Making salts

1 Plan and risk assessment

A metal oxide is a base: it neutralises an acid to make a salt and water. Copper(II) oxide is insoluble, so you add it until some is left over (in excess), filter that off, and crystallise the blue copper(II) sulfate from the solution.

CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Prediction

2 Apparatus

• ✓Safety goggles Protect your eyes from hot acid.
• ✓Dilute sulfuric acid, 1.0 mol/dm³ (irritant) The acid: 40 cm³ makes the salt.
• ✓50 cm³ measuring cylinder (resolution 1 cm³) Measures 40 cm³ of acid.
• ✓100 cm³ beaker and glass rod The reaction happens here; stir with the rod.
• ✓Bunsen burner, tripod, gauze and heatproof mat Warms the acid (and could heat the basin).
• ✓Copper(II) oxide powder (harmful), spatula and weighing boat The insoluble base, added until in excess.
• ✓Top-pan balance (resolution 0.01 g) Weighs the copper(II) oxide and the crystals.
• ✓Filter funnel, filter paper, clamp stand and conical flask Removes the excess copper(II) oxide.
• ✓Evaporating basin on a water bath (250 cm³ beaker) Evaporates water gently to the crystallisation point.
• ✓Crystallising dish The crystals grow here as the solution cools.
• ✓Watch glass and filter paper Dry and weigh the crystals.

3 Method: your choices

• Which version are you doing?: AQA required practical 1: a soluble salt, copper(II) sulfate crystalsCopper(II) oxide and warm sulfuric acid, filter, evaporate on a water bath and crystallise; a percentage yield.
• Put on safety gogglesSafety: DoneHot acid can spit or froth into your eyes, and long hair can catch in a Bunsen flame.
• Warm the acid: DoneThe copper(II) oxide reacts much faster with warm acid. Do not boil it: hot acid spits.
• Add the copper(II) oxide: A small spatula at a time, stirring with the glass rodEach portion reacts before the next goes in. No frothing.
• Decide when to stop adding: When some black powder stays at the bottom, however much I stirThe copper(II) oxide is in excess, so all the acid has reacted. The extra solid is filtered off.
• Filter the mixture: Slowly, down the glass rod, keeping the liquid below the top of the paperA clear blue filtrate. A little solution stays soaked in the paper.
• Concentrate the solution: On the water bath, until about half of the solution is leftThe solution reaches the crystallisation point. Blue crystals grow as it cools.
• Leave it to crystallise: Leave it in a cool place for 24 hoursSlow cooling (and a little more evaporation) grows large, regular blue crystals, 3–8 mm across, that are easy to dry.
• Dry the crystals: DoneSolution clinging to the crystals adds to their mass and leaves impurities as it dries.

Anomaly: No anomalous reading.

5 Graph

This practical has no graph: the results are in the table above.

6 Analysis

1. Moles of acid (Higher)

   moles = concentration (mol/dm³) × volume (dm³)

   Calculate the number of moles of sulfuric acid you used.

   0.040 mol Right first time

2. Copper(II) oxide that reacts

   mass = moles × Mr

   One mole of copper(II) oxide reacts with one mole of sulfuric acid. Calculate the mass of copper(II) oxide that reacts with all your acid.

   3.18 g Right first time

3. Copper(II) oxide you added

   mass added = mass weighed out − mass left on the boat

   Calculate the mass of copper(II) oxide you added to the acid.

   3.69 g Right first time

4. Was it in excess?

   You added 3.69 g of copper(II) oxide, and 3.18 g reacts with all your acid. Was the copper(II) oxide in excess?

   Yes: I added more than reacts with all the acid. Right first time

5. Maximum mass of crystals

   mass = moles × Mr

   One mole of acid makes one mole of copper(II) sulfate crystals, CuSO₄·5H₂O. Calculate the maximum mass of crystals your acid could make.

   9.98 g Right first time

6. Mass of crystals

   mass of crystals = mass of watch glass and crystals − mass of watch glass

   Calculate the mass of your dry crystals. If both weighings are used, take their mean first.

   4.46 g Right first time

7. Percentage yield (Chemistry only)

   percentage yield = mass of product made ÷ maximum theoretical mass × 100

   Calculate your percentage yield.

   45 % Right first time

7 Conclusion and evaluation

Conclusion

Sources of error

Improvement

Repeatable or reproducible

9 Exam questions on your data

On your own readings, self-marked against the mark scheme, with error carried forward where it applies. Total 30/30.

1. 01.1[2 marks]

   A student made copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.

   Complete the balanced equation for the reaction. Include state symbols.

