# Acids, bases and salt preparations

> Chemistry · Edexcel IGCSE 4CH1
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-acids-bases-and-salt-preparations/

This guide covers all required content for Edexcel IGCSE Chemistry 4CH1 sub-topic 2(g), including solubility rules, Brønsted acid-base definitions, standard acid reactions, and practical salt preparation methods for core and higher tiers.

**Prerequisites:** [Writing balanced chemical equations with state symbols](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s1-chemical-formulae-equations/); [pH scale and common indicators](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-ph-scale-indicators/)

## Learning objectives

- Recall mandatory solubility rules for common ionic compounds
- Define acids and bases using qualitative Brønsted proton transfer theory
- Write balanced symbol equations with state symbols for standard acid reactions
- Describe practical procedures to prepare pure, dry soluble and insoluble salts
- Recall steps for required practicals for hydrated copper(II) sulfate and lead(II) sulfate

## Solubility Rules for Ionic Compounds

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. 2. Apply the chloride rule: chlorides are insoluble for lead(II), so lead(II) chloride is insoluble.
3. 3. Apply the sulfate rule: sulfates are insoluble for barium, so barium sulfate is insoluble.
4. 4. 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.

## Brønsted Acid-Base Theory

**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: $2HCl(aq) + MgO(s) \rightarrow MgCl_2(aq) + H_2O(l)$

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

## Reactions of Common Acids

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 | $2HCl(aq) + Mg(s) \rightarrow MgCl_2(aq) + H_2(g)$ |
| Acid + base/metal oxide/hydroxide | Acid + Base → Salt + Water | $H_2SO_4(aq) + CuO(s) \rightarrow CuSO_4(aq) + H_2O(l)$ |
| Acid + metal carbonate | Acid + Carbonate → Salt + Water + Carbon Dioxide | $2HNO_3(aq) + Na_2CO_3(s) \rightarrow 2NaNO_3(aq) + H_2O(l) + CO_2(g)$ |

**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. 2. Write the unbalanced equation: $HNO_3(aq) + Ca(OH)_2(s) \rightarrow Ca(NO_3)_2(aq) + H_2O(l)$
3. 3. Balance the equation by adding coefficients: $2HNO_3(aq) + Ca(OH)_2(s) \rightarrow Ca(NO_3)_2(aq) + 2H_2O(l)$
4. 4. 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.

## Core Practical: Soluble Salt from Insoluble Reactant

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. 2. Filter the mixture to remove unreacted copper(II) oxide, collecting the blue copper sulfate solution in an evaporating basin.
3. 3. Heat the solution gently over a Bunsen burner to evaporate roughly 50% of the water, producing a concentrated solution.
4. 4. Leave the concentrated solution to cool and crystallise for 24–48 hours.
5. 5. Filter the resulting crystals, wash with a small volume of cold distilled water to remove impurities, then pat dry between filter papers.

*Calculator:* forbidden

## Higher Tier (Chemistry Only) Salt Preparation Methods

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. 2. 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. 3. Repeat the reaction using the same volumes of acid and alkali, without indicator, to produce pure sodium chloride solution.
4. 4. 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. 2. Filter the mixture to collect the lead(II) sulfate precipitate.
3. 3. Wash the precipitate with cold distilled water to remove any soluble impurity residues.
4. 4. 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.

## Common pitfalls

- **Wrong:** Stating all bases are alkalis
  - Why it fails: Only soluble bases are classified as alkalis; most metal oxides and hydroxides are insoluble bases, not alkalis
  - Correct: Define alkalis as a subset of soluble bases including NaOH, KOH, Ca(OH)₂, and aqueous ammonia
- **Wrong:** Using titration to make copper sulfate from copper oxide and sulfuric acid
  - Why it fails: Copper oxide is insoluble, so titration is only used for reactions between two soluble reactants (acid + alkali)
  - Correct: Use the excess insoluble reactant method, filter off unreacted copper oxide, then crystallise the product
- **Wrong:** Classifying calcium hydroxide as fully insoluble
  - Why it fails: The specification explicitly states calcium hydroxide is slightly soluble, which is a common multiple-choice distractor
  - Correct: Recall only Na/K hydroxides are fully soluble, Ca(OH)₂ is slightly soluble, all other hydroxides are insoluble
- **Wrong:** Writing reactions between nitric acid and metals
  - Why it fails: Nitric acid + metal reactions are explicitly excluded from the Edexcel IGCSE specification
  - Correct: Only write reactions for nitric acid with bases or metal carbonates to form nitrate salts
- **Wrong:** Evaporating all water from a salt solution to produce crystals
  - Why it fails: Rapid full evaporation breaks down hydrated crystals and leaves soluble impurities in the final product
  - Correct: Evaporate only 50% of the water to make a concentrated solution, then leave to cool and crystallise slowly

## 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 | 1. Add excess insoluble reactant to warm acid 2. Filter 3. Concentrate solution 4. Crystallise 5. Filter, wash, dry |
| Soluble Salt (acid + alkali, HL) Prep | 1. Titrate to find neutralisation volume 2. Repeat without indicator 3. Crystallise, dry |
| Insoluble Salt (HL) Prep | 1. Mix two soluble reactant solutions 2. Filter precipitate 3. Wash 4. Dry |

## 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](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-reactivity-series/)

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