5H₂O (hydrated, blue). Heating drives this water off, leaving anhydrous CuSO₄ (white powder). Anhydrous copper(II) sulfate turning blue is the standard chemical test for the presence of water. (0620 Extended, 5070 required) This chemically-combined water is formally named the water of crystallisation.

Exam traps

• Don’t confuse strong/weak (dissociation) with concentrated/dilute (amount per volume).
• An alkali is a soluble base — all alkalis are bases, not all bases are alkalis.
• Excess insoluble base is used so all the acid reacts, then filtered off — say both halves.
• In a titration, the indicator must be removed by repeating without it, or the salt is contaminated.
• Amphoteric oxides react with both — a common one-mark giveaway.
• Forgetting the fourth reaction of a base — with an ammonium salt, releasing ammonia gas on warming — when a question only expects the usual three.
• Confusing “hydrated” with “dissolved” — a hydrated salt is a solid crystal with water built into its structure, not a salt currently in solution.

Self-test

1. Write the ionic equation for neutralisation.
2. Name the salt from nitric acid and copper(II) oxide.
3. Which method prepares barium sulfate, and why?
4. Give the two amphoteric oxides.
5. Explain the difference between a weak acid and a dilute acid.
6. What gas is released when a base is warmed with an ammonium salt, and how would you test for it?
7. Describe the standard chemical test for the presence of water, using copper(II) sulfate.

Answers: 1. H⁺(aq) + OH⁻(aq) → H₂O(l). 2. Copper(II) nitrate. 3. Precipitation — barium sulfate is insoluble, so mixing two soluble solutions precipitates it for filtering. 4. Al₂O₃, ZnO. 5. Weak = only partially dissociates into ions; dilute = a small amount of acid per volume of solution. A weak acid can be concentrated, and a strong acid dilute. 6. Ammonia gas; test with damp red litmus paper, which turns blue. 7. Add the liquid to white anhydrous copper(II) sulfate; if water is present, it turns blue as the salt becomes hydrated (CuSO₄·5H₂O).