Acids, bases and salt preparations

Chemistry· 2.34–2.43 (sub-topic 2(g)) 2017 spec· 25 min read

1. Solubility Rules for Ionic Compounds★★☆☆☆⏱ 5 min

Solubility rules determine which preparation method you will use to make a pure salt, so you must recall all rules listed in the specification for the exam. The table below summarises all mandatory rules:

Compound Type	Solubility Rule
Sodium, potassium, ammonium compounds	All soluble
Nitrates	All soluble
Chlorides	Soluble except silver chloride, lead(II) chloride
Sulfates	Soluble except barium sulfate, calcium sulfate, lead(II) sulfate
Carbonates	Insoluble except sodium, potassium, ammonium carbonate
Hydroxides	Insoluble except sodium, potassium hydroxide; calcium hydroxide is slightly soluble

📐 Worked Example

Classify the following salts as soluble, insoluble, or slightly soluble: (a) sodium carbonate, (b) lead(II) chloride, (c) barium sulfate, (d) calcium hydroxide

1
1. Apply the rule for sodium compounds: all sodium salts are soluble, so sodium carbonate is soluble.
2
1. Apply the chloride rule: chlorides are insoluble for lead(II), so lead(II) chloride is insoluble.
3
1. Apply the sulfate rule: sulfates are insoluble for barium, so barium sulfate is insoluble.
4
1. Apply the hydroxide rule: calcium hydroxide is explicitly listed as slightly soluble.

Exam tip:

Solubility rules are tested either directly in multiple choice or as part of practical salt prep questions, so memorise the grouped rules to avoid common distractors.

2. Brønsted Acid-Base Theory★★☆☆☆⏱ 4 min

📘 Definition

Acids and Bases (Brønsted Definition)

An acid is a proton (H⁺ ion) donor that releases H⁺ ions in solution. A base is a proton acceptor that reacts with H⁺ ions to form neutral products. Bases include metal oxides, metal hydroxides, and ammonia. Alkalis are soluble bases that dissolve in water.

You only need to use this qualitative definition for the exam: quantitative acid strength calculations, conjugate pairs, and Lewis acid-base theory are out of scope for Edexcel IGCSE Chemistry.

📐 Worked Example

Identify the acid and base in the reaction between hydrochloric acid and magnesium oxide: $2 H C l (a q) + M g O (s) \to M g C l_{2} (a q) + H_{2} O (l)$

1
1. HCl donates H⁺ ions to form Cl⁻ ions in solution, so HCl is the acid.
2
1. MgO accepts H⁺ ions to form water, so MgO is the base.

3. Reactions of Common Acids★★★☆☆⏱ 6 min

The three acids tested in this topic are hydrochloric acid (forms chloride salts), sulfuric acid (forms sulfate salts), and nitric acid (forms nitrate salts). The three standard reaction types are summarised below:

Reaction Type	General Equation	Example
Acid + reactive metal	Acid + Metal → Salt + Hydrogen	$2 H C l (a q) + M g (s) \to M g C l_{2} (a q) + H_{2} (g)$
Acid + base/metal oxide/hydroxide	Acid + Base → Salt + Water	$H_{2} S O_{4} (a q) + C u O (s) \to C u S O_{4} (a q) + H_{2} O (l)$
Acid + metal carbonate	Acid + Carbonate → Salt + Water + Carbon Dioxide	$2 H N O_{3} (a q) + N a_{2} C O_{3} (s) \to 2 N a N O_{3} (a q) + H_{2} O (l) + C O_{2} (g)$

📐 Worked Example

Write a balanced symbol equation with state symbols for the reaction between nitric acid and calcium hydroxide

1
1. Identify the reaction type: acid + base → salt + water. Nitric acid forms nitrate salts, so the product is calcium nitrate.
2
1. Write the unbalanced equation: $H N O_{3} (a q) + C a (O H)_{2} (s) \to C a (N O_{3})_{2} (a q) + H_{2} O (l)$
3
1. Balance the equation by adding coefficients: $2 H N O_{3} (a q) + C a (O H)_{2} (s) \to C a (N O_{3})_{2} (a q) + 2 H_{2} O (l)$
4
1. Verify all atoms are equal on both sides and add correct state symbols as shown.

Exam tip:

Reactions of nitric acid with metals are explicitly excluded from the specification, so never write these in your answers even if you have studied them elsewhere.

4. Core Practical: Soluble Salt from Insoluble Reactant★★★☆☆⏱ 5 min

🚫 No Calculator

This method is used for all soluble salts where one reactant is insoluble (e.g. insoluble base, metal carbonate, or metal). You must recall the steps for the required practical to prepare hydrated copper(II) sulfate crystals from copper(II) oxide:

📐 Worked Example

Describe a practical method to prepare a pure, dry sample of hydrated copper(II) sulfate crystals starting from copper(II) oxide and dilute sulfuric acid

1
1. Warm dilute sulfuric acid in a beaker, then add excess copper(II) oxide while stirring, until no more solid dissolves (this confirms all acid is neutralised).
2
1. Filter the mixture to remove unreacted copper(II) oxide, collecting the blue copper sulfate solution in an evaporating basin.
3
1. Heat the solution gently over a Bunsen burner to evaporate roughly 50% of the water, producing a concentrated solution.
4
1. Leave the concentrated solution to cool and crystallise for 24–48 hours.
5
1. Filter the resulting crystals, wash with a small volume of cold distilled water to remove impurities, then pat dry between filter papers.

