The shape of the lesson
By the end of the lesson, learners can
- The lime cycle: Take limestone around the full cycle in the simulator, watching the mass change at each step.
- Reacting-mass calculations: Industrial uses of Group 2 compounds come with simple stoichiometry.
- Heat: CaCO3 → CaO + CO2.
- Slake: CaO + H2O → Ca(OH)2, exothermic.
- Bases neutralise acidic soil and stomach acid.
- BaSO4 is safe because it is insoluble.
One picture every idea returns to
Heating limestone (CaCO3) in a lime kiln gives quicklime (CaO) and CO2; adding water gives slaked lime (Ca(OH)2) in an exothermic reaction; Ca(OH)2 absorbs CO2 to re-form CaCO3, which is why limewater turns milky. Slaked lime and powdered limestone are bases used to neutralise acidic soils and lakes. Limestone is used in building and to make cement. Magnesium hydroxide is a mild base in indigestion remedies, and barium sulfate is safe to swallow for X-ray “barium meals” because it is insoluble.
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 farm to building site.. Limestone, quicklime and slaked lime are three forms of one cycle, and between them they sweeten acidic soils, make cement and test for carbon dioxide. 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 lime cycle | Take limestone around the full cycle in the simulator, watching the mass change at each step. Project tab 5 of the lab (Group 2 lab, the lime cycle) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Reacting-mass calculations | Industrial uses of Group 2 compounds come with simple stoichiometry. | Frayer Model |
| 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, 14 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 Reacting-mass calculations, 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.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
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 |
|---|---|
| Barium compounds in general are toxic. | The barium meal is safe only because BaSO 4 is insoluble; a soluble salt such as BaCl 2 must never be used. And slaking quicklime is strongly exothermic: the mixture can boil, so water must be added carefully. |
Uses of Group 2 compounds: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Write equations for the three steps of the lime cycle, starting with calcium carbonate.
Explain why farmers add slaked lime to acidic soil, with an ionic equation.
Calculate the mass of quicklime produced from 250 kg of limestone, assuming it is pure CaCO3. (Mr: CaCO3 100.1, CaO 56.1)
Explain why barium sulfate can be used in a barium meal but barium chloride cannot.
An indigestion tablet contains 0.290 g of Mg(OH)2 (Mr 58.3). Calculate the volume of 0.100 mol dm−3 HCl that it neutralises.
Suggest why powdered limestone, rather than quicklime, is often chosen to treat acidic lakes.
Answer key and marking notes
- CaCO3 → CaO + CO2.
- CaO + H2O → Ca(OH)2.
- Ca(OH)2 + CO2 → CaCO3 + H2O.
- Ca(OH)2 is a base that neutralises the acid, raising the soil pH so that crops grow better.
- OH− + H+ → H2O (or Ca(OH)2 + 2H+ → Ca2+ + 2H2O).
- Mass ratio CaO : CaCO3 = 56.1 : 100.1.
- 250 × 56.1 ÷ 100.1 = 140 kg.
- BaSO4 is insoluble, so it releases no Ba2+ ions that could be absorbed.
- BaCl2 is soluble, and the toxic Ba2+ ions would be absorbed into the body.
- n(Mg(OH)2) = 0.290 ÷ 58.3 = 4.97 × 10−3 mol.
- Mg(OH)2 + 2HCl, so n(HCl) = 9.95 × 10−3 mol.
- V = 9.95 × 10−3 ÷ 0.100 = 0.0995 dm3 = 99.5 cm3.
- Calcium carbonate is only sparingly soluble, so it neutralises acid slowly and does not overshoot to a high pH that would harm wildlife.
- Quicklime reacts exothermically and forms a strongly alkaline solution, and is hazardous to handle.
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.
Why do farmers spread powdered limestone (CaCO3) or slaked lime (Ca(OH)2) on some fields?
- Ato neutralise acidity in the soil, raising its pH so that crops grow well
- Bto make the soil more acidic, because most crops grow best below pH 5
- Cto add nitrogen to the soil as a fertiliser
- Dto dissolve and wash away barium sulfate from the soil
The diagram shows the lime cycle. Which statement about step 2 is correct?
- ACaO + H2O → Ca(OH)2; the reaction is strongly exothermic (slaking)
- BCaO + H2O → Ca(OH)2; the reaction is endothermic, so the mixture must be heated
- CCaO + H2O → CaCO3 + H2; the reaction is exothermic
- DCaCO3 → CaO + CO2; heat is given out
Patients are given a 'barium meal' of barium sulfate before an X-ray of the digestive system. Why is barium sulfate safe to use, even though barium ions are toxic?
- Abarium sulfate is insoluble, so very few Ba2+ ions dissolve and are absorbed into the body
- Bbarium sulfate is soluble, so it passes out of the body quickly before it can harm anyone
- Cbarium sulfate is toxic, so only a tiny dose is used
- Dbarium sulfate reacts with stomach acid to form harmless barium chloride
Milk of magnesia, a suspension of magnesium hydroxide, is used to treat indigestion. Which equation shows how it works?
- AMg(OH)2 + 2HCl → MgCl2 + 2H2O
- BMg(OH)2 → MgO + H2O
- CMg(OH)2 + H2SO4 → MgSO4 + H2
- DMg + 2HCl → MgCl2 + H2
Limewater is used to test for carbon dioxide. What is limewater and what happens in a positive test?
