2.4Unit 1 · Inorganic Chemistry
Acids, bases & salt preparations — Edexcel IGCSE Chemistry Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 7 parts Lesson 4 of 4 1 quiz
Lesson summary
Use the solubility rules to predict whether a salt is soluble.Acid + metal → salt + hydrogen.Acid + base (metal oxide or hydroxide) → salt + water.Acid + metal carbonate → salt + water + carbon dioxide.Ionic equation for neutralisationH+(aq) + OH−(aq) → H2O(l).Hydrochloric acid m…
- Methyl orange
- Acids, bases and protons
- Reactions of acids
- Soluble salt, method 1
- Soluble salt, method 2 (titration)
- Insoluble salt
Section 1 of 7Acids, bases & salt preparations
Essential knowledge
Solubility rules
Solubility rules all sodium, potassium and ammonium salts and all nitrates are soluble.
- Most chlorides are soluble except AgCl and PbCl2; most sulfates are soluble except BaSO4, CaSO4 and PbSO4.
- Most carbonates and hydroxides are insoluble except those of Na, K and NH4+.
Acids, bases and protons
Acids, bases and protons an acid is a proton donor and a base is a proton acceptor. In water, acids release H+ ions; alkalis (soluble bases) release OH− ions. Neutralisation is H+ + OH− → H2O.
Reactions of acids
Reactions of acids with reactive metals they give a salt and hydrogen (e.g. Mg + 2HCl → MgCl2 + H2); with bases they give a salt and water; with carbonates they give a salt, water and carbon dioxide (fizzing).
Bases
Bases include metal oxides (CuO), metal hydroxides (NaOH) and ammonia. Those that dissolve in water are called alkalis and have a pH above 7.
Soluble salt, method 1
Soluble salt, method 1 react the acid with an excess of an insoluble solid (metal, metal oxide, hydroxide or carbonate). Excess ensures all the acid is used up; the excess is removed by filtration and the filtrate is crystallised.
Soluble salt, method 2 (titration)
Soluble salt, method 2 (titration) for Na, K and ammonium salts the base is soluble, so excess cannot be filtered off. Titrate acid against alkali with an indicator to find the exact volumes, then repeat with the same volumes but no indicator, and crystallise.
Insoluble salt
Insoluble salt mix solutions of two soluble salts containing the required ions, e.g. BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq). Filter, wash the precipitate with distilled water and dry it.
Practical – copper(II) sulfate
Practical – copper(II) sulfate add excess copper(II) oxide to warm dilute sulfuric acid, filter, evaporate some water, cool to crystallise, then dry the blue crystals of CuSO4·5H2O. Do not heat to dryness or the water of crystallisation is lost.
Acids and alkalis in water
In water an acid ionises to release hydrogen ions, e.g. HCl(aq) → H+(aq) + Cl−(aq).
- An alkali such as sodium hydroxide releases hydroxide ions: NaOH → Na+(aq) + OH−(aq).
- The H+ ions make a solution acidic; the OH− ions make it alkaline.
Ammonia as an alkali
Ammonia gas dissolves in water and accepts a proton from water: NH3 + H2O ⇌ NH4+ + OH−. The hydroxide ions make the solution alkaline. Ammonia is the gas NH3; ammonium is the ion NH4+.
Which metals react with dilute acids?
Only metals above hydrogen in the reactivity series give hydrogen with dilute acids; copper, silver and gold do not react. Metals like potassium and sodium react explosively, so moderately reactive metals such as magnesium, zinc or iron are used to make salts.
Litmus
Red litmus paper turns blue in an alkali, and blue litmus turns red in an acid. All alkalis are bases, but not all bases are alkalis, because many bases (e.g. copper(II) oxide) do not dissolve in water.
Why all the acid must react
If acid were left over, it would become more and more concentrated as the water is evaporated, making the crystallisation step hazardous. Adding the solid in excess avoids this.
Crystal size
Heating drives off water quickly and gives small crystals. Leaving the filtered solution to evaporate slowly in a warm place over several days produces larger, well-shaped crystals.
Precipitation tips
The general pattern is AB(aq) + CD(aq) → AD(s) + CB(aq), sometimes called double decomposition. Add the second solution in slight excess so all of the first is precipitated. Lead(II) or silver nitrate is a good starting solution because all nitrates are soluble.
Section 2 of 7Process & sequence
1
Warm a measured volume of dilute sulfuric acid in a beaker.
2
Add copper(II) oxide a spatula at a time, stirring, until some remains unreacted (excess).
3
Filter the mixture to remove the excess copper(II) oxide.
4
Heat the blue filtrate in an evaporating basin until crystals form on a cold glass rod.
5
Leave the solution to cool and crystallise slowly.
6
Filter off the crystals and pat them dry between filter papers.
Making copper(II) sulfate crystalswrite these steps in this order; each step is a marking point.
1
Warm dilute sulfuric acid
2
Add excess copper(II) oxide
3
Filter off excess solid
4
Evaporate to crystallisation point
5
Cool, filter and dry crystals
Section 3 of 7Compare
| Compounds | Soluble? | Main exceptions |
|---|---|---|
| Sodium, potassium and ammonium salts | All soluble | none |
| Nitrates | All soluble | none |
| Chlorides | Mostly soluble | silver chloride, lead(II) chloride |
| Sulfates | Mostly soluble | barium, calcium and lead(II) sulfates |
| Carbonates | Mostly insoluble | sodium, potassium, ammonium carbonates |
| Hydroxides | Mostly insoluble | sodium, potassium, calcium (slightly) |
| Salt type | Reactants | Method |
|---|---|---|
| Soluble, from insoluble base/metal/carbonate | acid + excess solid | Add excess solid, filter, crystallise |
| Soluble sodium/potassium/ammonium salt | acid + alkali | Titration, repeat without indicator, crystallise |
| Insoluble salt | two soluble salt solutions | Precipitation, filter, wash, dry |
Section 4 of 7Worked examples
1Describe how to make a pure, dry sample of lead(II) sulfate.
- Lead(II) sulfate is insoluble, so use precipitation.
- Mix lead(II) nitrate solution with sodium sulfate (or dilute sulfuric acid).
- A white precipitate formsPb(NO3)2 + Na2SO4 → PbSO4 + 2NaNO3.
- Filter, wash the solid with distilled water to remove soluble ions, then dry in a warm oven or between filter papers.
2Why is excess copper(II) oxide used when making copper(II) sulfate?
- Using excess ensures that all the sulfuric acid reacts, so the final solution contains only copper(II) sulfate and water.
- The unreacted solid is easily removed by filtration, giving a pure product.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Acid · Base · Alkali · Neutralisation · Salt · Precipitate · Titration · Water of crystallisation · Indicator
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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