Acids, Bases and Salts

• Three routes, one question. Insoluble target → precipitate it. 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 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.
• 44 checks · 44 taught · 44 worked · 44 retrieved · 0 partial · 0 missing. The final mastery audit asks you to prove each one with an observable task rather than a feeling.
• Acid = proton donor. Base = proton acceptor, and in this syllabus that means a metal oxide or metal hydroxide. 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, nitric and sulfuric are the strong three; the carboxylic acids — ethanoic acid above all — are the weak ones.
• Acidic reacts with alkali only — SO2, CO2. Basic reacts with acid only — CuO, CaO. Amphoteric reacts with both — Al2O3 and ZnO, and nothing else on this syllabus. Neutral reacts with neither.

• 7.1.1 — I can state that aqueous solutions of acids contain H+ ions and that aqueous solutions of alkalis contain OH− ions.
• 7.1.10a — I can name hydrochloric acid, nitric acid and sulfuric acid as the strong acids required by this syllabus.
• 7.1.11a — I can state that carboxylic acids, such as ethanoic acid, are weak acids.
• 7.2.2a — I can state that sulfur dioxide and carbon dioxide are acidic oxides.
• 7.2.2b — I can state that copper(II) oxide and calcium oxide are basic oxides.
• 7.2.2c — I can state that aluminium oxide and zinc oxide are amphoteric oxides.
• 7.3.3a — I can state that all sodium, potassium and ammonium salts are soluble in water.
• 7.3.3b — I can state that all nitrates are soluble in water.
• 7.3.3c — I can state that chlorides are soluble except lead(II) chloride and silver chloride.
• 7.3.3d — I can state that sulfates are soluble except barium sulfate, calcium sulfate and lead(II) sulfate.
• 7.3.3e — I can state that carbonates are insoluble except sodium, potassium and ammonium carbonate.
• 7.3.3f — I can state that hydroxides are insoluble except sodium, potassium and ammonium hydroxide, with calcium hydroxide only partially soluble.
• 7.3.5b — I can state that CuSO4·5H2O is hydrated copper(II) sulfate and read its five water molecules from the formula.
• 7.3.5c — I can state that CoCl2·6H2O is hydrated cobalt(II) chloride and read its six water molecules from the formula.
• 7.1.2 — I can define an acid as a proton donor and a base as a proton acceptor.
• 7.1.3a — I can define a base as a metal oxide or a metal hydroxide.
• 7.1.3b — I can define an alkali as a soluble base, and explain why every alkali is a base but not every base is an alkali.
• 7.1.6 — I can define neutralisation as the reaction between an acid and a base.
• 7.1.9 — I can define a strong acid as one that is completely dissociated in aqueous solution and a weak acid as one that is only partially dissociated.
• 7.2.1 — I can define an amphoteric oxide as an oxide that reacts with acids and with bases to produce a salt and water.
• 7.3.4a — I can define a hydrated substance as one that is chemically combined with water.
• 7.3.4b — I can define an anhydrous substance as one that contains no water.
• 7.3.5a — I can define water of crystallisation as the water molecules present in hydrated crystals.
• 7.1.4a — I can describe the reaction of an acid with a reactive metal, giving a salt and hydrogen, with a balanced equation and the observation.
• 7.1.4b — I can describe the reaction of an acid with a base, giving a salt and water.
• 7.1.4c — I can describe the reaction of an acid with a carbonate, giving a salt, water and carbon dioxide.
• 7.1.5a — I can describe the reaction of a base with an acid from the base’s side of the equation.
• 7.1.5b — I can describe the reaction of a warmed alkali with an ammonium salt, giving a salt, water and ammonia.
• 7.1.7 — I can write the ionic equation H+(aq) + OH−(aq) → H2O(l) and derive it by cancelling spectator ions.
• 7.1.10b — I can write complete-dissociation equations for hydrochloric, nitric and sulfuric acid using a single forward arrow.
• 7.1.11b — I can write the partial-dissociation equation for ethanoic acid using an equilibrium arrow.
• 7.3.1a — I can describe, in order and with a reason for each step, the preparation of a soluble salt from an acid and an alkali by titration.
• 7.3.1b — I can describe the preparation of a soluble salt from an acid and an excess of a reactive metal.
• 7.3.1c — I can describe the preparation of a soluble salt from an acid and an excess of an insoluble base.
• 7.3.1d — I can describe the preparation of a soluble salt from an acid and an excess of an insoluble carbonate.
• 7.3.2 — I can describe the preparation of an insoluble salt by precipitation, including washing and drying the residue.
• 7.1.8a — I can describe the effect of acids and alkalis on litmus: red in acid, blue in alkali.
• 7.1.8b — I can describe the effect of acids and alkalis on thymolphthalein: colourless in acid, blue in alkali.
• 7.1.8c — I can describe the effect of acids and alkalis on methyl orange: red in acid, yellow in alkali.
• 7.1.12a — I can use universal indicator colour and pH to compare the relative hydrogen ion concentration of two aqueous solutions.
• 7.1.12b — I can identify a neutral solution as pH 7, green with universal indicator.
• 7.1.12c — I can compare the relative acidity of two solutions from their pH values.
• 7.1.12d — I can compare the relative alkalinity of two solutions from their pH values.
• 7.2.2d — I can relate the classification of the named oxides to the metallic or non-metallic character of the element, without over-claiming a universal rule.

Salt preparation is the most heavily practical part of the written papers. 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. And the acid–base language here — proton donor, proton acceptor, strong, weak, concentrated, dilute — is the vocabulary examiners use to test whether you are describing particles or just repeating a label.

