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IGCSE Chemistry 0620 · Chapter 7 notes · ~14 min read

Acids, bases & salts

The pH scale and indicators, what acids do to metals, bases and carbonates, how neutralisation works at ion level — and the right method for making any salt.

pH & indicatorsReactions of acidsOxidesMaking saltsSolubility rules
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Acids produce H⁺(aq). Alkalis produce OH⁻(aq). The pH scale runs from 0 (strong acid) through 7 (neutral) to 14 (strong alkali).
  • Acid + metal → salt + hydrogen. Acid + base → salt + water. Acid + carbonate → salt + water + carbon dioxide.
  • Neutralisation is H⁺(aq) + OH⁻(aq) → H₂O(l).
  • Pick the method for a salt from its solubility: soluble → acid + solid or titration; insoluble → precipitation.

By the end you can

  • Use indicators and the pH scale, and describe the typical reactions of acids and bases.
  • Write equations for neutralisation, and the ionic equation for it.
  • Classify oxides as acidic, basic or amphoteric.
  • Recall the solubility rules and choose a method to make a named salt.
  • Explain strong and weak acids, and the idea of proton donors and acceptors. EXT

1Acids, bases and alkalis

What they are

Acid — a substance that produces H⁺ ions in aqueous solution. EXT An acid is a proton donor.

Base — a substance that reacts with an acid to form a salt and water — usually a metal oxide or hydroxide. EXT A base is a proton acceptor.

Alkali — a base that dissolves in water to produce OH⁻ ions. Every alkali is a base, but most bases are insoluble in water.

Examples
acidshydrochloric HCl, sulfuric H₂SO₄, nitric HNO₃, ethanoic CH₃COOH
alkalissodium hydroxide NaOH, potassium hydroxide KOH, calcium hydroxide Ca(OH)₂, ammonia solution NH₃(aq)
bases that are not alkaliscopper(II) oxide CuO, magnesium oxide MgO, iron(III) oxide Fe₂O₃
  • EXT A proton is a hydrogen ion H⁺. In HCl + NH₃ → NH₄⁺ + Cl⁻, HCl gives an H⁺ (donor) and NH₃ takes it (acceptor).

pH and indicators

ACIDICNEUTRALALKALINE01234567891011121314hydrochloricacidlemonjuicevinegarpurewaterbakingsodaammoniasolutionsodiumhydroxidestronger acidstronger alkali(universal indicator colours)
The pH scale and the universal indicator colours. Lower pH = more acidic. Higher pH = more alkaline.
  • pH 0–6 acidic, pH 7 neutral, pH 8–14 alkaline.
  • Universal indicator is a mixture of dyes that gives a different colour at each pH. A pH meter gives a more accurate reading.
IndicatorIn acidNeutralIn alkali
litmusredpurpleblue
methyl orangeredyellowyellow
thymolphthaleincolourlesscolourlessblue
phenolphthaleincolourlesscolourlesspink
universal indicatorred, orange, yellowgreenblue, purple
Memory trickAcids turn litmus Red (A → R). Bases turn it Blue (B → B).
  • Methyl orange changes colour at about pH 4, so it can tell a strong acid from everything else, but it cannot tell a neutral solution from an alkaline one.
Exam tipLearn the colours of three indicators in acid and alkali: litmus, methyl orange and one of the "phthaleins". Say "turns red in acid" — not "goes red-ish".
Common mistake"Acids have a pH of 0 and alkalis of 14." pH 0 and pH 14 are the extremes. Any pH below 7 is acidic and any above 7 is alkaline.

