The shape of the lesson
By the end of the lesson, learners can
- Solubility rules: Compare the solubilities of the Group 2 hydroxides and sulfates in the simulator; they follow opposite trends, which is why barium sulfate is insoluble.
- Precipitation: Extended.
- Always soluble: Na, K, NH4 compounds and all nitrates.
- Chlorides: not Ag, Pb. Sulfates: not Ba, Ca, Pb.
- Carbonates, hydroxides: insoluble except Na, K, NH4.
- Precipitation: mix, filter, wash residue, dry.
One picture every idea returns to
General rules: all sodium, potassium and ammonium salts are soluble; all nitrates are soluble; chlorides are soluble except silver and lead(II) chlorides; sulfates are soluble except barium, calcium and lead(II) sulfates; carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium (calcium hydroxide is slightly soluble). Extended: an insoluble salt is made by precipitation: mix two solutions that contain its ions, then filter, wash and dry the precipitate.
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, Mix, then filter.. Whether a salt dissolves decides how you make it. 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: Solubility rules | Compare the solubilities of the Group 2 hydroxides and sulfates in the simulator; they follow opposite trends, which is why barium sulfate is insoluble. Project tab 4 of the lab (Group 2 lab, solubility trends) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Precipitation | Extended. | Quiz-Quiz-Trade |
| 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, 16 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 Precipitation, 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.
Quiz-Quiz-Trade
Each student holds a card with a question on the front and the answer on the back. They pair up, quiz each other, coach if needed, trade cards and find a new partner. Three rounds covers the whole set.
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 |
|---|---|
| For an insoluble salt the product you want is the residue on the filter paper, not the filtrate: the opposite of the soluble salt method. | Nitrates are useful starting materials because they are always soluble; for example, use lead(II) nitrate, not lead(II) chloride, as the source of lead ions. |
Insoluble salts and solubility rules: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
State whether each compound is soluble or insoluble in water: (a) lead(II) nitrate, (b) calcium carbonate, (c) potassium sulfate, (d) silver chloride.
(Extended) Describe how to prepare a pure, dry sample of barium sulfate.
(Extended) Name two solutions that could be mixed to make lead(II) iodide, and describe what is seen.
(Extended) Write the ionic equation, with state symbols, for the precipitation of barium sulfate.
(Extended) Explain why lead(II) nitrate, rather than lead(II) carbonate, is used as the source of lead ions when preparing lead(II) iodide.
Predict whether a precipitate forms when each pair of solutions is mixed: (a) sodium carbonate and calcium chloride, (b) potassium nitrate and sodium chloride.
Answer key and marking notes
- (a) soluble
- (b) insoluble
- (c) soluble
- (d) insoluble
- Mix aqueous barium chloride (or barium nitrate) with aqueous sodium sulfate (or dilute sulfuric acid).
- A white precipitate of barium sulfate forms.
- Filter the mixture and wash the residue with distilled water.
- Dry the residue in a warm oven or between filter papers.
- Lead(II) nitrate solution and potassium iodide (or sodium iodide) solution.
- A yellow precipitate forms.
- Ba2+(aq) + SO42−(aq) → BaSO4(s)
- One mark for the correct ions and product, one for state symbols.
- Lead(II) nitrate is soluble, so it provides Pb2+ ions in solution.
- Lead(II) carbonate is insoluble, so its lead ions are not free to react.
- (a) yes: insoluble calcium carbonate precipitates.
- (b) no: every possible product (potassium chloride, sodium nitrate) is soluble.
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.
Which compound is insoluble in water?
- Acopper(II) hydroxide
- Bsodium hydroxide
- Cammonium carbonate
- Dlead(II) nitrate
Which list contains only compounds that are soluble in water?
- Aammonium carbonate, silver nitrate, magnesium sulfate
- Bpotassium chloride, lead(II) chloride, zinc sulfate
- Csodium nitrate, calcium sulfate, copper(II) chloride
- Dpotassium carbonate, zinc carbonate, barium chloride
Which pair of solutions forms a precipitate when mixed?
- Acopper(II) sulfate and sodium hydroxide
- Bsodium chloride and potassium nitrate
- Cammonium sulfate and sodium nitrate
- Dmagnesium chloride and potassium sulfate
A student wants to prepare a sample of calcium carbonate by precipitation. Which pair of starting materials should be used?
- Aaqueous calcium nitrate and aqueous sodium carbonate
- Bsolid calcium oxide and dilute hydrochloric acid
- Caqueous calcium nitrate and aqueous sodium nitrate
- Dsolid calcium sulfate and aqueous sodium carbonate
Aqueous lead(II) nitrate is added to aqueous potassium iodide and a yellow precipitate forms. Which is the ionic equation for this reaction?
