Chemistry C7 of Cambridge IGCSE Co-ordinated Sciences 0654 links acidic and alkaline behaviour to reactions, indicators, oxides and salt preparation. Official sections C7.1 to C7.3 cover acid and base reactions, litmus, methyl orange, universal indicator and pH, neutralisation, acidic, basic and amphoteric oxides, three soluble-salt routes, precipitation, and hydrated or anhydrous substances. Proton-transfer definitions, acid-strength theory and general solubility rules are outside this boundary.
Characteristic acid reactions
Aqueous acids contain H⁺ ions. Their characteristic reactions include:
acid + reactive metal gives salt + hydrogen
acid + base gives salt + water
acid + carbonate gives salt + water + carbon dioxide
The acid name helps identify the salt: hydrochloric acid forms chlorides, sulfuric acid forms sulfates and nitric acid forms nitrates.
Bases and alkalis
Bases are oxides or hydroxides of metals in this syllabus definition. An alkali is a soluble base, so every alkali is a base but not every base is an alkali.
Aqueous alkalis contain OH⁻ ions. They neutralise acids. Bases also react with ammonium salts on warming to release ammonia:
NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
Ammonia turns damp red litmus paper blue. The paper must be damp so the gas can dissolve and show alkaline behaviour.
Indicators
Indicators distinguish acidic and alkaline conditions by colour.
Indicator
Acidic condition
Alkaline condition
litmus
red
blue
Check this topic from memory
Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
At intermediate transition conditions, an indicator may show a mixed colour. Use the expected endpoint colour for the named indicator rather than transferring colours from a different indicator.
Universal indicator paper gives an approximate pH through a colour scale. Low pH means greater hydrogen-ion concentration and stronger acidic condition for the compared solutions. pH 7 is neutral. High pH means more alkaline condition.
Neutralisation
Acid and alkali neutralisation produces a salt and water. Name the salt from the acid anion and the metal or ammonium ion supplied by the alkali. The ionic equation is not required in C7.
Neutral does not always mean that equal volumes were mixed. Volumes depend on concentrations and stoichiometric ratio.
Acidic, basic and amphoteric oxides
Acidic oxides react with bases. Non-metal oxides such as SO₂ and CO₂ are classified as acidic.
Basic oxides react with acids. Metal oxides such as CuO and CaO are basic.
Amphoteric oxides react with both acids and bases to produce salt and water. Aluminium oxide, Al₂O₃, and zinc oxide, ZnO, are the required examples.
Metallic or non-metallic character gives the general acidic or basic pattern, but amphoteric examples must be remembered as exceptions to a simple two-way classification.
Prepare a soluble salt using excess solid
If a soluble salt can be made from an acid and an excess metal, insoluble base or insoluble carbonate:
Warm the dilute acid gently where suitable.
Add the solid in small portions until no more reacts and some remains.
Filter to remove unreacted excess solid.
Heat the filtrate to concentrate it to near saturation.
Allow it to cool so crystals form.
Filter, wash with a little cold distilled water and dry the crystals.
Excess solid ensures all acid is consumed. Filtration works because the excess reagent is insoluble. Do not evaporate to complete dryness when hydrated crystals or well-formed crystals are required.
For copper(II) sulfate, use sulfuric acid with excess copper(II) oxide or carbonate, not copper metal because copper does not react suitably with dilute sulfuric acid in this context.
Prepare a soluble salt by titration
If both reactants are soluble, excess cannot be removed by filtration. To prepare a pure sodium, potassium or ammonium salt from acid and alkali, first use titration to find the exact neutralising volumes.
Repeat using those measured volumes without indicator, because indicator would contaminate the crystals. Then concentrate, cool, crystallise, filter, wash and dry.
Titration is selected because neither soluble reagent can simply be filtered away. Technique, concordant titres and apparatus ownership belong in the practical note.
Prepare an insoluble salt by precipitation
An insoluble salt is made by mixing two suitable soluble solutions that supply its ions. The solid precipitate forms immediately.
For silver chloride:
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Filter the precipitate, wash it with distilled water to remove soluble ions and dry it. Crystallisation is not used because the desired product is already the insoluble solid.
