Acids, Bases and Salts

• Two Core routes, one question. Soluble target with a dissolved alkali → titrate it. Soluble target with an insoluble solid → add that solid in excess and filter it off. Everything else in Core Topic 7 is the evidence you need to answer that question honestly.
• If checks 3 and 4 felt uncertain, do them again before section 7.3. The whole of salt preparation is built on knowing that filtration removes an undissolved solid and nothing else.
• 12 Core · 8 Supplement · 20 taught · 20 practised · 20 retrieved · 0 partial · 0 missing. If you are on the Core route, the twelve Core rows are your whole examinable scope for Topic 7 and nothing in them requires you to open a Supplement block. The final mastery audit asks you to prove each statement with an observable task rather than a feeling.
• Aqueous acid → H+(aq). Aqueous alkali → OH−(aq). Base = an oxide or a hydroxide of a metal. Alkali = a base that dissolves. The word “soluble” is the entire difference between the last two.
• Metal → hydrogen. Base → water only. Carbonate → water and carbon dioxide. Ammonium salt with a warmed alkali → water and ammonia. Underneath the first three, whenever an alkali is involved, sits one line: H+(aq) + OH−(aq) → H2O(l).
• Six colours, three pairs: litmus red / blue, thymolphthalein colourless / blue, methyl orange red / yellow. Universal indicator is the only one that ranks: lower pH → more H+(aq) → more acidic.
• Strong and weak describe what fraction has ionised. Concentrated and dilute describe how much is there. Single arrow for strong; equilibrium arrow for weak. Hydrochloric acid is the named strong acid and ethanoic acid is the named weak acid — two acids, two equations, two arrows.
• Acidic reacts with alkali only — SO2, CO2, both oxides of non-metals. Basic reacts with acid only — CuO, CaO, both oxides of metals. Those two classes and the metallic-character link are the whole of Core 7.2.1, and every verdict is an experimental result rather than a position on a table. An oxide that reacts with neither is simply neither of them.

• Core 7.1.1 — I can describe what an acid does to (a) a reactive metal, (b) a base and (c) a carbonate, giving the products, a balanced equation and the observation in each case.
• Core 7.1.2 — I can describe the effect of an acid on litmus, on thymolphthalein and on methyl orange.
• Core 7.1.3 — I can state that bases are oxides or hydroxides of metals, that alkalis are soluble bases, and explain why every alkali is a base but most bases are not alkalis.
• Core 7.1.4 — I can describe what a base does to (a) an acid and (b) an ammonium salt, including the ammonia released on warming and the test that identifies it.
• Core 7.1.5 — I can describe the effect of an alkali on litmus, on thymolphthalein and on methyl orange.
• Core 7.1.6 — I can state that aqueous solutions of acids contain H+ ions and that aqueous solutions of alkalis contain OH− ions.
• Core 7.1.7 — I can use universal indicator paper colour and pH to compare hydrogen ion concentration, to identify neutrality, and to compare relative acidity and relative alkalinity — with no calculation.
• Core 7.1.8 — I can describe the neutralisation of an acid by an alkali to produce water, and write H+(aq) + OH−(aq) → H2O(l).
• Core 7.2.1 — I can classify an oxide as acidic (including SO2 and CO2) or basic (including CuO and CaO), and relate the classification to metallic and non-metallic character without over-claiming a law.
• Core 7.3.1 — I can describe the preparation, separation and purification of a soluble salt by reacting an acid with (a) an alkali by titration, (b) excess metal, (c) excess insoluble base and (d) excess insoluble carbonate, giving a reason for every step.
• Core 7.3.2 — I can state all six general solubility rules with their exact exceptions, including that calcium hydroxide is only partially soluble.
• Core 7.3.3 — I can define a hydrated substance as one chemically combined with water, and an anhydrous substance as one containing no water.
• Supplement 7.1.9 — I can define an acid as a proton donor and a base as a proton acceptor, and connect that to the H+ and OH− ions of Core 7.1.6.
• Supplement 7.1.10 — I can define a strong acid as one completely dissociated in aqueous solution and a weak acid as one only partially dissociated, and keep strength apart from concentration.
• Supplement 7.1.11 — I can state that hydrochloric acid is a strong acid and show it with the equation HCl(aq) → H+(aq) + Cl−(aq).
• Supplement 7.1.12 — I can state that ethanoic acid is a weak acid and show it with the equation CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq), and justify the equilibrium arrow.
• Supplement 7.2.2 — I can describe an amphoteric oxide as one that reacts with acids and with bases to produce a salt and water.
• Supplement 7.2.3 — I can classify Al2O3 and ZnO as amphoteric oxides, and give the evidence that decides it.
• Supplement 7.3.4 — I can describe the preparation of an insoluble salt by precipitation, including choosing the two solutions, filtering, washing and drying.
• Supplement 7.3.5 — I can define water of crystallisation as the water molecules present in hydrated crystals, and read CuSO4·5H2O and CoCl2·6H2O correctly.