   CuO(s) + H₂SO₄(aq) → ________ + ________

   Your answer

   CuSO4(aq) + H2O(l)

   2/2 self-marked

   • CuSO₄ and H₂O (correct formulae)
   • State symbols: CuSO₄(aq) and H₂O(l)

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

2. 01.2[1 mark]

   You warmed the acid before adding the copper(II) oxide.

   Why is the sulfuric acid warmed before the copper(II) oxide is added?

   Tick (✓) one box.

   • ✓ To make the reaction faster (your choice)
   • To make more copper(II) sulfate
   • To evaporate the water from the acid
   • To make the acid less hazardous

   Your answer

   To make the reaction faster

   1/1

   • To make the reaction faster

   Full marks. Clear and precise.

   Model answer: To make the reaction faster (to speed up the reaction).

3. 01.3[2 marks]

   The dilute sulfuric acid is heated until it is almost boiling.

   Suggest one safety precaution the student should take when heating the acid. Give a reason for your answer.

   Your answer

   Wear safety goggles because hot acid could splash into your eyes and damage them.

   2/2 self-marked

   • A precaution: wear goggles (eye protection); do not let the acid boil; turn the Bunsen off before adding the oxide; add the oxide in small portions; tie hair back
   • Linked reason: hot acid could splash or spit (into the eyes) and burn or damage them (the acid is an irritant)

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Wear safety goggles, because the hot acid could spit or splash into the eyes and damage them.

4. 01.4[1 mark]

   The student added copper(II) oxide to the warm acid a little at a time, stirring after each addition.

   How would the student know when the copper(II) oxide was in excess?

   Your answer

   Black solid remains at the bottom of the beaker and no longer disappears when stirred.

   1/1 self-marked

   • (Black) solid remains / no longer disappears (allow copper(II) oxide stays at the bottom)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: Some black solid (copper(II) oxide) stays at the bottom of the beaker and does not disappear when stirred.

5. 01.5[2 marks]

   Explain why copper(II) oxide is added until it is in excess.

   Your answer

   To make sure all the acid reacts. The excess copper oxide is insoluble so it can be filtered off.

   2/2 self-marked

   • So that all the acid reacts (is used up / neutralised)
   • The excess (insoluble) solid can be removed by filtering

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: So that all of the acid reacts (none is left in the solution), and the excess copper(II) oxide is insoluble so it can be removed by filtering.

6. 01.6[1 mark]

   Why is the mixture filtered?

   Your answer

   To remove the excess copper(II) oxide.

   1/1 self-marked

   • To remove the excess (unreacted) copper(II) oxide (ignore “to remove impurities”)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: To remove the excess (unreacted) copper(II) oxide.

7. 01.7[1 mark]

   A student made copper(II) sulfate by adding copper(II) oxide to warm dilute sulfuric acid until no more would react.

   Name one other copper compound that could be added to sulfuric acid to make copper(II) sulfate.

   Your answer

   copper carbonate

   1/1 self-marked

   • Copper(II) carbonate or copper(II) hydroxide (allow CuCO₃ or Cu(OH)₂; do not accept copper, or another salt such as copper chloride)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: Copper(II) carbonate (or copper(II) hydroxide).

8. 02.1[2 marks]

   After filtering, the student has a blue solution of copper(II) sulfate.

   Describe how the student should get copper(II) sulfate crystals from the solution.

   Your answer

   Heat the solution on a water bath to evaporate some of the water until crystals start to form. Then leave it to cool and crystallise.

   2/2 self-marked

   • Heat the solution to evaporate some of the water / until the crystallisation point is reached (do not accept evaporating the acid or the salt, or heating to dryness)
   • Leave the solution to cool / to crystallise

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Heat the solution (on a water bath) to evaporate some of the water until the crystallisation point is reached. Then leave the solution to cool so that crystals form.

9. 02.2[1 mark]

   The student heated the solution in an evaporating basin on a water bath.

   Give one reason why a water bath is used to heat the solution, rather than heating it directly with a Bunsen burner.

   Your answer

   It heats the solution gently and cannot go above 100 °C so the solution does not spit.

   1/1 self-marked

   • It heats gently / cannot go above 100 °C, so the solution does not spit, boil dry, or the crystals do not break down (lose their water)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: The water bath heats the solution gently (it cannot go above 100 °C), so it does not spit or boil dry and the crystals are not decomposed.

10. 02.3[1 mark]

    Another student wants to grow larger crystals.

    What should the student do?

    Tick (✓) one box.

    • Cool the solution quickly in iced water
    • ✓ Leave the solution to cool slowly (your choice)
    • Evaporate the solution to dryness
    • Add more copper(II) oxide to the acid

    Your answer

    Leave the solution to cool slowly

    1/1

    • Leave the solution to cool slowly

    Full marks. Clear and precise.

    Model answer: Leave the solution to cool slowly.