Choose reactants that are soluble when their solutions are mixed. General solubility rules do not have to be recalled for this topic.
Hydrated and anhydrous substances
A hydrated substance is chemically combined with water. An anhydrous substance contains no water. Hydrated does not merely mean that a solid has a wet surface; the water is chemically combined in the substance.
Worked application: choose routes from solubility
To prepare copper(II) sulfate crystals, the target is soluble and copper(II) oxide is insoluble, so add excess CuO to warm dilute sulfuric acid, filter, concentrate and cool. To prepare sodium sulfate crystals, both sodium hydroxide and sulfuric acid are soluble, so first titrate for exact volumes, repeat without indicator, then crystallise. To prepare barium sulfate, the target is insoluble, so mix soluble barium nitrate and sodium sulfate, then filter, wash and dry the precipitate. Washing removes soluble spectator ions without redissolving much product. One decision, target solubility and reagent removability, selects three different methods.
Common misconceptions and corrections
Calling every base an alkali. Only soluble bases are alkalis.
Saying acids contain OH⁻ in water. Aqueous acids contain H⁺.
Saying alkalis contain H⁺ as their characteristic ion. They contain OH⁻.
Predicting oxygen from acid plus metal. Hydrogen forms.
Omitting water from acid-carbonate products. Salt, water and carbon dioxide form.
Using dry litmus for ammonia gas. Damp paper is required.
Giving methyl orange the litmus colours. It is red in acid and yellow in alkali.
Calling pH 7 strongly acidic. It is neutral.
Equating neutralisation with equal volumes. Amounts and ratios matter.
Using a single arrow for partial ethanoic-acid dissociation. Use a reversible arrow.
Calling all metal oxides basic. Al₂O₃ and ZnO are amphoteric.
Calling amphoteric oxides neutral. They react with both acids and bases.
Saying all chlorides are soluble. Lead and silver chlorides are exceptions.
Saying all sulfates are soluble. Barium, calcium and lead sulfates are exceptions.
Calling calcium hydroxide fully insoluble. It is partially soluble.
Choosing titration when excess insoluble solid can be filtered. Use the simpler excess route.
Using excess alkali and filtering it off. A soluble excess remains in solution.
Stopping solid addition before excess remains. Acid may remain and contaminate product.
Filtering before excess solid is present. Completion has not been assured.
Evaporating a crystal solution to dryness. Concentrate and cool.
Keeping indicator in a preparative titration. Repeat exact volumes without it.
Crystallising an insoluble precipitate from solution. Filter it directly.
Using insoluble starting salts for precipitation. Reactant solutions must supply mobile ions.
Calling every wet solid hydrated. Hydrated means chemically combined with water.
Reading the dot in CuSO₄·5H₂O as multiplication. It denotes hydrated composition.
Assessment guidance
For reaction questions, name every product and include a balanced equation where requested. Distinguish litmus or methyl-orange colour from universal-indicator pH evidence. Oxide classification should cite reaction with acid, base or both. For salt preparation, first state whether the target is soluble, then justify titration, excess solid or precipitation from whether the reagents can be separated. Write the sequence through filtration, concentration, cooling, washing and drying, explaining why each stage removes an impurity. Hydration answers must distinguish water chemically combined in a substance from ordinary liquid water on its surface.
Retrieval practice
Complete word and symbol equations for each characteristic acid and base reaction. Recall litmus and methyl-orange colour pairs and compare five universal-indicator pH results. Classify six oxides, including Al₂O₃ and ZnO as amphoteric. Choose and justify preparation methods for soluble salts made by titration or excess solid and insoluble salts made by precipitation. Explain every filtration, washing, concentration, crystallisation and drying step.
Theory and practical ownership
This theory note owns characteristic reactions, indicator interpretation, relative pH, oxide classification, hydrated and anhydrous definitions and preparation-route selection. The dedicated Co-ordinated Sciences practical series owns titration technique, heating, addition, filtration, crystallisation, washing, drying, hazards, observations and evaluation.
