The shape of the lesson
By the end of the lesson, learners can
- The excess solid method: Use this method when the other reactant is an insoluble solid, so any unreacted solid can be filtered off.
- The titration method: Sodium, potassium and ammonium compounds are soluble, so an excess cannot be filtered off.
- Insoluble reactant: excess solid method.
- Na, K, NH4 salts: titration method.
- Point of crystallisation, then cool; never to dryness.
- Filter, wash, dry the crystals at the end.
One picture every idea returns to
A salt forms when the hydrogen of an acid is replaced by a metal or ammonium ion. To make a soluble salt from an insoluble solid: add excess solid (metal, base or carbonate) to warm acid, filter off the excess, heat the filtrate to the point of crystallisation, leave to cool, then filter, wash and dry the crystals. For sodium, potassium and ammonium salts, react an acid with an alkali using a titration, then repeat without the indicator and crystallise.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, From acid to crystals.. Pure, dry crystals of a soluble salt can be made in two ways. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: The excess solid method | Use this method when the other reactant is an insoluble solid, so any unreacted solid can be filtered off. | Whole class, then pairs |
| 17 to 30 min | Teach and check: The titration method | Sodium, potassium and ammonium compounds are soluble, so an excess cannot be filtered off. | Rally Robin |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 18 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The titration method, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Rally Robin
Pairs take turns to say one item each, back and forth: a definition, a unit, a step of the method. Keep it fast and oral; the point is retrieval, not writing.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Filter before crystallising, to remove the excess solid; then filter again after crystallising, to separate the crystals from the remaining solution. | And the salt is in the filtrate in the first filtration, not in the residue. |
Preparing soluble salts: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how to prepare pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
Explain why an excess of copper(II) oxide is used, and how you would know it was in excess.
Describe how to prepare a pure sample of potassium nitrate crystals from potassium hydroxide solution and dilute nitric acid.
Name the acid and a suitable insoluble solid to prepare (a) zinc chloride and (b) magnesium sulfate, and name the method.
Why is the salt solution heated only to the point of crystallisation and not evaporated to dryness?
Write a balanced equation for the reaction of zinc carbonate with dilute sulfuric acid, and state one observation.
Answer key and marking notes
- Warm dilute sulfuric acid in a beaker.
- Add copper(II) oxide a little at a time, stirring, until some remains unreacted (excess).
- Filter to remove the excess copper(II) oxide.
- Heat the filtrate until it reaches the point of crystallisation, then leave it to cool.
- Filter off the crystals, wash with a little cold water and dry between filter papers.
- To make sure all the sulfuric acid reacts, so the salt is not contaminated with acid.
- Black solid remains undissolved at the bottom of the beaker.
- Titrate: pipette potassium hydroxide into a flask, add indicator, add nitric acid from a burette until the colour changes; record the volume.
- Repeat with the same volumes without indicator.
- Heat the solution to the point of crystallisation and leave to cool.
- Filter off, wash and dry the crystals.
- (a) hydrochloric acid with zinc (or zinc oxide or zinc carbonate).
- (b) sulfuric acid with magnesium (or magnesium oxide or magnesium carbonate).
- Both by the excess solid method (react, filter, crystallise).
- Evaporating to dryness can decompose the salt or drive off its water of crystallisation.
- Cooling a saturated solution slowly forms pure, well-shaped crystals.
- ZnCO3 + H2SO4 → ZnSO4 + H2O + CO2
- Fizzing (a gas that turns limewater milky); the solid disappears.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Excess black copper(II) oxide was warmed with dilute sulfuric acid. The mixture is now being filtered, as shown. Where is the copper(II) sulfate at this stage?
- Adissolved in the filtrate, Q
- Bon the filter paper as the residue, P
- Cshared equally between P and Q
- Dit has not been formed yet
A student plans to make copper(II) chloride by adding excess copper metal to dilute hydrochloric acid. Why will this not work?
- ACopper does not react with dilute hydrochloric acid.
- BCopper dissolves in water, so the excess cannot be filtered off.
- CCopper would produce carbon dioxide, which contaminates the salt.
- DCopper chloride is insoluble, so it would be filtered off with the excess.
Magnesium carbonate powder is added a spatula at a time to warm dilute sulfuric acid while stirring. How can the student tell that the acid has all been used up?
- AThe fizzing stops and some solid stays undissolved.
- BThe solution turns blue.
- CThe mixture starts to fizz.
- DAll the solid dissolves to leave a clear solution.
At the end of a salt preparation, the crystals are filtered off and rinsed with a small amount of cold distilled water. Why?
- Ato remove the remaining solution from their surface while dissolving very little of the salt
- Bto give the crystals their water of crystallisation
- Cto neutralise any unreacted solid base
- Dto make the crystals grow larger
A student makes sodium sulfate crystals from aqueous sodium hydroxide and dilute sulfuric acid. Which is the correct order of steps?
