At a glance
- Cambridge section
- 0620 topic 7 (7.1 to 7.3)
- Edexcel section
- 4CH1 statements 2.28 to 2.43C
- Neutralisation
- H+(aq) + OH-(aq) -> H2O(l)
- Indicators named
- Cambridge: litmus, thymolphthalein, methyl orange; Edexcel: litmus, phenolphthalein, methyl orange
- Edexcel Paper 2 only
- titration (2.33C), acid plus alkali salts (2.40C), insoluble salts (2.41C, 2.43C)
Indicator colours
| Indicator | In acid | In alkali | Board |
|---|---|---|---|
| Litmus | Red | Blue | Both |
| Methyl orange | Red | Yellow | Both |
| Thymolphthalein | Colourless | Blue | Cambridge |
| Phenolphthalein | Colourless | Pink | Edexcel |
| Universal indicator | Red (pH 0 to 3), orange to yellow (weak acid) | Blue to purple (pH 11 to 14 is purple) | Both |
Edexcel 2.29 classifies pH 0 to 3 as strongly acidic, 4 to 6 weakly acidic, 7 neutral, 8 to 10 weakly alkaline and 11 to 14 strongly alkaline.
Key ideas board by board
Cambridge 0620 Core covers the reactions of acids with metals, bases and carbonates, indicator colours, bases as metal oxides and hydroxides, alkalis as soluble bases, the reaction of bases with ammonium salts (giving ammonia), H+ and OH- ions, pH with universal indicator, the ionic equation for neutralisation, acidic oxides (SO2, CO2) and basic oxides (CuO, CaO), preparing soluble salts four ways, the solubility rules, and hydrated and anhydrous substances. Supplement adds acids as proton donors and bases as proton acceptors, strong acids (completely dissociated, such as HCl) and weak acids (partially dissociated, such as ethanoic acid), amphoteric oxides (Al2O3 and ZnO), preparing insoluble salts by precipitation, and water of crystallisation in CuSO4.5H2O and CoCl2.6H2O.
Edexcel 4CH1 covers indicators, the pH scale, acids as sources of H+ and alkalis as sources of OH-, proton transfer, reactions of hydrochloric, sulfuric and nitric acids with metals, bases and carbonates (not nitric acid with metals), the solubility rules, and preparing a soluble salt from an insoluble reactant, including the required practical making hydrated copper(II) sulfate crystals from copper(II) oxide (2.42). Titration, salts from an acid and an alkali, insoluble salt preparation and the lead(II) sulfate practical carry C references and are examined in Paper 2.
Which salt preparation method?
| Salt you want | Method | Example |
|---|---|---|
| Soluble salt, starting material insoluble (metal, oxide, carbonate) | Add excess solid to warm acid, filter off the excess, evaporate to the point of crystallisation, cool, filter and dry the crystals | Copper(II) oxide + sulfuric acid -> copper(II) sulfate + water |
| Soluble salt of sodium, potassium or ammonium | Titration: find the exact volumes with an indicator, repeat without indicator, then crystallise | Sodium hydroxide + hydrochloric acid -> sodium chloride + water |
| Insoluble salt | Precipitation: mix two solutions, filter, wash the residue with distilled water, dry | Barium chloride + sodium sulfate -> barium sulfate + sodium chloride |
Worked example 1: copper(II) sulfate crystals
Equation: CuO(s) + H2SO4(aq) -> CuSO4(aq) + H2O(l). Method: warm dilute sulfuric acid in a beaker. Add copper(II) oxide a spatula at a time, stirring, until some black solid remains: this shows the acid has all reacted. Filter to remove the excess copper(II) oxide; the filtrate is blue copper(II) sulfate solution. Heat the filtrate to evaporate some of the water until crystals start to form on a cold glass rod dipped in it, then leave to cool so crystals grow. Filter the crystals and dry them between filter papers.
Why excess? Using excess solid guarantees no acid remains, and the solid is easy to filter out. Why not evaporate to dryness? It would decompose the hydrated crystals and drive off the water of crystallisation.
Worked example 2: equations and an insoluble salt
Calcium carbonate and hydrochloric acid: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g). Observation: effervescence, and the solid disappears. Zinc and sulfuric acid: Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g).
Barium sulfate is insoluble (sulfates are soluble except barium, calcium and lead), so make it by precipitation: BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq). The ionic equation is Ba2+(aq) + SO4 2-(aq) -> BaSO4(s). Filter, wash the white precipitate with distilled water to remove soluble sodium chloride, and dry it in a warm oven.
Common mistakes
- Saying a strong acid is a concentrated acid. Strength is about how completely the acid dissociates; concentration is about how much acid is in a volume.
- Forgetting carbon dioxide in acid plus carbonate equations, or hydrogen in acid plus metal.
- Using the titration method for a salt made from an insoluble base. Titration is for alkalis, which are soluble.
- Heating salt solutions to dryness when the question asks for crystals.
- Not washing a precipitate, so the product is contaminated with the soluble salt.
- Mixing up thymolphthalein (Cambridge) and phenolphthalein (Edexcel) colours.
Exam technique
Salt preparation is a favourite six-mark question. Mark schemes look for the reactants named, excess solid added, a reason (to make sure all the acid reacts), filtration to remove the excess, heating to the crystallisation point, cooling, and drying. Number the steps and keep each one short.
For "explain why hydrochloric acid has a lower pH than ethanoic acid of the same concentration" (Cambridge Supplement), write that hydrochloric acid is completely dissociated while ethanoic acid is only partially dissociated, so hydrochloric acid has a higher concentration of H+ ions. Always include state symbols when the question says so.
How one-to-one lessons help
This topic mixes recall (rules and colours) with method writing, and students often know the facts but write vague methods. A tutor sets the salt-choice decision as a quick drill, then has the student write full methods that the tutor marks line by line against past mark schemes, adding the reasons examiners look for. Equation writing with state symbols gets the same treatment.
Self-check
- Write word and symbol equations for an acid with a metal, a base and a carbonate.
- Give indicator colours in acid and alkali for your board's indicators.
- State the solubility rules and use them to choose a preparation method.
- Describe making copper(II) sulfate crystals from copper(II) oxide, with reasons.
- Describe making an insoluble salt by precipitation, with an ionic equation.
- Explain the difference between strong and weak, and concentrated and dilute.
- (Cambridge Supplement) Name two amphoteric oxides.
Common questions
Which indicators do I need?
Cambridge 0620 names litmus, thymolphthalein and methyl orange, plus universal indicator. Edexcel 4CH1 names litmus, phenolphthalein and methyl orange, plus universal indicator.
Is titration on both boards?
Yes. Cambridge describes acid-base titration in topic 12.2 and uses it for soluble salts; titration calculations are Supplement. In Edexcel, carrying out a titration (2.33C) is Paper 2 content.
Do I need the reaction of nitric acid with metals?
Edexcel 2.37 specifically excludes reactions between nitric acid and metals. Stick to hydrochloric and sulfuric acids for metal reactions.
How much do LiveTutor chemistry lessons cost?
$15 a lesson for every subject and level, on a weekly plan of 1 to 5 lessons billed monthly. Lessons are 60 minutes, one to one and online, and the first lesson is a free trial.
What is an amphoteric oxide?
An oxide that reacts with both acids and bases to give a salt and water. Cambridge Supplement names aluminium oxide and zinc oxide.
Sources
Dates and figures on this page come from these official and published sources. Always confirm deadlines on the official page before acting on them.