Sign in

Playbook

Acids, Bases and Salts

33 q · 6% HARD. pH-scale classification (acidic < 7, neutral = 7, basic > 7), common-acid recall (citric in lemons, oxalic in tomatoes, lactic in milk, acetic in vinegar), Arrhenius/Brønsted/Lewis theory comparisons, oxides (acidic/basic/amphoteric/neutral), salts (acidic/basic/normal), and water of crystallisation counts.

questions in the bank
33
tagged HARD
6%
subtopic(s)
5
worked examples
2

When you’ll see it

A pH-scale classification (acidic/basic/neutral), a common-acid recall (citric/oxalic/lactic/acetic), an acid-base theory match (Arrhenius/Brønsted/Lewis), an oxide classification (acidic/basic/amphoteric/neutral), or a 'water of crystallisation' formula.

How this chapter is tested

33 q in 10 years · 6% HARD. Five subtopics, each rule-of-thumb sized. pH Scale (8 q): 0–6 acidic (lemon ~2, vinegar ~3, soft drinks ~3), 7 neutral (pure water, blood ~7.4), 8–14 basic (soap ~10, NaOH ~14). Common Acids (8 q · 12% HARD) overlaps heavily with /common-compounds: citric (lemons), oxalic (tomatoes, spinach), lactic (sour milk, muscle fatigue), acetic (vinegar), malic (apples), tartaric (grapes), formic (ant sting), HCl (gastric juice), H₂SO₄ (battery), HNO₃ (fertilisers, explosives).

Acid-Base Theory (7 q · 14% HARD): Arrhenius — acid releases H⁺ in water, base releases OH⁻. Brønsted-Lowry — acid donates H⁺, base accepts H⁺ (broader, includes NH₃ as base). Lewis — acid accepts e⁻ pair, base donates (broadest; includes AlCl₃, BF₃, FeCl₃ as Lewis acids despite no H⁺). Oxides: BASIC (Na₂O, MgO — metal oxides), ACIDIC (CO₂, SO₃ — non-metal oxides), AMPHOTERIC (Al₂O₃, ZnO — react with both acid and base), NEUTRAL (CO, NO, N₂O, H₂O — don't react).

Salts (7 q): formed by acid + base neutralisation. Normal salt (NaCl from HCl + NaOH). Acidic salt (NaHSO₄ — has replaceable H⁺ still). Basic salt (basic Cu carbonate — has OH⁻ still). Water of crystallisation (3 q): the H₂O molecules built into a salt's crystal lattice. CuSO₄·5H₂O (blue, dehydrate → white). Na₂CO₃·10H₂O (washing soda). MgSO₄·7H₂O (Epsom salt). FeSO₄·7H₂O (green vitriol). Number after the dot = molecules of water per formula unit.

The sub-skills

The rules and habits that decide whether you get a question right.

  • pH classification + common values

    pH = −log[H⁺]. Acidic: 0–6. Neutral: 7 (pure water, blood ~7.4). Basic: 8–14. Lemon 2, vinegar 3, soda 3, milk 6.5, blood 7.4, sea water 8, soap 9–10, NaOH 14. Stronger acid = lower pH.

  • Common acid recall

    Citric (lemons, oranges). Oxalic (tomatoes, spinach, rhubarb). Lactic (sour milk, fatigued muscles). Acetic (vinegar). Malic (apples). Tartaric (grapes). Formic (ant sting). HCl (gastric juice). H₂SO₄ (battery). HNO₃ (fertiliser, explosives). H₃PO₄ (soft drinks acidulant).

  • Acid-base theory selection

    Arrhenius (water-only, H⁺/OH⁻). Brønsted (broader, H⁺ donor/acceptor; NH₃ is a Brønsted base). Lewis (broadest, e⁻-pair acceptor/donor; AlCl₃, BF₃, FeCl₃, H⁺ are Lewis acids despite some having no H). Lewis-acid examples in NDA include AlCl₃, BF₃, FeCl₃, Cu²⁺.

  • Oxide classification

    Basic: metal + O (Na₂O, CaO, MgO, K₂O). Acidic: non-metal + O (CO₂, SO₂, SO₃, NO₂, P₂O₅). Amphoteric: react with both acid + base (Al₂O₃, ZnO, PbO, SnO, BeO). Neutral: don't react with acid or base (CO, NO, N₂O, H₂O).

  • Water of crystallisation count

    The H₂O after the dot = molecules per formula unit. CuSO₄·5H₂O has 5. Na₂CO₃·10H₂O has 10. MgSO₄·7H₂O has 7. Na₂SO₄·10H₂O has 10 (Glauber's salt). Heating drives water off (CuSO₄·5H₂O → CuSO₄ white).

2 worked examples from the bank

Real past-year questions illustrating the playbook. Click to reveal options + solution.

Example 1Acids, Bases and SaltsHARD
Which one of the following acids is predominantly found in tomatoes ?

[Q85 · Apr · 2021]

Example 2Acids, Bases and SaltsMODERATE
Consider the following: I. AlCl3\text{AlCl}_3 II. NH3\text{NH}_3 III. BF3\text{BF}_3 IV. FeCl3\text{FeCl}_3 How many of the above are Lewis acids?

[Q87 · Apr · 2026]

Traps to expect

Distractor shapes specific to this chapter. The page-wide Traps section covers the bank-level patterns.

  • NH₃ as Arrhenius base

    NH₃ is NOT an Arrhenius base (it doesn't release OH⁻ directly). It IS a Brønsted base (accepts H⁺ from water → NH₄⁺ + OH⁻ indirectly) and a Lewis base (lone pair donor). Wrong option treats all bases as Arrhenius.

  • Al₂O₃ as 'just acidic' or 'just basic'

    Al₂O₃ is AMPHOTERIC — reacts with HCl (→ AlCl₃, behaves as base) AND with NaOH (→ NaAlO₂, behaves as acid). Wrong option places it in one category only. Same for ZnO, PbO, BeO.

  • CO as 'acidic oxide'

    CO (carbon monoxide) is a NEUTRAL oxide — it doesn't react with acid or base. CO₂ is acidic (forms H₂CO₃ with water). Wrong option treats both as acidic because both contain carbon.

Drill every acids, bases and salts question

33 questions from the bank, scoped to 5 bundled subtopics.

Drill the 33 questions

Related playbooks

Often paired with this one — drill these next if you found the worked examples above tractable.