C5 RP Required Practical: Making a Pure Salt

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: required practical: preparing a pure, dry sample of a soluble salt from an insoluble oxide or carbonate. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Choose suitable reactants, then describe and explain each step in making pure, dry crystals of a soluble salt from an insoluble oxide or carbonate
• Carry out a risk assessment for the practical
• Process class results (mean, anomalies, uncertainty) and (Higher tier) calculate the maximum mass of salt that can form
• Evaluate a method and suggest improvements, and identify variables in a related investigation

Key words

• Excess: More of a reactant than is needed, so some is left over and the other reactant is used up.
• Filtrate: The solution that passes through the filter paper (here, the salt solution).
• Residue: The solid left on the filter paper (here, the unreacted oxide or carbonate).
• Water bath: A container of hot water used to heat something gently and evenly.
• Hydrated salt: A salt whose crystals contain water, e.g. CuSO₄·5H₂O (blue).
• Point of crystallisation: The point at which a hot solution is so concentrated that crystals form as soon as it cools a little.

Aim, equipment and the reaction

Aim: to make pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.

copper(II) oxide + sulfuric acid → copper(II) sulfate + water

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) black solid → blue solution

With copper(II) carbonate: CuCO₃(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) + CO₂(g)

Choosing the reactants. Pick an insoluble oxide or carbonate of the metal you want, and the acid that gives the negative ion you want. For example, to make zinc sulfate use zinc carbonate (or zinc oxide) and dilute sulfuric acid: ZnCO₃(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂O(l) + CO₂(g). Use the oxide or carbonate rather than the metal when the metal is below hydrogen (copper) or too reactive to add to acid safely.

Equipment: 100 cm³ beaker, measuring cylinder, dilute sulfuric acid, copper(II) oxide powder, spatula, glass rod, Bunsen burner, tripod, gauze and heatproof mat, filter funnel and filter paper, conical flask, evaporating basin, beaker of water for a water bath (or an electric heater), eye protection.

Method

• Step 1: Measure 40 cm³ of dilute sulfuric acid into a beaker with a measuring cylinder.
• Step 2: Warm the acid gently over a Bunsen burner (to about 50 °C – do not boil). Turn off the Bunsen.
• Step 3: Add copper(II) oxide one spatula at a time, stirring with a glass rod, until some black solid stays undissolved. (With a carbonate, add until the fizzing stops and some solid remains.)
• Step 4: Filter the warm mixture into a conical flask to remove the excess copper(II) oxide.
• Step 5: Pour the blue filtrate into an evaporating basin. Heat it over a water bath until crystals start to form at the edge (about half the water gone). To test, dip a cold glass rod in and lift it out: if tiny crystals form on the rod as it cools, the solution has reached its point of crystallisation – stop heating.
• Step 6: Leave the basin in a cool place to crystallise (for a day or more for larger crystals).
• Step 7: Pour off any remaining liquid. Pat the crystals dry between filter papers, or leave them in a warm place to dry.

Key fact: How do you know the acid has all been used up? With an oxide: some black solid stays undissolved however long you stir. With a carbonate: the fizzing stops and some solid is left. You can also touch a drop of the mixture on universal indicator paper – it should no longer turn red (strongly acidic).

Why each step matters

• warm the acid; Reason speeds up the reaction, so the oxide dissolves faster
• add the oxide in excess; Reason makes sure all the acid reacts, so no acid is left to contaminate the salt
• filter; Reason removes the unreacted (excess) solid, which is insoluble
• heat over a water bath; Reason gentle, even heating; stops the solution spitting and the crystals overheating
• do not evaporate to dryness; Reason crystals would lose their water of crystallisation (white powder) and could be contaminated by any soluble impurities
• cool slowly; Reason gives larger, well-shaped crystals

Variables in a related investigation. A student investigates how the cooling temperature affects crystal size. Independent: cooling temperature (e.g. fridge, room, warm cupboard). Dependent: size (length) of the largest crystals. Control: volume and concentration of the solution, volume evaporated, time left to crystallise, shape of container.

Risk assessment

• dilute sulfuric acid (irritant); Risk irritates eyes and skin; Control measure wear eye protection; wash splashes off with water
• copper(II) oxide and copper(II) sulfate (harmful; toxic to aquatic life); Risk harmful if swallowed; irritates eyes; Control measure eye protection; wash hands; dispose of residues as instructed, not down the sink
• hot apparatus and Bunsen flame; Risk burns; Control measure use tongs; let apparatus cool; tie hair back
• spitting hot solution; Risk burns to skin or eyes; Control measure heat gently over a water bath; stand up; eye protection

Processing results and evaluating

Worked example: (Higher tier) maximum mass of crystals

A student added 3.90 g of CuO. After filtering, the dried excess CuO had a mass of 0.72 g, so 3.90 − 0.72 = 3.18 g reacted.
moles of CuO = 3.18 ÷ 79.5 = 0.040 mol. CuO : CuSO₄ = 1 : 1, so 0.040 mol of hydrated crystals, CuSO₄·5H₂O (Mr = 249.5), can form.
maximum mass = 0.040 × 249.5 = 9.98 g
The student collected 7.20 g of dry crystals – less than the maximum.

Answer: maximum 9.98 g; 7.20 g obtained

Worked example: mean and uncertainty of the class results

Leave out D (5.3 g). mean = (7.6 + 7.4 + 7.7 + 7.5) ÷ 4 = 30.2 ÷ 4 = 7.55 g
range = 7.7 − 7.4 = 0.3 g, so uncertainty = ± 0.15 g

Answer: 7.55 ± 0.15 g

Why less than the maximum mass is collected: some solution stays in the filter paper and beaker; some salt stays dissolved in the liquid left after crystallising; crystals are lost when transferring. Why a mass might be too high: crystals not fully dry. Resolution and precision: weigh the crystals on a balance that reads to 0.01 g (its resolution); repeat results that are close together are precise. Improvements: rinse the beaker and filter paper with a little distilled water; dry the crystals to constant mass; evaporate more slowly and leave longer to crystallise.

Common mistakes

• Saying the excess solid is added to “make more salt”. It makes sure all the acid is used up.
• Forgetting to filter, so the crystals are mixed with black copper(II) oxide.
• Boiling the solution to dryness on a direct flame. This causes spitting and gives a white powder instead of blue crystals.
• Using the whole mass of oxide added in a calculation. Only the oxide that reacted (added minus excess left over) makes salt.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows the first stage of making copper(II) sulfate. Why is the dilute sulfuric acid warmed? (Choose from: to speed up the reaction, to evaporate the water, to make the copper(II) oxide soluble in water, to make crystals form.)

Answer: to speed up the reaction

Working: Warming the acid makes the copper(II) oxide react faster. It is only warmed, not boiled, so it does not spit.

Question 2. Which technique removes the excess copper(II) oxide from the copper(II) sulfate solution? (Choose from: filtration, crystallisation, distillation, chromatography.)

Answer: filtration

Working: Filtration: the insoluble oxide stays on the filter paper and the salt solution passes through.

Question 3. True or false? The copper(II) sulfate solution should be heated until all the water has evaporated.

Answer: False

Working: False. Heat only until crystals start to form, then leave it to cool. Heating to dryness gives a white powder (water of crystallisation lost) and can cause spitting.

Related lessons

• Previous lesson: C5.8 Strong and Weak Acids
• Next lesson: C6.1 Introduction to Electrolysis
• In this chapter: C5.1 The Reactivity Series
• In this chapter: C5.2 Displacement Reactions
• In this chapter: C5.3 Extracting Metals

Continue in the full course

This lesson is section C5 RP (AGCSC2208) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.