- APb2+(aq) + 2I−(aq) → PbI2(s)
- BPb2+(aq) + I−(aq) → PbI(s)
- CPb2+(aq) + 2I−(aq) → PbI2(aq)
- DK+(aq) + NO3−(aq) → KNO3(s)
Aqueous barium nitrate is mixed with aqueous potassium sulfate and a white precipitate forms. Which ions are spectator ions?
- AK+ and NO3−
- BBa2+ and SO42−
- CBa2+ and NO3−
- DK+ and SO42−
Aqueous lead(II) nitrate and aqueous sodium iodide are mixed. Which sequence gives a pure, dry sample of lead(II) iodide?
- Afilter; wash the residue with distilled water; dry it in a warm oven
- Bfilter; keep the filtrate; heat it to the point of crystallisation
- Cheat the whole mixture to dryness
- Dfilter; wash the residue with more sodium iodide solution; dry it
A solution containing 0.030 mol of calcium chloride is mixed with excess aqueous sodium carbonate. What mass of calcium carbonate, CaCO3, is precipitated? (Ar: Ca 40, C 12, O 16)
- A3.0 g
- B1.5 g
- C3.3 g
- D30 g
Quiz key
Hydroxides are insoluble except those of sodium, potassium and ammonium (calcium hydroxide is slightly soluble), so copper(II) hydroxide is insoluble; it forms as a blue precipitate. Sodium hydroxide and ammonium carbonate are soluble because every sodium and ammonium compound is soluble. Lead(II) nitrate is soluble because all nitrates are soluble, even though many other lead(II) salts are not.
Ammonium compounds and nitrates are always soluble, and magnesium sulfate is not one of the insoluble sulfates (barium, calcium, lead(II)), so all three in the first list are soluble. Lead(II) chloride is one of the insoluble chlorides. Calcium sulfate is one of the insoluble sulfates. Zinc carbonate is insoluble, because only sodium, potassium and ammonium carbonates dissolve.
Copper(II) sulfate and sodium hydroxide could form copper(II) hydroxide and sodium sulfate. Copper(II) hydroxide is insoluble, so it appears as a blue precipitate. Swapping partners in the other pairs gives only soluble compounds: potassium chloride and sodium nitrate; ammonium nitrate and sodium sulfate; magnesium sulfate and potassium chloride. All sodium, potassium, ammonium and nitrate compounds are soluble, and magnesium sulfate is soluble, so no precipitate forms in those pairs.
Calcium nitrate is soluble (all nitrates are) and provides Ca2+(aq); sodium carbonate is soluble and provides CO32−(aq). When they are mixed, insoluble calcium carbonate precipitates. Calcium oxide with hydrochloric acid makes calcium chloride, a soluble salt, and supplies no carbonate. Sodium nitrate supplies no carbonate ions. Calcium sulfate is insoluble, so its calcium ions are not free in solution to react.
The yellow precipitate is lead(II) iodide. A Pb2+ ion needs two I− ions to balance its charge, giving PbI2, and it is a solid, so (s) is correct. PbI with one iodide leaves a charge of +1 unbalanced. Writing PbI2(aq) contradicts the fact that it precipitates. Potassium and nitrate ions are spectator ions: potassium nitrate is soluble and stays in solution, so it does not appear in the ionic equation.
The precipitate is barium sulfate, so Ba2+ and SO42− are the ions that react: Ba2+(aq) + SO42−(aq) → BaSO4(s). The K+ and NO3− ions stay dissolved as potassium nitrate, which is soluble, so they are the spectators. The other pairs each include at least one ion that is locked into the barium sulfate precipitate.
Lead(II) iodide is insoluble, so it is trapped on the filter paper as the residue. Washing with distilled water removes the soluble sodium nitrate, and drying gives the pure salt. Keeping the filtrate and crystallising is the soluble salt method; here the filtrate contains only sodium nitrate. Evaporating the whole mixture leaves sodium nitrate mixed with the product. Washing with sodium iodide solution would leave a new soluble impurity behind.
The mole ratio of CaCl2 to CaCO3 is 1 : 1, so 0.030 mol of calcium carbonate forms. Mr of CaCO3 = 40 + 12 + (3 × 16) = 100, so the mass is 0.030 × 100 = 3.0 g. 1.5 g comes from halving the moles. 3.3 g is the mass of the calcium chloride (0.030 × 111), not of the precipitate. 30 g is a factor-of-ten slip.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
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.