OxfordAQA International GCSE Chemistry: The properties of acids and bases, and preparation of salts (9202) – Revision Notes

These revision notes condense acids, bases and the preparation of salts for OxfordAQA International GCSE Chemistry (9202). They follow the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, and cover sections 3.5.1 (statements a to g) and 3.5.2 (statements a to c). With no tiers in this qualification, every candidate needs all of it. The full teaching, with worked examples, is in the acids, bases and salts study guide.

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Key definitions (3.5.1 a, b, f)

Term	Meaning
Base	A metal oxide or metal hydroxide
Alkali	A soluble hydroxide (a base that dissolves in water)
Salt	The compound formed when the hydrogen of an acid is replaced by a metal (or ammonium) ion
Neutralisation	The reaction of an acid with a base or alkali to form a salt (and usually water)
Acidic solution	Contains H⁺(aq) ions; pH below 7
Alkaline solution	Contains OH⁻(aq) ions; pH above 7
Neutral	pH 7
Precipitate	An insoluble solid formed when two solutions are mixed

Naming salts (3.5.1 c)

Salt name = metal from the base or alkali + ending from the acid.

Acid	Salt ending	Example
Hydrochloric, HCl	chloride	zinc + hydrochloric acid → zinc chloride
Nitric, HNO₃	nitrate	calcium hydroxide + nitric acid → calcium nitrate
Sulfuric, H₂SO₄	sulfate	nickel(II) oxide + sulfuric acid → nickel(II) sulfate

Worked reminders

• NiO + H₂SO₄ → NiSO₄ + H₂O
• Al(OH)₃ + 3HNO₃ → Al(NO₃)₃ + 3H₂O
• Ca(OH)₂ + 2HNO₃ → Ca(NO₃)₂ + 2H₂O

Balancing tip: count the H⁺ the acid must supply. A hydroxide with three OH groups needs three HNO₃ or three HCl.

Word equations to know

Reaction type	General word equation
Acid + metal	salt + hydrogen
Acid + metal oxide	salt + water
Acid + metal hydroxide (or alkali)	salt + water
Acid + ammonia	ammonium salt (no water)

Ammonia and limewater (3.5.1 d, e)

• Ammonia dissolves in water to give an alkaline solution: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
• Ammonia + acid → ammonium salt. Ammonium chloride, ammonium nitrate and ammonium sulfate all come from ammonia and the matching acid. Ammonium nitrate and ammonium sulfate are fertilisers.
• Limewater = calcium hydroxide solution.
• Limewater + carbon dioxide → calcium carbonate + water: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate is insoluble, so the solution turns cloudy white.

pH and universal indicator (3.5.1 f)

pH	0–2	3–6	7	8–11	12–14
Description	very acidic	weakly acidic	neutral	weakly alkaline	very alkaline
Typical universal indicator colour	red	orange / yellow	green	blue	purple

Colours are approximate and differ slightly between brands.

Method in steps: approximate pH with universal indicator

1. Place a few drops of the test solution on a spotting tile (or in a test tube).
2. Add a drop of universal indicator, or dab the solution onto universal indicator paper with a glass rod.
3. Match the colour against the chart; read the pH to the nearest whole number.

Neutralisation equation (3.5.1 g)

H⁺(aq) + OH⁻(aq) → H₂O(l)

• The H⁺ comes from the acid; the OH⁻ from the alkali.
• All other ions are spectators; they stay in solution and form the salt when the water is removed.
• Adding alkali to acid: pH rises → 7 at exact neutralisation → above 7 when alkali is in excess.

Soluble salts (3.5.2 a, b)

Which route?

Starting material	Route	How you know the acid is used up
Moderately reactive metal (Mg, Zn, Fe)	Add excess metal to acid; filter	Bubbling stops; metal left over
Insoluble base (most metal oxides and hydroxides)	Add excess base to acid; filter	Solid stays undissolved
Alkali (soluble hydroxide, e.g. Na, K, Li)	Indicator method with burette and pipette	Indicator changes colour

Metal route limits:

• Too reactive: potassium, sodium, lithium; the reaction is violent and dangerous.
• Not reactive enough: copper, silver, gold; no reaction with dilute hydrochloric or sulfuric acid.

Method in steps: excess solid (metal or insoluble base)

1. Warm the acid gently in a beaker.
2. Stir in small portions of the solid; stop once some refuses to react.
3. Filter off the excess solid.
4. Crystallise the filtrate.

Method in steps: alkali + acid

1. Use a pipette to transfer a measured volume of alkali to a conical flask, then add indicator.
2. Run in acid from a burette until the colour just changes; record the volume.
3. Repeat with the same volumes and no indicator.
4. Crystallise the solution.