Reactions of acids

Acid + …ProductsObservations
a metalsalt + hydrogenfizzing, the metal dissolves; hydrogen gives a squeaky pop
a base (metal oxide or hydroxide)salt + waterblack CuO dissolves to give a blue solution
a carbonatesalt + water + carbon dioxidefizzing; CO₂ turns limewater milky
  • The salt depends on the acid: hydrochloric acid → chlorides, sulfuric acid → sulfates, nitric acid → nitrates, ethanoic acid → ethanoates.
  • A base can also react with an ammonium salt: ammonium salt + alkali → salt + water + ammonia gas (pungent; turns damp red litmus blue). This releases ammonia from fertilisers.
Worked example

Complete and balance: (a) Mg + H₂SO₄ → (b) CuO + HNO₃ → (c) Na₂CO₃ + HCl →

  1. (a) metal + acid gives salt + hydrogen: Mg + H₂SO₄ → MgSO₄ + H₂
  2. (b) base + acid gives salt + water: CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O
  3. (c) carbonate + acid gives salt + water + CO₂: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
Worked example

Write the equation for the reaction between ammonium chloride and sodium hydroxide.

  1. ammonium salt + alkali → salt + water + ammonia
  2. NH₄Cl + NaOH → NaCl + H₂O + NH₃

Neutralisation

neutralisation: what really happensH⁺+OH⁻OH₂Ospectator ions (unchanged)Na⁺Cl⁻stay dissolved:NaCl forms whenwater is removedacid + alkali → salt + waterHCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)it is exothermic: the temperature risesneutral when H⁺ and OH⁻ are in equal amounts (pH 7)
Neutralisation. H⁺ from the acid and OH⁻ from the alkali combine to form water. The other ions (Na⁺ and Cl⁻) are spectator ions.
H⁺(aq) + OH⁻(aq) → H₂O(l)
ionic equation for neutralisation (acid + alkali)
  • Neutralisation is exothermic — the temperature rises.
  • The solution is exactly neutral (pH 7) when the amounts of H⁺ and OH⁻ are equal.
  • Uses: indigestion tablets (bases neutralise stomach acid), adding lime (calcium hydroxide) to acidic soil or lakes, treating acidic waste.

Strong and weak acidsEXTENDED

STRONG acid: HClWEAK acid: CH₃COOHevery molecule splits into ionsHCl → H⁺ + Cl⁻only a few molecules split into ionsCH₃COOH ⇌ H⁺ + CH₃COO⁻H⁺ ionanionacid moleculestrong/weak = how much it ionises · concentrated/dilute = how much acid is dissolved
A strong acid splits up completely into ions. A weak acid splits up only a little (a reversible reaction). Strength is not the same as concentration.
  • Strong acid: completely ionised (dissociated) in water, for example HCl → H⁺ + Cl⁻. Others: H₂SO₄, HNO₃.
  • Weak acid: only partially ionised. A reversible reaction: CH₃COOH ⇌ H⁺ + CH₃COO⁻. Others: citric acid, carbonic acid.
  • For the same concentration, a strong acid has a lower pH, conducts better and reacts faster with magnesium than a weak acid.
  • Strong/weak says how much the acid ionises. Concentrated/dilute says how much acid is dissolved. They are not the same thing.
Worked example

EXT 25 cm³ of 0.10 mol/dm³ hydrochloric acid and 25 cm³ of 0.10 mol/dm³ ethanoic acid each react with excess magnesium. Compare (a) the starting rate and (b) the final volume of hydrogen.

  1. (a) hydrochloric acid starts faster — it is fully ionised, so it has a higher H⁺ concentration
  2. (b) both have the same number of moles of acid (0.0025 mol)
  3. each gives the same final volume of hydrogen, because the weak acid keeps ionising as H⁺ is used up
Common mistake"A strong acid is a concentrated acid." A dilute acid can still be strong. Strength is about ionisation.

2Oxides

Acidic, basic and amphoteric oxides

Acidic oxide — an oxide of a non-metal. It reacts with bases and alkalis to form a salt. Examples: CO₂, SO₂, NO₂, SiO₂, P₄O₁₀.

Basic oxide — an oxide of a metal. It reacts with acids to form a salt and water. Examples: CuO, MgO, CaO, Na₂O.

Amphoteric oxide EXT — an oxide that reacts with both acids and bases. Examples: aluminium oxide (Al₂O₃) and zinc oxide (ZnO).