Salt preparation is where this topic turns into laboratory work, and the practical component is taken by Core and Extended candidates alike. The same six solubility rules reappear whenever you are asked to explain why a solid formed on mixing, why a residue was washed, or why a solution was not simply boiled dry. The acid–base language here is also the vocabulary that separates describing particles from repeating a label. At Core that language is hydrogen ion, hydroxide ion, base, alkali and soluble; at Supplement it grows to include proton donor, proton acceptor, strong and weak.

• “A base is the same thing as an alkali.” Repair A base is an oxide or a hydroxide of a metal. An alkali is a base that dissolves in water. Copper(II) oxide is a base — it neutralises acids — but it is insoluble, so it is not an alkali and no solution of it exists to turn litmus blue.
• “Neutralisation always ends at pH 7.” Repair Neutralisation is defined by what reacts — an acid with an alkali, producing water — not by a number. A mixture only sits at pH 7 when exactly enough of each has been added and the salt formed does not itself make the solution acidic or alkaline. Add one drop too much acid and the reaction was still a neutralisation.
• “Acid plus carbonate gives hydrogen — you can see it fizzing.” Repair Fizzing tells you a gas is made, not which gas. Acid + carbonate gives carbon dioxide, along with a salt and water. Only acid + a reactive metal gives hydrogen.
• “Methyl orange is purple in alkali.” Repair Methyl orange is red in acid and yellow in alkali. Purple belongs to litmus in a neutral solution. Learn each indicator as a complete pair; half a pair is worth nothing in a multiple-choice question.
• “Thymolphthalein is the pink one.” Repair That is phenolphthalein, a different indicator and not one of the three this syllabus names. Thymolphthalein is colourless in acid and blue in alkali. The three named indicators are litmus, thymolphthalein and methyl orange.
• “Universal indicator just tells you acid or alkali.” Repair Litmus does that. Universal indicator gives a graded colour, so it lets you compare how acidic or alkaline two solutions are, and match the colour to an approximate pH. That is what Core statement 7.1.7 is asking for.
• “Mix the acid and the alkali, then filter off the salt.” Repair There is nothing solid to filter. Both reactants are dissolved and so is the salt. That is precisely why an acid–alkali preparation must be done by titration — you have to measure the right volume, because you cannot remove an excess afterwards.
• “Titrate to the end point, then evaporate that flask to get the crystals.” Repair That flask contains indicator, which would be trapped in the crystals and colour the product. Record the volume from the titration, then repeat with the same measured volumes and no indicator, and crystallise that solution.
• “Filter the solution to remove the dissolved salt from the water.” Repair Filtration only separates an undissolved solid from a liquid. Dissolved ions pass straight through the paper. To recover a dissolved salt you must concentrate the solution by gentle evaporation and then let it cool and crystallise.
• “All chlorides are soluble — and all sulfates, and all hydroxides.” Repair Chlorides are soluble except lead(II) and silver. Sulfates are soluble except barium, calcium and lead(II). Hydroxides are the other way round: insoluble except sodium, potassium and ammonium. A rule without its exceptions cannot predict anything.
• “Calcium hydroxide is soluble — limewater is a solution, isn’t it?” Repair Calcium hydroxide is partially soluble. Limewater is a dilute solution of the small amount that does dissolve. Write “partially soluble” — that is the syllabus’s own wording in rule 6, and both “soluble” and “insoluble” misstate it.
• “A strong acid is a concentrated acid, or a corrosive one.” Repair Strong describes the extent of dissociation in aqueous solution: complete for a strong acid, partial for a weak one. Concentrated describes how much acid there is per unit volume. They are independent, so a dilute strong acid and a concentrated weak acid both exist.
• “Amphoteric means neutral — it sits in the middle.” Repair The opposite. An amphoteric oxide reacts with both acids and bases to give a salt and water. A neutral oxide reacts with neither. Aluminium oxide and zinc oxide are the two you need; they are defined by evidence of reaction, never by position on a table.
• “Evaporate the mixture to make the precipitate appear.” Repair A precipitate forms the instant the two solutions meet, because the ions of an insoluble salt cannot stay in solution together. Evaporating would only dump every dissolved spectator salt on top of your product. Mix, filter, wash the residue with distilled water, dry.
• “Water of crystallisation is water clinging to the outside of the crystals.” Repair Surface water is damp crystals, and it comes off by drying between filter papers. Water of crystallisation is chemically combined inside the crystal, in a fixed number of molecules per formula unit, and removing it changes the substance — blue crystals become a white powder.
• “CuSO4·5H2O means CuSO4 multiplied by 5H2O.” Repair The dot is not multiplication. It reads “combined with”: one formula unit of copper(II) sulfate held together with five water molecules in the crystal. Read CoCl2·6H2O the same way — six water molecules, not six times anything.
• “Magnesium + hydrochloric acid → magnesium chloride + water.” Repair A metal has no oxygen and no hydroxide to give, so no water can be made. The hydrogen released by the acid has nowhere to go except into H2(g). Products: salt + hydrogen.
• “Copper(II) oxide + sulfuric acid → copper(II) sulfate + hydrogen.” Repair The oxide ion accepts the hydrogen ions, so they end up as water, not as hydrogen gas. Copper is also below hydrogen in the reactivity series and would not displace it anyway. Products: salt + water, and no effervescence at all.
• “It fizzed, so calcium carbonate + acid must give hydrogen.” Repair The carbonate ion CO32− supplies the carbon and oxygen for carbon dioxide. Test it: the gas turns limewater milky and does not pop. Products: salt + water + carbon dioxide.
• “Ammonium chloride + sodium hydroxide → sodium chloride + ammonium hydroxide.” Repair There is no such compound as ammonium hydroxide in an answer at this level. On warming, an alkali reacts with an ammonium salt to give a salt, water and ammonia gas. Write NH3(g) and H2O(l) as two separate products.

