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5.9 Required Practicals: Chemical Changes

GCSE Chemistry · AQA · Chapter 5: Chemical Changes

A free GCSE Chemistry lesson for AQA (8462) from OBA School: required practical 1: 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
  • Calculate the theoretical yield and percentage yield of the salt
  • 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).
  • Percentage yield: (actual mass of product ÷ maximum theoretical mass) × 100

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 (about 1 mol/dm³), 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: percentage yield

40 cm³ of 1.0 mol/dm³ H₂SO₄ was used, so moles of acid = 1.0 × (40 ÷ 1000) = 0.040 mol
Acid : salt = 1 : 1, so 0.040 mol of hydrated crystals, CuSO₄·5H₂O (Mr = 249.5), can form.
Theoretical yield = 0.040 × 249.5 = 9.98 g
Mass of dry crystals obtained = 7.20 g. Percentage yield = (7.20 ÷ 9.98) × 100 = 72.1%

Answer: 72.1%

Why the yield is below 100%: 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. 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 mass of the oxide instead of the moles of the limiting reactant (the acid) when the oxide is in excess.

Revision questions

Try these, then check your answers below each one.

Question 1. In the preparation of copper(II) sulfate, why is the dilute sulfuric acid warmed first? (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 (dissolve) faster. The acid is not boiled.

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 separates an insoluble solid (the residue) from a solution (the filtrate).

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

Answer: False

Working: False. Heat gently (over a water bath) until crystals start to form, then leave to cool. Heating to dryness causes spitting and gives a white powder instead of blue crystals.

Related lessons

Continue in the full course

This lesson is section 5.9 (ARPCH0509) of our free GCSE Chemistry 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 Chemistry course.

 

 

 

 

 

 

 

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