  • Metal oxides are basic, non-metal oxides are acidic. A few non-metal oxides (carbon monoxide CO, nitrogen monoxide NO and water) are neutral.
  • Soluble acidic oxides make acids: SO₂ + H₂O → H₂SO₃ (this is part of acid rain). Soluble basic oxides make alkalis: Na₂O + H₂O → 2NaOH.
  • CO₂ + 2NaOH → Na₂CO₃ + H₂O (an acidic oxide reacting with an alkali).
  • EXT Amphoteric: ZnO + 2HCl → ZnCl₂ + H₂O (acting as a base), and it also dissolves in sodium hydroxide solution (acting as an acid). You do not need the full equation for the alkali reaction.
Exam tipThe idea to remember: position in the Periodic Table decides the oxide type — far left basic, middle amphoteric (Al), right acidic. See the Periodic Table chapter.
Common mistakeWriting "all metal oxides are basic". Aluminium and zinc oxides are amphoteric.

3Making salts

What is a salt?

Salt — a compound formed when the H⁺ of an acid is replaced by a metal ion or the ammonium ion (NH₄⁺).

Solubility rules

CompoundSolubility in water
all sodium, potassium and ammonium saltssoluble
all nitratessoluble
chloridessoluble, except silver chloride and lead(II) chloride
sulfatessoluble, except barium, calcium and lead(II) sulfates
carbonatesinsoluble, except sodium, potassium and ammonium carbonates
hydroxidesinsoluble, except sodium, potassium (and calcium, slightly soluble)
Memory trickSPAN — Sodium, Potassium, Ammonium and Nitrates: always soluble.

Which method?

Is the salt soluble in water?NOYESPRECIPITATIONmix two soluble solutionsthat hold the ions you needfilter off the solidwash with distilled waterdry between filter paperWhat is the other reactant?insoluble solid:metal, base orcarbonateadd excess to warmdilute acid, filteroff the excess,crystallisesoluble alkali(NaOH, KOH)titration: acid +alkali + indicator,repeat withoutindicator,crystallisecrystallise: heat to evaporate about halfthe water, leave to cool, filter thecrystals, wash with cold water, dryalways make sure the acid is fully used up or the solid is in excess
Choosing a method. First ask whether the salt is soluble. If yes, the method depends on the other reactant. If no, use precipitation.
  • Method A — a soluble salt from an insoluble solid (metal, base or carbonate): warm some dilute acid and add the solid in excess until no more reacts (no more fizzing or the solid stays). Filter off the excess solid. Crystallise the filtrate (below).
  • Method B — a soluble salt from an alkali (titration): use a pipette to put a known volume (25.0 cm³) of alkali in a flask with an indicator. Add acid from a burette until the indicator changes colour. Note the volume. Repeat without indicator using exactly the same volumes, then crystallise.
  • Method C — an insoluble salt (precipitation) EXT: mix two soluble salt solutions that contain the ions you need. The insoluble salt forms as a precipitate. Filter, wash with distilled water to remove soluble impurities and dry between filter papers.
  • Crystallisation: heat the solution to evaporate about half the water (until a drop on a cold glass rod forms crystals), then leave it to cool slowly. Filter off the crystals, wash with a little cold water and dry.
  • Metals only work if they are moderately reactive (Mg, Zn, Fe). Potassium and sodium are far too dangerous, and copper does not react with dilute acids.
Worked example

Name the method and the chemicals you would use to make (a) copper(II) sulfate, (b) sodium nitrate, (c) barium sulfate.

  1. (a) copper(II) oxide is an insoluble base, and copper(II) sulfate is soluble → Method A: excess CuO + warm dilute sulfuric acid, filter, crystallise. CuO + H₂SO₄ → CuSO₄ + H₂O
  2. (b) sodium hydroxide is a soluble alkali → Method B: titrate NaOH with nitric acid, repeat without indicator, crystallise. NaOH + HNO₃ → NaNO₃ + H₂O
  3. (c) barium sulfate is insoluble → Method C, precipitation: barium chloride solution + sodium sulfate solution. BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl
Worked example

EXT Write the ionic equation for the precipitation of barium sulfate.

