The shape of the lesson
By the end of the lesson, learners can
- Is it water, and is it pure?: A chemical test tells you water is there.
- What is in natural water?: Water from rivers, lakes and underground is never just H 2 O.
- Treating the water supply: Water from a reservoir passes through the same stages in every treatment works.
- Water present: CuSO4 white → blue; CoCl2 blue → pink.
- Water pure: boils at 100 °C, melts at 0 °C.
- Nitrates and phosphates: algae, decay, deoxygenation.
- Treatment: sedimentation, filtration, carbon (tastes, odours), chlorine (microbes).
One picture every idea returns to
Water is detected with anhydrous copper(II) sulfate, which turns from white to blue, or anhydrous cobalt(II) chloride, which turns from blue to pink. These tests show water is present, not that it is pure: pure water boils at exactly 100 °C and melts at exactly 0 °C. Distilled water is used in practical chemistry because it contains no dissolved substances that could interfere with results. Natural water contains some beneficial substances (dissolved oxygen for aquatic life, some metal compounds as minerals) and some harmful ones (toxic metal compounds, plastics, sewage, harmful microbes, and nitrates and phosphates from fertilisers that lead to deoxygenation of water). Domestic water is treated by sedimentation and filtration to remove solids, carbon to remove tastes and odours, and chlorination to kill microbes.
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, Clean water, tested and treated.. Two coloured powders tell you whether water is present, and two temperatures tell you whether it is pure. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Is it water, and is it pure? | A chemical test tells you water is there. | Whole class, then pairs |
| 13 to 21 min | Teach and check: What is in natural water? | Water from rivers, lakes and underground is never just H 2 O. | Pairs |
| 21 to 30 min | Teach and check: Treating the water supply | Water from a reservoir passes through the same stages in every treatment works. | Jot Thoughts |
| 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 Treating the water supply, 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.
Jot Thoughts
Teams write ideas on slips, one idea per slip, as many as they can in two minutes, saying each aloud as they place it in the middle. Then they sort the slips, which is where the structure of the topic appears.
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 |
|---|---|
| Filtration removes insoluble solids only: dissolved salts, nitrates and microbes small enough to pass through are not removed by it. | Chlorine kills microbes; it does not remove solids, tastes or dissolved substances. And a positive copper(II) sulfate test shows that water is present, never that it is pure. |
Water: tests, treatment and pollution: worksheet
6 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
Describe a chemical test for water. Give the reagent and the colour change.
A student says a sample of rain water is pure because it turns anhydrous copper(II) sulfate blue. Explain why the student is wrong and describe how to test whether the water is pure.
Name the four stages in the treatment of the domestic water supply described in the syllabus and state the purpose of each.
Explain how nitrates from fertilisers washed into a river can cause fish to die.
Natural water contains substances that are beneficial and substances that are harmful. Give one beneficial and two harmful substances, with a reason for each.
Suggest why chlorination is the last stage of water treatment rather than the first.
Answer key and marking notes
- Add the liquid to anhydrous copper(II) sulfate.
- It turns from white to blue. (Or: anhydrous cobalt(II) chloride turns from blue to pink.)
- The colour change only shows that water is present; impure water gives the same result.
- Measure the boiling point (or melting point).
- Pure water boils at exactly 100 °C (and melts at exactly 0 °C).
- Sedimentation: large insoluble particles settle out.
- Filtration: sand and gravel trap smaller insoluble particles.
- Carbon: removes tastes and odours.
- Chlorination: kills microbes.
- Nitrates help algae and water plants grow rapidly.
- The algae die (they block light and run short of nutrients).
- Bacteria decompose the dead algae and use up dissolved oxygen.
- The water is deoxygenated, so fish cannot get enough oxygen and die.
- Beneficial: dissolved oxygen, needed by aquatic life (or some metal compounds that provide essential minerals).
- Harmful: sewage, which contains harmful microbes that cause disease.
- Harmful: plastics, which harm aquatic life (or toxic metal compounds, or nitrates and phosphates that cause deoxygenation).
- Solids are removed first, so less chlorine is used up and microbes cannot shelter in particles.
- Adding chlorine last means some remains in the water as it travels to homes, keeping it free of microbes.
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.
A few drops of a colourless liquid are added to anhydrous copper(II) sulfate. The solid turns blue. What does this show?
