Acids, Bases and Indicators

Acids and bases supply 0620 with its most reused marking points: the three characteristic reactions of acids appear in salt preparation, gas tests and even organic chemistry, and “all acids react the same three ways” is the pattern that makes them predictable. The marks leak at the definitions (base vs alkali, strong vs concentrated), where one wrong word converts a right idea into a zero.

What acids and bases do (Core)

Acids in aqueous solution are a source of hydrogen ions, H+. Alkalis are a source of hydroxide ions, OH−. The three characteristic reactions of acids, each worth knowing as a general equation:

Acid + …	Products	Example
Metal	Salt + hydrogen	Mg + 2HCl → MgCl2 + H2
Base (metal oxide/hydroxide)	Salt + water	CuO + H2SO4 → CuSO4 + H2O
Carbonate	Salt + water + carbon dioxide	CaCO3 + 2HCl → CaCl2 + H2O + CO2

The gases identify themselves: hydrogen pops a lighted splint, carbon dioxide turns limewater milky, the same tests catalogued in qualitative analysis ion and gas tests.

Base vs alkali is a definition pair examiners test directly. A base neutralises an acid to form a salt and water; an alkali is a soluble base, releasing OH− ions in solution. Copper(II) oxide is a base but not an alkali (insoluble); sodium hydroxide is both. Neutralisation itself reduces to one ionic equation: H+ + OH− → H2O.

Indicators (Core)

Three indicators carry fixed colours:

Indicator	In acid	In alkali
Litmus	Red	Blue
Thymolphthalein	Colourless	Blue
Methyl orange	Red	Yellow

Litmus answers “is it acidic or alkaline?”; the other two give the sharp colour change needed in titrations. The full-spectrum indicator, universal indicator, measures how acidic, and belongs with the pH scale.

Proton transfer (Supplement)

The Extended definitions upgrade the picture: an acid is a proton (H+) donor; a base is a proton (H+) acceptor. A hydrogen ion is just a proton, so the two phrasings are interchangeable.

Given an equation, identify the donor and acceptor by following the H+. In NH3 + HCl → NH4Cl, the HCl donates a proton (acid) and the ammonia accepts it (base). In HCl + H2O → H3O+ + Cl−, water is the acceptor. The command word is usually “identify… and explain”, so name the species and say which way the proton moved.

Strong and weak acids (Supplement)

A strong acid is completely dissociated into ions in aqueous solution: HCl → H+ + Cl−. A weak acid is only partially dissociated, written with the equilibrium arrow: CH3COOH ⇌ CH3COO− + H+. Hydrochloric, sulfuric and nitric acids are strong; ethanoic acid is the stock weak acid.

Consequences the exam tests: at the same concentration, the weak acid has a higher pH (fewer H+ ions), conducts electricity less well, and reacts more slowly with magnesium or carbonates, though it eventually yields the same volume of gas, because the equilibrium replaces H+ ions as they are used.

Strength is not concentration. Strong/weak describes the degree of dissociation; concentrated/dilute describes how much acid is dissolved. A concentrated weak acid and a dilute strong acid are both possible, and confusing the vocabularies is a named examiner-report error.

Worked exam question

Ethanoic acid is a weak acid; hydrochloric acid is a strong acid. (a) Define a weak acid. [1] (b) Equal volumes of 0.1 mol/dm³ ethanoic acid and 0.1 mol/dm³ hydrochloric acid are tested. Predict which has the higher pH and explain why. [2] (c) In the reaction CH3COOH + H2O ⇌ CH3COO− + H3O+, identify the base and explain your answer. [2]

Model answer: (a) An acid that is only partially dissociated in aqueous solution (1). (b) Ethanoic acid (1), because it is only partially dissociated, so the concentration of H+ ions is lower (1). (c) Water (1); it accepts a proton (H+) from the ethanoic acid (1).

Mark-by-mark: (a) needs “partially dissociated”: “not very strong” or “doesn’t react much” score zero. (b) ties the prediction to H+ concentration; pH reasoning without mentioning H+ usually loses the second mark. (c) requires the direction of proton transfer, not just the name.

The mistakes that cost marks

1. Using base and alkali interchangeably. Copper(II) oxide is a base, never an alkali. Solubility is the dividing line.
2. “Weak means dilute.” Strength = degree of dissociation; concentration = amount dissolved. Different axes entirely.
3. Writing weak-acid dissociation with a full arrow. Partial dissociation demands ⇌.
4. Naming the proton acceptor without saying it accepts a proton. The explanation mark is in the transfer, not the label.

How examiners want it phrased

Student wording	Mark-scheme wording
”Acids are corrosive things"	"An acid is a proton donor / a source of H+ ions in solution"
"Ethanoic acid is weaker"	"Ethanoic acid is only partially dissociated, so the H+ concentration is lower"
"It cancels out the acid"	"The base neutralises the acid: H+ + OH− → H2O"
"Ammonia grabs the hydrogen"	"Ammonia accepts a proton from HCl, so it acts as a base”

These definitions feed directly into preparation of salts, where the three characteristic reactions become methods, and the whole section is mapped on the Acids, Bases and Salts pillar. If base/alkali and strong/concentrated still blur under time pressure, a free 1-hour trial lesson with a Chemistry specialist will separate them for good with targeted past-paper drills.

Test yourself

Cover the page above, answer all three, then click to mark yourself.

Q1 (2 marks). Dilute sulfuric acid is added to zinc. Write the word equation and state one observation.

Show answer

• zinc + sulfuric acid → zinc sulfate + hydrogen [1] • effervescence / bubbles of gas, and the zinc gets smaller; the gas pops with a lighted splint [1]

Q2 (2 marks). Explain why copper(II) oxide is described as a base but not as an alkali.

Show answer

• it neutralises acids to form a salt and water, so it is a base [1] • it is insoluble in water, and an alkali must be a soluble base releasing OH− ions [1]

Q3 (2 marks). (Extended) Ammonia reacts with hydrogen chloride: NH3 + HCl → NH4Cl. Identify the acid and the base in this reaction and explain your answer in terms of proton transfer.

Show answer

• HCl is the acid: it donates a proton (H+) [1] • NH3 is the base: it accepts the proton, forming NH4+ [1]