- Atitrate with indicator; repeat with the same volumes and no indicator; heat to the point of crystallisation; cool and filter off the crystals
- Badd excess sodium hydroxide; filter; heat to the point of crystallisation; cool and filter off the crystals
- Ctitrate with indicator; heat that solution to dryness; wash the solid with acid
- Drepeat with no indicator; titrate with indicator; filter off the crystals; heat to the point of crystallisation
Which is the most suitable way to prepare potassium sulfate?
- Atitrate aqueous potassium hydroxide with dilute sulfuric acid
- Badd excess potassium metal to dilute sulfuric acid, then filter
- Cadd excess solid potassium carbonate to dilute sulfuric acid, then filter off the excess
- Dmix aqueous potassium chloride with dilute sulfuric acid and filter off the precipitate
Excess copper(II) oxide is added to a solution containing 0.050 mol of sulfuric acid. What is the maximum mass of anhydrous copper(II) sulfate, CuSO4, that could form? (Ar: Cu 64, S 32, O 16)
- A8.0 g
- B4.0 g
- C16 g
- D80 g
Zinc sulfate is made by adding excess zinc granules to dilute sulfuric acid. Which gas is given off, and why should there be no flames nearby?
- Ahydrogen, because it is flammable
- Bcarbon dioxide, because it is flammable
- Coxygen, because it makes things burn more fiercely
- Dsulfur dioxide, because it explodes when mixed with air
Quiz key
Copper(II) sulfate is soluble, so it is dissolved in the solution and passes through the filter paper into the flask: it is in the blue filtrate, Q. The black residue, P, is the unreacted excess copper(II) oxide, which is insoluble and is thrown away. The salt is not split between the two, because only undissolved solid is trapped. The salt formed in step 1, when the oxide reacted with the acid, so it already exists before filtering.
Copper is below hydrogen in the reactivity series, so it does not react with dilute hydrochloric acid and no salt forms. The student should use copper(II) oxide or copper(II) carbonate instead. Copper metal is insoluble in water, so filtering would not be the problem. A metal never produces carbon dioxide with an acid; that comes from carbonates. Copper(II) chloride is soluble, so it would not be lost on the filter paper.
While acid is present, the carbonate reacts, giving off carbon dioxide (fizzing) and dissolving. When the acid is used up, extra carbonate cannot react: fizzing stops and solid is left at the bottom, showing the carbonate is in excess. Magnesium sulfate solution is colourless, so no blue colour appears; blue belongs to copper(II) salts. Fizzing starting only shows that the reaction has begun. If all the solid dissolves, some acid may still be left.
Rinsing washes away the leftover solution (which may hold soluble impurities) from the surface of the crystals. Using only a small amount of cold water keeps the loss of the soluble salt low, because less salt dissolves in cold water. The water of crystallisation was built into the crystals as they formed from solution, not added by rinsing. Any excess solid base was already removed by the first filtration. Rinsing does not make crystals grow.
Because sodium hydroxide and sodium sulfate are both soluble, the titration method is needed: first titrate with indicator to find the volume of acid needed, then mix the same volumes without indicator so the salt is not contaminated, then concentrate, cool and collect the crystals. Adding excess sodium hydroxide leaves an alkali that cannot be filtered out of the solution. Heating the indicator-containing solution to dryness gives an impure, possibly decomposed solid. The last sequence repeats before knowing the volumes and filters before any crystals have formed.
Potassium hydroxide is an alkali, so it is titrated with sulfuric acid to find the exact volumes; the salt is then crystallised from the neutral solution. Potassium metal reacts dangerously with acids and must never be used. Potassium carbonate is soluble, so any excess would dissolve and could not be filtered off, contaminating the salt. Potassium sulfate is soluble, so mixing solutions would give no precipitate to collect.
The copper(II) oxide is in excess, so the acid decides the amount of salt. One mole of H2SO4 gives one mole of CuSO4, so 0.050 mol of salt forms. Mr of CuSO4 = 64 + 32 + (4 × 16) = 160, so the mass is 0.050 × 160 = 8.0 g. 4.0 g comes from wrongly halving the moles, 16 g from doubling them, and 80 g from a factor-of-ten slip (0.50 × 160).
A metal reacting with an acid gives a salt and hydrogen: zinc + sulfuric acid → zinc sulfate + hydrogen. Hydrogen is flammable and a hydrogen and air mixture can explode, so flames must be kept away. Carbon dioxide comes from carbonates and does not burn. Oxygen is not produced by any acid reaction here. Sulfur dioxide is not formed when zinc reacts with dilute sulfuric acid.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.