The shape of the lesson
By the end of the lesson, learners can
- Tests for anions: Add acidified silver nitrate to each halide solution in the simulator and compare the three precipitate colours.
- Tests for cations: Add a few drops of reagent, note the precipitate, then add the reagent in excess and note whether it dissolves.
- Flame tests: Dip a clean nichrome or platinum wire (cleaned in concentrated hydrochloric acid) into the solid and hold it in a roaring blue Bunsen flame.
- Halides: HNO3 then AgNO3: Cl− white, Br− cream, I− yellow.
- Sulfate: acidify, Ba(NO3)2: white ppt. Sulfite: acidified KMnO4 purple to colourless.
- NaOH: Cu2+ light blue, Fe2+ green, Fe3+ red-brown; Al3+, Zn2+, Cr3+ dissolve in excess.
- Flames: Li red, Na yellow, K lilac, Ca orange-red, Ba light green, Cu blue-green.
One picture every idea returns to
Anions: carbonate fizzes with dilute acid, giving CO2 that turns limewater milky; halides are acidified with dilute nitric acid and tested with aqueous silver nitrate (chloride white, bromide cream, iodide yellow precipitate); sulfate is acidified and gives a white precipitate with aqueous barium nitrate; sulfite turns acidified aqueous potassium manganate(VII) from purple to colourless; nitrate warmed with aqueous sodium hydroxide and aluminium foil gives ammonia. Cations are tested with aqueous sodium hydroxide and aqueous ammonia: the colour of the precipitate, and whether it dissolves in excess, identifies the ion. Flame tests identify Li+, Na+, K+, Ca2+, Ba2+ and Cu2+ by colour.
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, What is in the test tube?. A white powder could be almost anything. 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: Tests for anions | Add acidified silver nitrate to each halide solution in the simulator and compare the three precipitate colours. Project tab 5 of the lab (Group 17 lab, halide tests) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Tests for cations | Add a few drops of reagent, note the precipitate, then add the reagent in excess and note whether it dissolves. | Pairs |
| 21 to 30 min | Teach and check: Flame tests | Dip a clean nichrome or platinum wire (cleaned in concentrated hydrochloric acid) into the solid and hold it in a roaring blue Bunsen flame. | 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 Flame tests, 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 |
|---|---|
| Halides are acidified with nitric acid, never hydrochloric (it adds chloride ions) or sulfuric acid. | For sulfate, use barium nitrate after acidifying. Fe 2+ gives a green precipitate and Fe 3+ a red-brown one; do not swap them. Copper(II) hydroxide is light blue; the dark blue solution appears only with excess ammonia. Sodium's strong yellow flame can hide other colours, so the wire must be clean. |
Tests for anions and cations: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Describe a test for chloride ions in a solution. Give the result.
Describe how to show that a solid contains carbonate ions.
Solution J gives a green precipitate with aqueous sodium hydroxide. The precipitate does not dissolve in excess. With acidified aqueous barium nitrate it gives a white precipitate. Identify J.
Two white solids each give a white precipitate with aqueous sodium hydroxide that dissolves in excess. Describe a test to tell aluminium ions from zinc ions.
Describe how to carry out a flame test, and give the flame colours for lithium, calcium and barium ions.
Describe the test for ammonium ions and explain the result.
Answer key and marking notes
- Acidify the solution with dilute nitric acid.
- Add aqueous silver nitrate.
- A white precipitate forms.
- Add dilute acid (for example hydrochloric acid).
- Effervescence (fizzing) is seen.
- Bubble the gas through limewater, which turns milky, showing carbon dioxide.
- Green precipitate insoluble in excess NaOH: iron(II) ions, Fe2+ (chromium(III) would dissolve in excess).
- White precipitate with barium nitrate: sulfate ions.
- J is iron(II) sulfate, FeSO4.
- Add aqueous ammonia, then excess.
- Zinc: white precipitate dissolves in excess ammonia. Aluminium: white precipitate does not dissolve.
- Clean a nichrome wire in concentrated hydrochloric acid, dip it in the solid and hold it in a hot (blue) Bunsen flame.
- Lithium: red. Calcium: orange-red.
- Barium: light green.
- Add aqueous sodium hydroxide and warm gently.
- Ammonia gas is given off, which turns damp red litmus paper blue.