5. Higher Tier (Chemistry Only) Salt Preparation Methods★★★★☆HL only⏱ 5 min

Higher tier students must recall two additional preparation methods: titration for soluble salts made from an acid and soluble alkali (both reactants are soluble, so excess reactant cannot be filtered out), and precipitation for insoluble salts.

📐 Worked Example

Describe the titration method to prepare a pure, dry sample of sodium chloride from dilute hydrochloric acid and sodium hydroxide solution

1
1. Use a pipette to add a fixed volume of sodium hydroxide solution to a conical flask, add 2–3 drops of phenolphthalein indicator.
2
1. Add hydrochloric acid from a burette slowly, swirling the flask, until the pink indicator just turns colourless (neutralisation point). Record the volume of acid used.
3
1. Repeat the reaction using the same volumes of acid and alkali, without indicator, to produce pure sodium chloride solution.
4
1. Evaporate the solution to concentrate it, leave to crystallise, then filter and dry the sodium chloride crystals.

📐 Worked Example

Describe the method to prepare a pure, dry sample of insoluble lead(II) sulfate

1
1. Mix equal volumes of dilute soluble lead nitrate solution and dilute soluble sodium sulfate solution in a beaker: a white precipitate of lead(II) sulfate forms immediately.
2
1. Filter the mixture to collect the lead(II) sulfate precipitate.
3
1. Wash the precipitate with cold distilled water to remove any soluble impurity residues.
4
1. Dry the precipitate in a warm oven or between clean filter papers.

Exam tip:

Always first use solubility rules to classify your target salt as soluble or insoluble before selecting a preparation method for higher tier exam questions.

6. Common Pitfalls

Wrong move:

Stating all bases are alkalis

Why:

Only soluble bases are classified as alkalis; most metal oxides and hydroxides are insoluble bases, not alkalis

Correct move:

Define alkalis as a subset of soluble bases including NaOH, KOH, Ca(OH)₂, and aqueous ammonia

Wrong move:

Using titration to make copper sulfate from copper oxide and sulfuric acid

Why:

Copper oxide is insoluble, so titration is only used for reactions between two soluble reactants (acid + alkali)

Correct move:

Use the excess insoluble reactant method, filter off unreacted copper oxide, then crystallise the product

Wrong move:

Classifying calcium hydroxide as fully insoluble

Why:

The specification explicitly states calcium hydroxide is slightly soluble, which is a common multiple-choice distractor

Correct move:

Recall only Na/K hydroxides are fully soluble, Ca(OH)₂ is slightly soluble, all other hydroxides are insoluble

Wrong move:

Writing reactions between nitric acid and metals

Why:

Nitric acid + metal reactions are explicitly excluded from the Edexcel IGCSE specification

Correct move:

Only write reactions for nitric acid with bases or metal carbonates to form nitrate salts

Wrong move:

Evaporating all water from a salt solution to produce crystals

Why:

Rapid full evaporation breaks down hydrated crystals and leaves soluble impurities in the final product

Correct move:

Evaporate only 50% of the water to make a concentrated solution, then leave to cool and crystallise slowly

7. Quick Reference Cheatsheet

Category	Key Exam Information
Solubility Rules	All Na/K/ammonium/nitrate salts soluble; Chlorides soluble except AgCl, PbCl₂; Sulfates soluble except BaSO₄, CaSO₄, PbSO₄; Carbonates insoluble except Na/K/ammonium; Hydroxides insoluble except Na/K, Ca(OH)₂ slightly soluble
Acid Definition	Proton (H⁺) donor
Base Definition	Proton (H⁺) acceptor
Acid + Metal →	Salt + Hydrogen (excludes nitric acid + metals)
Acid + Base →	Salt + Water
Acid + Metal Carbonate →	Salt + Water + Carbon Dioxide
Soluble Salt (insoluble reactant) Prep	Add excess insoluble reactant to warm acid 2. Filter 3. Concentrate solution 4. Crystallise 5. Filter, wash, dry
Soluble Salt (acid + alkali, HL) Prep	Titrate to find neutralisation volume 2. Repeat without indicator 3. Crystallise, dry
Insoluble Salt (HL) Prep	Mix two soluble reactant solutions 2. Filter precipitate 3. Wash 4. Dry

8. Frequently Asked

Do I need to memorise solubility rules for the exam?

Yes, solubility rules are not provided on the Edexcel IGCSE Chemistry data sheet, so you must recall them to select the correct salt preparation method and answer multiple-choice questions.

What is the difference between a base and an alkali?

A base is any proton acceptor, including insoluble metal oxides and hydroxides. An alkali is a subset of bases that dissolve in water, e.g. sodium hydroxide, potassium hydroxide, aqueous ammonia.

Why do we add excess insoluble reactant when making a soluble salt?

Adding excess insoluble base, carbonate or metal ensures all the acid is fully neutralised, so no unreacted acid remains in your final salt product to contaminate it.

What's Next

Now that you have mastered acids, bases and salt preparations, you are ready to move on to related inorganic chemistry topics frequently tested alongside this sub-topic in Edexcel IGCSE Chemistry exams. The next core topic is chemical tests for anions and cations, which you will use to identify the salts you have learned to prepare. For higher tier students, consolidate your knowledge of titration calculations to fully answer all 6-mark practical questions on salt preparation. Finally, revise the reactivity series to understand which metals react safely with dilute acids to form soluble salts.

→Reactivity Series of Metals