Method in steps: crystallisation

1. Heat to evaporate some water until the solution is saturated (crystals form on a cooled glass rod).
2. Leave to cool slowly.
3. Filter off the crystals, rinse with a little cold water, and dry between filter papers.

Example: zinc chloride from zinc. Zn + 2HCl → ZnCl₂ + H₂. Add zinc granules to hydrochloric acid until fizzing stops and zinc remains, filter, crystallise.

Choosing a route for a named soluble salt: ask three questions

1. Which acid gives the right ending? (chloride → HCl, nitrate → HNO₃, sulfate → H₂SO₄)
2. Is the metal safe and reactive enough to use directly? If yes, the metal route works.
3. If not, is its oxide or hydroxide insoluble (excess-and-filter route) or soluble (indicator route)?

Example: sodium nitrate. Sodium is too reactive, and sodium hydroxide is an alkali, so use the indicator route with nitric acid: NaOH + HNO₃ → NaNO₃ + H₂O.

Insoluble salts (3.5.2 c)

• Mix two solutions: one supplies the positive ion, the other the negative ion.
• Choose soluble reagents. Useful facts: all nitrates are soluble; all sodium, potassium and ammonium compounds are soluble.
• Method: mix, filter, wash the residue with distilled water, dry.
• Example: barium sulfate from barium nitrate solution and potassium sulfate solution. Ba(NO₃)₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KNO₃(aq)

Precipitation and water treatment: precipitation removes unwanted ions from solution. For drinking water, calcium ions in hard water can be removed as calcium carbonate. For effluent, dissolved metal ions can be removed as insoluble hydroxides, and phosphate as an insoluble phosphate, before the solid is separated.

Must-know distinctions

• Base vs alkali: all alkalis are bases; only soluble hydroxides are alkalis.
• Excess solid vs indicator: use excess and filter when the reactant is insoluble; use an indicator when both reactants are solutions.
• Filtrate vs residue: for a soluble salt you keep the filtrate; for an insoluble salt you keep the residue.
• Crystallise vs evaporate to dryness: crystallise; do not heat to dryness.
• Ammonia vs ammonium: ammonia is NH₃ (a gas, alkaline in water); ammonium is the NH₄⁺ ion in its salts.

Quick self-test

1. Name the salt formed when potassium oxide reacts with hydrochloric acid.
2. Which of these is an alkali: copper(II) hydroxide, lithium hydroxide, iron(III) oxide?
3. Write the ionic equation for neutralisation, with state symbols.
4. Universal indicator turns blue in a solution. Give a likely pH range and name the ion present in excess.
5. Name the product of ammonia with sulfuric acid, and give its formula.
6. State what forms when carbon dioxide is bubbled through limewater.
7. Why can copper metal not be used to make copper(II) sulfate from dilute sulfuric acid?
8. When making a salt from an insoluble base, how do you know all the acid has reacted?
9. Name two solutions you could mix to make lead(II) iodide.
10. Why is the alkali and acid reaction repeated without indicator?
11. Give one use of precipitation in water treatment.
12. Balance: K₂O + HCl → KCl + H₂O

Answers

1. Potassium chloride.
2. Lithium hydroxide (the only soluble one).
3. H⁺(aq) + OH⁻(aq) → H₂O(l)
4. About pH 8 to 11; hydroxide ions, OH⁻.
5. Ammonium sulfate, (NH₄)₂SO₄.
6. Calcium carbonate (and water); the limewater turns cloudy white.
7. Copper is below hydrogen in the reactivity series, so it does not react with dilute sulfuric acid.
8. Some solid base stays undissolved after stirring.
9. Lead(II) nitrate solution and potassium iodide solution (or sodium iodide solution).
10. So the salt is not contaminated with indicator.
11. Removing metal ions or phosphate from effluent, or removing calcium ions from hard water.
12. K₂O + 2HCl → 2KCl + H₂O

Where marks are usually lost

• Writing “copper oxide is an alkali” because it neutralises acid; it is a base but insoluble.
• Giving “hydrogen” as a product of an acid with an oxide or hydroxide; hydrogen only forms with a metal.
• Adding water as a product of ammonia + acid.
• Describing pH 7 as “weak”; it is neutral.
• Stating a single exact pH from universal indicator; it gives an approximate value from a colour chart.
• Suggesting the metal route for copper, silver or a Group 1 metal.
• Leaving out filtering the excess solid, or keeping the residue when the salt is in the filtrate.
• Naming an insoluble compound (for example, a metal oxide) as a reagent for precipitation; both reagents must be solutions.
• Forgetting to wash the precipitate, so it is contaminated with the other soluble salt.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. These notes cover section 3.5.1 The properties of acids and bases and section 3.5.2 Preparation of salts.