11. 02.4[3 marks]

    You used 40 cm³ of 1.0 mol/dm³ sulfuric acid.

    One mole of sulfuric acid makes one mole of copper(II) sulfate crystals, CuSO₄·5H₂O.

    • Relative formula mass (Mr) of CuSO₄·5H₂O: 249.5

    GCSE Chemistry only, Higher tier.

    Calculate the maximum mass of copper(II) sulfate crystals that could be made.

    Your answer

    9.98

    Working: moles of acid = 1.0 × 40 ÷ 1000 = 0.04 mol mass = 0.04 × 249.5 mass = 9.98 g

    3/3

    • Moles of acid = 1.0 × 40 ÷ 1000 = 0.04 mol
    • Mass = moles × 249.5
    • Maximum mass = 9.98 g

    Full marks. That is exactly what the mark scheme is looking for.

    Model answer: moles of acid = 1.0 × 40 ÷ 1000 = 0.04 mol mass = 0.04 × 249.5 mass = 9.98 g

12. 02.5[3 marks]

    Table 1 shows the masses you recorded for your dry crystals.

    Table 1

    Mass of watch glass in g	Mass of watch glass and dry crystals in g
    33.32	37.77

    GCSE Chemistry only, Higher tier.

    Calculate your percentage yield of copper(II) sulfate crystals.

    Use your answer to the previous question.

    Your answer

    44.6

    Working: mass of crystals = 37.77 − 33.32 = 4.45 g percentage yield = 4.45 ÷ 9.98 × 100 percentage yield = 44.6 %

    3/3

    • Mass of crystals = 37.77 − 33.32 = 4.45 g
    • 4.45 ÷ 9.98 × 100 (allow their maximum mass)
    • Percentage yield = 44.6 % (allow an answer consistent with their maximum mass)

    Full marks. Clear and precise.

    Model answer: mass of crystals = 37.77 − 33.32 = 4.45 g percentage yield = 4.45 ÷ 9.98 × 100 percentage yield = 44.6 %

13. 02.6[1 mark]

    Every group in the class made copper(II) sulfate crystals from 40 cm³ of the same acid. The maximum mass of crystals was 9.98 g.

    Table 2 shows the class results.

    Which result is anomalous?

    Select one row.

    Table 2

    Group	Mass of crystals in g	Percentage yield in %
    You	4.45	45
    A	4.89	49
    B	4.49	45
    C	6.19	62
    D	4.29	43
    E	10.68	107

    Your answer

    group E

    1/1

    • Group E: a yield above 100 %

    Full marks. Your answer meets all the points for this question. Group E’s yield is over 100 %: more crystals than the acid could possibly make.

    Model answer: Group E. Its yield is over 100 %, which is impossible.

14. 02.7[1 mark]

    Suggest why group E’s percentage yield is more than 100 %.

    Your answer

    The crystals were still wet when they were weighed.

    1/1 self-marked

    • The crystals were still wet (not dried) when they were weighed (allow contained water or solution)

    Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

    Model answer: The crystals were still wet when they were weighed, so the solution on them added to their mass.

15. 02.8[2 marks]

    Give two reasons why the percentage yield of dry crystals is less than 100 %.

    Your answer

    Some copper sulfate stays dissolved in the solution and does not crystallise. Some solution is lost on the filter paper and left in the beaker.

    2/2 self-marked

    • Some copper(II) sulfate stays dissolved in the solution (not all of it crystallises)
    • Some solution or crystals are lost: left on the filter paper or glassware, or lost when transferring
    • Not all the crystals were collected (some left in the dish or lost when drying)

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: Some copper(II) sulfate stays dissolved in the solution left in the dish. Some solution is lost on the filter paper and the glassware.

16. 03[6 marks]

    Magnesium sulfate is a soluble salt. It can be made from magnesium oxide, which is insoluble, and dilute sulfuric acid.

    Describe a method to make pure, dry crystals of magnesium sulfate from magnesium oxide and dilute sulfuric acid.

    Your answer

    1. Measure about 40 cm³ of dilute sulfuric acid into a beaker. 2. Warm the acid gently with a Bunsen burner, then turn it off. 3. Add magnesium oxide a little at a time, stirring with a glass rod. 4. Keep adding until the magnesium oxide is in excess: some solid stays and does not disappear. 5. Filter the mixture with a filter funnel and filter paper to remove the excess magnesium oxide. 6. Heat the filtrate in an evaporating basin on a water bath to evaporate some of the water, until the crystallisation point. 7. Leave the solution to cool and crystallise. 8. Remove the crystals and pat them dry with filter paper.