- Athe liquid contains water
- Bthe liquid is pure water
- Cthe liquid is an acid
- Dthe liquid contains copper ions
Which row gives the correct colour change for each test for water?
| anhydrous copper(II) sulfate | anhydrous cobalt(II) chloride | |
|---|---|---|
| A | white to blue | blue to pink |
| B | blue to white | pink to blue |
| C | white to blue | pink to blue |
| D | blue to white | blue to pink |
- AA
- BB
- CC
- DD
The graph shows the temperature of a water sample as it is heated. What can be concluded about the sample?
- Ait is not pure, because pure water boils at exactly 100 °C
- Bit is pure water, because it boils at a fixed temperature
- Cit is not water, because water boils at 104 °C
- Dit is pure, because it boils above 100 °C
In the treatment of the domestic water supply, what is the purpose of passing the water through carbon?
- Ato remove tastes and odours
- Bto kill microbes
- Cto remove large insoluble particles
- Dto remove dissolved salts such as sodium chloride
Which substance found in natural water is beneficial rather than harmful?
- Adissolved oxygen
- Bharmful microbes from sewage
- Cplastics
- Dnitrates from fertiliser run-off
Large amounts of fertiliser are washed into a lake. Which sequence of events leads to the death of fish?
- Aalgae grow rapidly → algae die → bacteria decompose them, using up dissolved oxygen → fish suffocate
- Bnitrates react with oxygen in the water → nitrogen dioxide forms → the lake becomes acidic → fish die
- Calgae grow rapidly → algae use up all the nitrates → fish have no nitrates to eat → fish starve
- Dphosphates are toxic metal compounds → they poison the algae → the fish are poisoned in turn
A sample of river water is filtered through sand. Which substances could still be present in the filtered water?
- Adissolved salts and microbes
- Bsand and mud only
- Cnothing, because filtered water is pure
- Dlarge insoluble particles such as twigs
A student makes up a solution to test for chloride ions using silver nitrate. She dissolves the sample in tap water and gets a white precipitate. Why might her result be unreliable?
- Atap water contains dissolved chloride ions, so she should have used distilled water
- Bsilver nitrate does not react with chloride ions
- Ctap water is pure, so the result must come from the sample
- Ddistilled water would also give a white precipitate with silver nitrate
Quiz key
Anhydrous copper(II) sulfate turns from white to blue when it gains water, so the liquid contains water. It cannot show purity: salt water or any aqueous solution gives the same blue colour; purity needs a boiling or melting point test. The colour change is not a test for acids. The blue colour comes from the copper(II) sulfate itself becoming hydrated, not from copper ions in the liquid.
Anhydrous copper(II) sulfate is white and becomes blue with water; anhydrous cobalt(II) chloride is blue and becomes pink. Row A has both. Rows B and D give the copper(II) sulfate change in reverse, which is what happens when blue crystals are heated to drive the water off. Rows B and C give the cobalt(II) chloride change in reverse, which is drying the hydrated pink form.
Pure water boils at exactly 100 °C. This sample boils at 104 °C, so it contains dissolved substances, which raise the boiling point. A fixed boiling temperature alone does not prove purity: the value must be exactly 100 °C. Water does not boil at 104 °C, and boiling above 100 °C is a sign of impurity, not purity.
Activated carbon adsorbs the substances that give water an unpleasant taste or smell. Microbes are killed by chlorination, not by carbon. Large particles are removed by sedimentation and filtration earlier in the process. Dissolved salts such as sodium chloride are not removed by any of the normal treatment stages.
Dissolved oxygen is needed by fish and other aquatic life, so it is beneficial. Sewage contains harmful microbes that cause disease. Plastics harm aquatic life, which can swallow them or become trapped. Nitrates from fertilisers lead to deoxygenation of the water, which kills aquatic life.
Nitrates and phosphates make algae grow rapidly; the algae then die, and bacteria decomposing them use up the dissolved oxygen, so fish suffocate. This is deoxygenation of the water. Nitrates do not react directly with oxygen to give nitrogen dioxide in a lake. Fish do not feed on nitrates, so a shortage of nitrates does not starve them. Phosphates are not toxic metal compounds and do not poison the algae; they help them grow.
Filtration removes insoluble solids only. Dissolved salts pass straight through, and many microbes are small enough to pass too, which is why chlorination follows. Sand and mud are trapped, not passed. Filtered water is not pure because dissolved substances remain. Large particles such as twigs are removed by screening and sedimentation and would not pass through sand.
Tap water contains dissolved ions, including chloride from chlorination and from natural minerals, which also give a white precipitate with silver nitrate. Using distilled water removes this source of error. Silver nitrate does react with chloride ions; that is the basis of the test. Tap water is not pure. Distilled water contains no chloride ions, so it would not give a precipitate.
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.