  1. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
  2. Cl⁻ and Na⁺ are the same on both sides (spectator ions)
  3. Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Exam tipExplain why you use excess solid: it makes sure all of the acid is used up, so the filtrate is the pure salt solution. The excess can be filtered off because it does not dissolve.
Common mistake"Evaporate to dryness." If you heat until it is dry, you may lose the water of crystallisation and the crystals will be small and powdery. Crystallise by evaporating only part of the water.
Common mistakeNot repeating the titration without the indicator. The indicator would contaminate the salt.

Hydrated and anhydrous salts

Hydrated — containing water of crystallisation (water bonded into the crystal in a fixed ratio). Example: CuSO₄·5H₂O.

Anhydrous — containing no water of crystallisation. Example: CuSO₄ (white).

  • Hydrated crystals lose their water of crystallisation when heated, and the colour may change: blue CuSO₄·5H₂O → white CuSO₄ (see reversible reactions).
Worked example

EXT 2.46 g of hydrated magnesium sulfate, MgSO₄·xH₂O, is heated until all of the water has gone. 1.20 g of anhydrous MgSO₄ is left. Find x. (Mr of MgSO₄ = 120, Mr of H₂O = 18)

  1. mass of water lost = 2.46 − 1.20 = 1.26 g
  2. moles of MgSO₄ = 1.20 ÷ 120 = 0.010 mol
  3. moles of H₂O = 1.26 ÷ 18 = 0.070 mol
  4. ratio MgSO₄ : H₂O = 0.010 : 0.070 = 1 : 7
  5. x = 7, so MgSO₄·7H₂O

4Test yourself

Test yourself — cover the answers first

Quick check

  1. What ion do all acids produce in water?
  2. What colour does litmus turn in alkali?
  3. Complete: acid + carbonate → ?
  4. Give the ionic equation for neutralisation.
  5. Are metal oxides acidic or basic?
  6. Is silver chloride soluble or insoluble?
  7. EXT Name an amphoteric oxide.
  8. EXT What is the difference between a strong acid and a concentrated acid?

Exam-style

  1. Describe how to make crystals of zinc sulfate from zinc oxide. [5]
  2. EXT Describe how to make a pure, dry sample of lead(II) iodide. [4]
  3. EXT Explain why a solution of 0.1 mol/dm³ ethanoic acid has a higher pH than 0.1 mol/dm³ hydrochloric acid. [2]
  4. Magnesium reacts with dilute hydrochloric acid. Name the gas, describe the test, and write the balanced equation. [3]
Show answers
  1. H⁺
  2. blue
  3. salt + water + carbon dioxide
  4. H⁺(aq) + OH⁻(aq) → H₂O(l)
  5. basic (except amphoteric ones)
  6. insoluble
  7. aluminium oxide / zinc oxide
  8. strong = fully ionised in water; concentrated = a lot of acid dissolved per dm³
  9. warm dilute sulfuric acid, and add zinc oxide until in excess
    filter off the unreacted zinc oxide
    heat the filtrate to evaporate about half the water
    leave to cool so that crystals form
    filter, wash and dry the crystals
  10. mix lead(II) nitrate solution and potassium iodide solution
    lead(II) iodide forms as a yellow precipitate
    filter it off and wash with distilled water
    dry in a warm oven or between filter papers
  11. ethanoic acid is a weak acid — only partly ionised, so a lower concentration of H⁺ ions
    hydrochloric acid is strong and fully ionised
  12. hydrogen
    lit splint gives a squeaky pop
    Mg + 2HCl → MgCl₂ + H₂
Practise this chapter
Acids & basesOxidesPreparation of saltsConcentration & titration
Next chapterThe Periodic Table →
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