    6/6 self-marked

    • Put dilute sulfuric acid in a beaker (measure a volume)
    • Warm the acid
    • Add magnesium oxide (a little at a time) and stir
    • Until the magnesium oxide is in excess / solid remains
    • Filter (with filter paper and funnel) to remove the excess magnesium oxide
    • Heat the solution (in an evaporating basin, on a water bath or electric heater) to evaporate some water / to the crystallisation point
    • Leave to cool and crystallise
    • Pat the crystals dry with filter paper (or leave in a warm place to dry)

    Self-marked: Level 3, 6 of 6 marks. You ticked 8 of 8 indicative points. This is your own judgement against the mark scheme.

    Model answer: 1. Measure about 40 cm³ of dilute sulfuric acid into a beaker. 2. Warm the acid gently with a Bunsen burner, then turn it off. 3. Add magnesium oxide a little at a time, stirring with a glass rod. 4. Keep adding until the magnesium oxide is in excess: some solid stays and does not disappear. 5. Filter the mixture with a filter funnel and filter paper to remove the excess magnesium oxide. 6. Heat the filtrate in an evaporating basin on a water bath to evaporate some of the water, until the crystallisation point. 7. Leave the solution to cool and crystallise. 8. Remove the crystals and pat them dry with filter paper.

Unseen data: the same skills, new apparatus

1. 01[2 marks]

   A student makes magnesium sulfate crystals. The empty evaporating dish has a mass of 31.06 g. The crystals are dried in a warm oven and weighed in the dish every 10 minutes.

   Mass of evaporating dish and crystals while drying

   Weighing	Mass in g
   1	35.67
   2	35.39
   3	35.22
   4	35.19
   5	35.19

   Calculate the mass of the dry magnesium sulfate crystals.

   Your answer

   4.13

   Working: 35.19 - 31.06

   2/2

   • Uses the constant mass, 35.19 g.
   • Subtracts the dish: the dry crystals in g.

   Full marks. Your answer meets all the points for this question.

   Model answer: The mass stopped changing at 35.19 g (weighings 4 and 5 agree), so the crystals are dry. Mass of crystals = 35.19 − 31.06 = 4.13 g.

2. 02[2 marks]

   A student has a filtered solution of zinc chloride, made from zinc oxide and dilute hydrochloric acid. They want pure, dry crystals of zinc chloride.

   Which TWO steps should they do? Tick (✓) two boxes.

   • ✓ Heat the solution in an evaporating basin on a water bath until crystals start to form, then leave it to cool. (your choice)
   • ✓ Filter off the crystals and pat them dry between pieces of filter paper. (your choice)
   • Heat the solution strongly with a Bunsen burner until all the liquid has gone.
   • Heat the solution to boil off the hydrochloric acid, leaving the zinc chloride behind.
   • Weigh the crystals straight from the solution, so that none are lost.

   Your answer

   Heat the solution in an evaporating basin on a water bath until crystals start to form, then leave it to cool.; Filter off the crystals and pat them dry between pieces of filter paper.

   2/2

   • Heat the solution in an evaporating basin on a water bath until crystals start to form, then leave it to cool.
   • Filter off the crystals and pat them dry between pieces of filter paper.

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: Gentle heating (a water bath or an electric heater) evaporates some of the water until the solution reaches its crystallisation point; crystals grow as it cools. Filter them off and pat them dry. Strong heating to dryness can decompose the salt or drive off its water of crystallisation; it is the water that evaporates, not the acid.

3. 03[2 marks]

   A student makes nickel(II) chloride solution by adding green nickel(II) carbonate powder, a spatula at a time, to dilute hydrochloric acid. Bubbles of carbon dioxide form as it reacts.

   Which TWO statements are correct? Tick (✓) two boxes.

   • ✓ The nickel(II) carbonate is in excess when green solid stays in the beaker however much it is stirred, and no more bubbles form. (your choice)
   • ✓ Using an excess makes sure all the acid reacts; the extra solid can then be filtered off. (your choice)
   • The nickel(II) carbonate is in excess as soon as the solution turns green.
   • Using an excess makes the crystals grow larger.
   • The extra nickel(II) carbonate removes impurities from the acid.

   Your answer

   The nickel(II) carbonate is in excess when green solid stays in the beaker however much it is stirred, and no more bubbles form.; Using an excess makes sure all the acid reacts; the extra solid can then be filtered off.

   2/2

   • The nickel(II) carbonate is in excess when green solid stays in the beaker however much it is stirred, and no more bubbles form.
   • Using an excess makes sure all the acid reacts; the extra solid can then be filtered off.

   Full marks. Your answer meets all the points for this question.

   Model answer: Solid that stays and no more fizzing are visible signs that no acid is left to react. The excess makes sure all the acid is used up, and being insoluble it is easily filtered off. The solution turns green as soon as some carbonate reacts, long before the acid is used up.