- Aaqueous calcium hydroxide; it turns milky because insoluble CaCO3 forms: Ca(OH)2 + CO2 → CaCO3 + H2O
- Baqueous calcium oxide; it turns milky because calcium metal is precipitated
- Caqueous calcium carbonate; it turns clear because CO2 dissolves the carbonate
- Daqueous calcium hydroxide; it turns milky because insoluble CaSO4 forms
In a lime kiln, limestone is decomposed: CaCO3 → CaO + CO2. What mass of calcium oxide can be made from 1.00 tonne of pure calcium carbonate? (Mr: CaCO3 100.1, CaO 56.1)
- A0.560 tonne
- B0.440 tonne
- C1.78 tonne
- D0.740 tonne
5.00 g of calcium carbonate is heated until it is completely decomposed. What volume of carbon dioxide is produced, measured at room conditions? (Mr CaCO3 = 100.1; molar gas volume = 24.0 dm3 mol−1)
- A1.20 dm3
- B2.73 dm3
- C0.600 dm3
- D120 dm3
A field contains acid equivalent to 2.45 kg of sulfuric acid. What mass of calcium hydroxide is needed to neutralise it? Ca(OH)2 + H2SO4 → CaSO4 + 2H2O (Mr: H2SO4 98.1, Ca(OH)2 74.1)
- A1.85 kg
- B3.70 kg
- C1.40 kg
- D0.925 kg
Quiz key
Both compounds are bases: Ca(OH)2 + 2H+ → Ca2+ + 2H2O and CaCO3 + 2H+ → Ca2+ + CO2 + H2O. Spreading them on acidic soil raises the pH towards the range most crops need. Bases cannot make soil more acidic, and most crops grow poorly in very acidic soil. Neither compound contains nitrogen, so they are not nitrogen fertilisers. Barium sulfate is not found in farm soil and is insoluble; liming has nothing to do with it.
Step 2 is slaking: CaO + H2O → Ca(OH)2. It releases so much heat that the water can boil, so it is strongly exothermic. It is not endothermic; no heating is needed once water is added. There is no carbon in CaO or water, so CaCO3 cannot form in step 2, and no hydrogen is given off. CaCO3 → CaO + CO2 is step 1, which is endothermic (heat is taken in, in a lime kiln), not step 2.
BaSO4 is insoluble (the least soluble of the Group 2 sulfates), so almost no Ba2+ ions enter solution to be absorbed, while the dense barium atoms still block X-rays and outline the gut. A soluble barium compound would release toxic Ba2+ ions, so solubility would make it dangerous, not safe. The meal is not toxic in this insoluble form, and a large dose is swallowed to coat the gut. BaSO4 does not react with hydrochloric acid, and BaCl2 is soluble and toxic, not harmless.
Mg(OH)2 is a base that neutralises excess stomach acid (HCl), giving a salt and water: Mg(OH)2 + 2HCl → MgCl2 + 2H2O. Its low solubility means the suspension is only weakly alkaline, so it is safe to swallow. Decomposition to MgO needs strong heating and does not occur in the stomach. Stomach acid is hydrochloric acid, and a hydroxide plus an acid gives water, not hydrogen. The medicine contains magnesium hydroxide, not magnesium metal.
Limewater is a saturated solution of Ca(OH)2. Carbon dioxide reacts to give a white precipitate of calcium carbonate: Ca(OH)2 + CO2 → CaCO3 + H2O, so the solution turns milky. CaO reacts with water to form Ca(OH)2, so 'aqueous CaO' is really the hydroxide, and no calcium metal can form. CaCO3 is insoluble, so it cannot be the clear limewater reagent (excess CO2 can later re-dissolve the precipitate, but that is not the test). There is no sulfur in the test, so CaSO4 cannot form.
1 mol CaCO3 (100.1 g) gives 1 mol CaO (56.1 g), so 1.00 tonne gives 1.00 × 56.1 / 100.1 = 0.560 tonne (scaling grams to tonnes does not change the ratio). 0.440 tonne is the mass of CO2 lost, not the CaO left. 1.78 tonne inverts the ratio (100.1 / 56.1), which would give more product than starting material. 0.740 tonne uses the Mr of Ca(OH)2 (74.1) instead of CaO.
n(CaCO3) = 5.00 / 100.1 = 0.0500 mol (0.04995), so n(CO2) = 0.0500 mol and V = 0.04995 × 24.0 = 1.20 dm3. 2.73 dm3 divides 5.00 g by the Mr of CO2 (44.0) instead of CaCO3; the 5.00 g is the mass of carbonate, not of gas. 0.600 dm3 halves the mole ratio, as if 2 mol carbonate gave 1 mol CO2. 120 dm3 multiplies the mass by 24.0 without converting to moles.
n(H2SO4) = 2.45 / 98.1 = 0.0250 kmol (0.02497). The ratio is 1 : 1, so n(Ca(OH)2) = 0.02497 kmol and mass = 0.02497 × 74.1 = 1.85 kg. 3.70 kg uses two moles of Ca(OH)2 per mole of acid, confusing the 2H2O (or the two H+ ions, which one Ca(OH)2 already neutralises with its two OH−) with the base ratio. 1.40 kg uses the Mr of CaO (56.1) instead of Ca(OH)2. 0.925 kg halves the base, as if each Ca(OH)2 neutralised two H2SO4.
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.