Acids, alkalis and indicator colours
Acids: pH below 7, turn litmus red, and react with reactive metals (→ salt + hydrogen), carbonates (→ salt + water + carbon dioxide) and bases (neutralisation → salt + water only). Alkalis (soluble bases): pH above 7, turn litmus blue, neutralise acids to salt + water. pH 7 is neutral (pure water). Indicator colours (acid/alkali): litmus red/blue; screened methyl orange red/green; phenolphthalein colourless/pink; universal red-orange to green to blue-violet.
Neutralisation: acid + base → salt + water
Neutralisation (mark-scheme form): an acid reacts with a base to give a salt and water only. The word "only" matters — if carbon dioxide is also produced (acid + carbonate) it is not simple neutralisation. Pattern: acid + base → salt + water, e.g. HCl + NaOH → NaCl + water. The net ionic equation for a strong acid with a soluble alkali is H⁺(aq) + OH⁻(aq) → H₂O(l): a proton joins a hydroxide ion, and spectator ions (Na⁺, Cl⁻) are omitted.
(Extended) Strong vs weak acids; proton transfer
(Extended) A strong acid is completely (fully) ionised — HCl, H₂SO₄, HNO₃ — shown by a single arrow: HCl → H⁺ + Cl⁻. A weak acid is partially ionised — ethanoic and carbonic acids — a reversible arrow shows the equilibrium: CH₃COOH ⇌ CH₃COO⁻ + H⁺. At equal concentration the strong acid has the lower pH, as full ionisation gives more H⁺ per dm³ while the weak gives far fewer. Brønsted–Lowry: the acid is the proton (H⁺) donor, the base the acceptor.
Drawn from real examiner reports.
Methyl orange is green in alkali, not pink
Screened methyl orange is red in acid and green in alkali. The common error is to write "pink" for alkali — pink is the alkaline colour of phenolphthalein, not methyl orange. Nor is it "yellow" (that is unscreened methyl orange, not the screened indicator used here). Memory hook: methyl orange is "Red in Acid, Green in Alkali" (RAG).
W22 P42 Q2(c)(iii); S22 P31 Q4(a)
Phenolphthalein is colourless in acid
Phenolphthalein is colourless in both acidic and neutral solutions and turns pink (magenta) only in alkali. A frequent error is to give it a colour — often "red" — in acid; it has none. A colourless result therefore shows only that a solution is not alkaline, not that it is neutral; use litmus or universal indicator to tell acidic from neutral.
Metal + acid vs metal oxide + acid
Zinc metal and zinc oxide behave differently with acid. Zn + HCl → zinc chloride + hydrogen (bubbles/fizzing). ZnO + HCl → zinc chloride + water, with no gas — the solid just dissolves. Students wrongly claim hydrogen or fizzing from ZnO + HCl, but an oxide has no hydrogen source. Rule: metal + acid gives hydrogen; metal oxide + acid gives water only.
W22 P31 Q6(a); W22 P32 Q6(a)
Describe observations, not product names
When a question says "describe what you observe/see", give only what the senses detect — bubbles of colourless gas, a solid dissolving, a colour change, warming — not the names of products. "Zinc sulfate is formed" or "hydrogen is produced" names products and scores zero. Instead write "bubbles of colourless gas", "the solid dissolves", "the mixture warms".
W22 P31 Q6(a); W22 P32 Q6(a)
Write the hydroxide ion as OH⁻
The hydroxide ion carries a single negative charge and must be written OH⁻. Omitting the charge ("OH") or writing "OH²⁻" gains no credit in an ionic equation. It appears correctly in the neutralisation ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l), where one H⁺ and one OH⁻ combine to give water. Always check ion charges before balancing an ionic equation.
S23 P32 Q4(e)(ii)
(Extended) "Strong" is not "concentrated"
(Extended) Strength and concentration differ. Strong/weak is degree of ionisation; concentrated/dilute is amount dissolved per dm³. A dilute strong acid (0.001 mol/dm³ HCl) is still strong; a concentrated weak acid (glacial ethanoic acid) is still weak. Defining "strong" as corrosive or concentrated scores zero — say "completely ionised".
Writing neutralisation equations
Name the salt first: metal (or ammonium) from the base, ending from the acid — chloride/HCl, sulfate/H₂SO₄, nitrate/HNO₃. Write acid + base → salt + water (+ CO₂ if carbonate), balance and add state symbols, e.g. H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.
Read the command word
Match the answer to the command word. "Describe what is seen" wants observations only — fizzing, dissolving, a colour change — never product names. "Write an equation" wants balanced reactants and products with state symbols. "State" wants a single word or short fact.
(Extended) Explaining strong vs weak pH
(Extended) Explain low pH by ionisation, not concentration. Strong acid fully ionised → higher [H⁺] → lower pH; weak acid partially ionised → lower [H⁺] → higher pH. Trap: equal concentration does not make the strong acid "more concentrated".
Acid — a substance that produces hydrogen ions () in aqueous solution, has a pH below 7, turns litmus red, and reacts characteristically with metals, carbonates and bases.
Base — a substance that reacts with an acid to produce a salt and water only. Metal oxides and metal hydroxides are bases.
Alkali — a base that is soluble in water; produces hydroxide ions () in aqueous solution; has pH above 7; turns litmus blue.
Neutralisation (mark-scheme definition): The reaction between an acid and a base to produce a salt and water only.
Define neutralisation.
Copper(II) oxide reacts with dilute hydrochloric acid in a neutralisation reaction.
(a) Write a balanced symbol equation for this reaction, including state symbols.
(b) State the colour change observed when the reaction is complete.