• The one habit worth building for this topic Whenever you write a preparation step, immediately write the word because after it. “Filter the mixture because the excess copper(II) oxide is insoluble and the copper(II) sulfate is dissolved.” The command word for most of statement 7.3.1 is describe, and a described method is one where each step is tied to the property that makes it work; a step with no reason is a recipe, and a recipe cannot be adapted to a salt you have not met.
• The two sentences that carry the whole method “An excess of an alkali cannot be removed by filtration, because it is dissolved — so the volumes must be measured instead.” and “The titration is repeated without indicator so that the indicator does not contaminate the crystals.” Those two ideas are what separate a described method from a memorised one: each names the property of the reagents that forces the step.
• Reading the rules in both directions Forwards: “Is silver chloride soluble?” → rule 3 → no, it is one of the two exceptions. Backwards: “I need a solid chloride — which metal?” → rule 3 read the other way → lead(II) or silver. Design questions need the backwards reading, and it is the one most students have never practised.
• The pattern across all eighteen Twelve of them are the same failure in different clothing: a rule was remembered without the condition attached to it. “Filter to get a solid” — if it is undissolved. “Sulfates are soluble” — except three. “Evaporate to get crystals” — to the crystallisation point, not to dryness. When you write a rule down in revision, write its condition on the same line.
• Marking yourself honestly Give the mark only when your wording carries the same information, not merely the same topic. “It is soluble” does not earn the mark that “sodium hydroxide is soluble, so the excess cannot be filtered off” earns. Core candidates, out of 58: under 40 — work the Core prompts of the retrieval ladder again before re-attempting. 40–49 — the gaps are almost always in the reason half of a method answer. Extended candidates, out of 79: under 55 — work the retrieval ladder again. 55–68 — same diagnosis as above, and check Question 5 and Question 9 separately: if the Supplement marks are where the losses cluster, the problem is the Supplement statements rather than the method writing.

• Cambridge IGCSE Chemistry 0620 is examined on two routes. Core candidates study the Core statements only and are eligible for grades C to G. Extended candidates study Core plus Supplement — Extended is not a separate syllabus — and are eligible for grades A* to G. Topic 7 has twelve Core statements and eight Supplement statements, so an Extended candidate meets all twenty and a Core candidate meets the first twelve.
• Your route decides your theory papers. It does not change your practical component: experimental skills are assessed for Core and Extended candidates alike, and both routes take either Paper 5 or Paper 6.
• On weightings Those percentages are the weightings of the papers, across the whole syllabus. They are not the weighting of Topic 7, and no topic of this syllabus has a published mark share of its own.
• Extended papers assess Core content as well as Supplement content, because Extended means Core plus Supplement. Nothing in this chapter is Supplement-only revision for an Extended candidate: the twelve Core statements are examined on Papers 2 and 4 as well.
• One decisive fact per question: the colour of a named indicator in a named solution, which of four salts is insoluble, which gas is released by an acid and a carbonate. There is no credit for method here, so the six solubility rules and the three indicator pairs must be instant. Extended papers add the strong/weak distinction and the amphoteric oxides.
• Balanced symbol equations with state symbols, the ionic neutralisation equation, and an ordered salt preparation with a reason attached to each step. Extended theory adds dissociation equations, the proton definitions, amphoteric behaviour and preparation by precipitation.