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 solution is acidified with dilute nitric acid and aqueous silver nitrate is added. A yellow precipitate forms. Which ion is present?
- Aiodide
- Bchloride
- Cbromide
- Dsulfate
Aqueous sodium hydroxide is added to a solution and a green precipitate forms that does not dissolve in excess. Which ion is present?
- Airon(II)
- Biron(III)
- Ccopper(II)
- Dchromium(III)
A compound gives a lilac flame in a flame test. Which ion does it contain?
- Apotassium
- Bsodium
- Clithium
- Dbarium
Which test shows that a solution contains sulfate ions?
- Aacidify with dilute nitric acid, add aqueous barium nitrate: white precipitate
- Bacidify with dilute nitric acid, add aqueous silver nitrate: white precipitate
- Cadd dilute acid: gas turns limewater milky
- Dadd acidified aqueous potassium manganate(VII): purple to colourless
A blue solution gives a light blue precipitate with a few drops of aqueous ammonia. With excess aqueous ammonia, the precipitate dissolves to give a dark blue solution. Which ion is present?
- Acopper(II)
- Biron(II)
- Czinc
- Daluminium
Solution Y gives a white precipitate with aqueous sodium hydroxide; the precipitate dissolves in excess. With aqueous ammonia, Y gives a white precipitate that also dissolves in excess. With acidified silver nitrate, Y gives a white precipitate. What is Y?
- Azinc chloride
- Baluminium chloride
- Ccalcium chloride
- Dzinc iodide
When testing a solution for chloride ions, why is it acidified with dilute nitric acid and not with dilute hydrochloric acid?
- Ahydrochloric acid contains chloride ions, which would give a white precipitate whatever the sample
- Bnitric acid makes silver chloride more soluble
- Chydrochloric acid would react with the silver nitrate to give a gas
- Dnitric acid turns the precipitate yellow so it is easier to see
A white solid gives no precipitate with acidified barium nitrate. Warmed with aqueous sodium hydroxide and aluminium foil, it gives a gas that turns damp red litmus blue. In a flame test it gives a yellow flame. What is the solid?
- Asodium nitrate
- Bsodium sulfate
- Cpotassium nitrate
- Dammonium chloride
Quiz key
Iodide ions give a yellow precipitate of silver iodide. Chloride gives a white precipitate and bromide a cream precipitate. Sulfate is tested with barium nitrate, not silver nitrate.
Iron(II) gives a green precipitate of iron(II) hydroxide that is insoluble in excess sodium hydroxide. Iron(III) gives a red-brown precipitate. Copper(II) gives a light blue precipitate. Chromium(III) also gives a green precipitate, but it dissolves in excess sodium hydroxide.
Potassium ions give a lilac flame. Sodium gives yellow, lithium gives red and barium gives light green.
Sulfate ions form a white precipitate of barium sulfate with barium nitrate in acid solution. A white precipitate with silver nitrate shows chloride. A gas that turns limewater milky shows carbonate. Decolourising acidified potassium manganate(VII) shows sulfite, not sulfate.
Copper(II) gives a light blue precipitate with ammonia that dissolves in excess to form a dark blue solution. Iron(II) gives a green precipitate that does not dissolve. Zinc gives a white precipitate that dissolves to a colourless solution. Aluminium gives a white precipitate that does not dissolve in excess ammonia.
A white precipitate soluble in excess of both sodium hydroxide and ammonia identifies zinc ions, and a white precipitate with acidified silver nitrate identifies chloride, so Y is zinc chloride. Aluminium hydroxide does not dissolve in excess ammonia. Calcium hydroxide does not dissolve in excess sodium hydroxide. Iodide would give a yellow precipitate, not white.
Hydrochloric acid would add chloride ions, so a white precipitate would form even if the sample had none. Nitric acid adds only nitrate ions, which do not precipitate with silver ions; it is used to remove carbonate ions that would also give a precipitate. Nitric acid does not dissolve silver chloride, no gas is formed with hydrochloric acid, and the acid does not change the precipitate colour.
Ammonia from sodium hydroxide and aluminium foil identifies nitrate, and a yellow flame identifies sodium, so the solid is sodium nitrate. Sodium sulfate would give a white precipitate with barium nitrate. Potassium gives a lilac flame. Ammonium chloride would give ammonia with sodium hydroxide alone, without needing aluminium, and would not give a yellow flame.
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.