Acid

A substance that acts as a proton donor. In aqueous solution an acid produces hydrogen ions, H+(aq), and it reacts with reactive metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide.

Anhydrous Substance

A substance that contains no water. Anhydrous copper(II) sulfate, CuSO4, is a white powder and anhydrous cobalt(II) chloride, CoCl2, is blue; each is formed by heating the corresponding hydrated crystals.

Acidic Oxide

A non-metal oxide that reacts with an alkali to produce a salt and water, and does not react with acids. Sulfur dioxide and carbon dioxide are the acidic oxides named by this syllabus.

Universal Indicator

A mixture of indicators that shows a range of colours across the pH scale rather than a single change, so it can be used to estimate pH and to compare how acidic or how alkaline two solutions are. It is red or orange in strongly acidic solution, green at about pH 7, and blue or purple in strongly alkaline solution.

Base

A substance that acts as a proton acceptor. In this syllabus a base is a metal oxide or a metal hydroxide, and it reacts with an acid to give a salt and water. A base does not have to dissolve in water.

Indicator

A substance that has one colour in acidic solution and a different colour in alkaline solution, so that its colour reports which of the two is present. Litmus, thymolphthalein and methyl orange are the three named in this syllabus.

Hydrated Substance

A substance that is chemically combined with water. Hydrated copper(II) sulfate, CuSO4.5H2O, is blue and holds five water molecules per formula unit; hydrated cobalt(II) chloride, CoCl2.6H2O, is pink and holds six.

Soluble Salt

A salt that dissolves in water, so that after it has been made it is present in the filtrate as a solution and must be recovered by evaporating the solution to the crystallisation point and letting it cool and crystallise.

Alkali

A base that is soluble in water. An alkali dissolves to give an aqueous solution containing hydroxide ions, OH-(aq); sodium hydroxide and potassium hydroxide are alkalis, while insoluble bases such as copper(II) oxide are not.

Spectator ion

An ion that is present in the solution before and after a reaction in exactly the same form, taking no part in the change. Spectator ions are cancelled from both sides of a full ionic equation to leave the net ionic equation.

Amphoteric Oxide

An oxide that reacts with acids and also with bases, in each case producing a salt and water. Aluminium oxide and zinc oxide are the amphoteric oxides named by this syllabus.

Weak Acid

An acid that is only partially dissociated into ions in aqueous solution, so that most of the acid remains as un-ionised molecules at any moment. Carboxylic acids such as ethanoic acid are weak acids, and their dissociation is written with an equilibrium arrow.

Neutralisation

The reaction between an acid and a base. The hydrogen ions from the acid are accepted by the base, so a salt and water are formed; in aqueous solution the change is summarised by the ionic equation H+(aq) + OH-(aq) to H2O(l).

Strong Acid

An acid that is completely dissociated into ions in aqueous solution, so that every acid particle has released its hydrogen ion. Hydrochloric acid, nitric acid and sulfuric acid are the strong acids named by this syllabus, and their dissociation is written with a single forward arrow.

Basic Oxide

A metal oxide that reacts with an acid to produce a salt and water, and does not react with alkalis. Copper(II) oxide and calcium oxide are the basic oxides named by this syllabus.

Water of Crystallisation

The water molecules present in hydrated crystals. They are chemically combined in the crystal in a fixed number per formula unit, which is why the formula of a hydrated salt states that number after a dot.

Precipitation

The formation of an insoluble solid when two aqueous solutions are mixed, because the ions of the insoluble salt cannot remain dissolved together. The solid is separated by filtration as the residue, then washed with distilled water and dried.

• “A base is the same thing as an alkali.” Repair A base is a metal oxide or metal hydroxide. 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.
• “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.
• “Neutralisation always ends at pH 7.” Repair Neutralisation is defined by what reacts — an acid with a base — 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 requires. Thymolphthalein is colourless in acid and blue in alkali. The three required 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 statement 7.1.12 is asking for.
• “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.
• “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 select a precipitation reaction.
• “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” — both “soluble” and “insoluble” are marked wrong.
• “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 On warming, the ammonium ion gives up a proton to the hydroxide ion, so the products are 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.” Method marks in this topic are almost always awarded for the reason, not the apparatus — a step with no reason reads as a memorised recipe.
• The coefficient that gets dropped Sulfuric acid releases two hydrogen ions per formula unit, so the coefficient 2 in front of H+(aq) is not decoration — without it the charges do not balance, because SO42− carries 2−. Check charge as well as atoms on every ionic equation you write.
• The two sentences examiners are looking for “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.
• 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. If you scored under 55, work the retrieval ladder again before re-attempting; if you scored 55–68, the gaps are almost always in the reason half of a method answer.

• Cambridge O Level Chemistry 5070 is assessed by Paper 1 (multiple choice), Paper 2 (theory) and a practical paper. Topic 7 material can appear in any of them. What follows describes the kind of output each command word wants — it does not claim how often the topic appears or predict what will be set.
• One decisive fact per question: the colour of a named indicator in a named solution, which of four oxides is amphoteric, 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.
• Balanced symbol equations with state symbols, the ionic neutralisation equation, an explanation of complete versus partial dissociation, an ordered salt preparation with a reason attached to each step, and classification questions that ask for the evidence as well as the category.
• Choosing and ordering apparatus; explaining why a solid was added in excess; identifying the residue and the filtrate; explaining why a titration is repeated without indicator; why a precipitate is washed with distilled water; and why a solution is not evaporated to dryness.
• 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.” Method marks in this topic are almost always awarded for the reason, not the apparatus — a step with no reason reads as a memorised recipe.