The shape of the lesson
By the end of the lesson, learners can
- The six tests: Learn the test and the result as a pair.
- Ammonia and where gases come from: Warm an ammonium salt with alkali in the simulator and hold damp red litmus at the mouth of the tube.
- H2: lighted splint, squeaky pop. O2: glowing splint, relights.
- CO2: limewater turns milky.
- NH3: damp red litmus turns blue. Cl2: bleaches damp litmus.
- SO2: acidified KMnO4(aq) purple to colourless.
One picture every idea returns to
Ammonia turns damp red litmus paper blue. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus paper. Hydrogen gives a squeaky pop with a lighted splint. Oxygen relights a glowing splint. Sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless. The litmus must be damp, because a gas has to dissolve in water before it can act as an alkali, acid or bleach.
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, Six gases, six tests.. Colourless gases look identical, but each one reacts in a way that gives it away: a squeaky pop, a splint bursting into flame, limewater turning milky. 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 six tests | Learn the test and the result as a pair. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Ammonia and where gases come from | Warm an ammonium salt with alkali in the simulator and hold damp red litmus at the mouth of the tube. Project tab 1 of the lab (Nitrogen and sulfur lab, ammonia and NH4+) and let a student drive it while the class predicts. | Mix-Pair-Share |
| 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, 13 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 Ammonia and where gases come from, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| A lighted splint is for hydrogen (squeaky pop); a glowing splint is for oxygen (relights). | It is the limewater that turns milky, not the gas. Ammonia turns red litmus blue, so blue litmus would show no change. Chlorine bleaches litmus; "turns red" alone does not score. |
Tests for gases: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the tests for hydrogen and for oxygen, giving the result of each.
Describe the test for chlorine and give the result.
A white solid is warmed with aqueous sodium hydroxide. A gas is produced that turns damp red litmus paper blue. Name the gas and the ion present in the solid.
Explain why limewater turns milky when carbon dioxide is bubbled through it. Include an equation.
Name the gas produced in each reaction and state how you would test for it: (a) magnesium with dilute hydrochloric acid, (b) calcium carbonate with dilute nitric acid, (c) sodium sulfite with dilute hydrochloric acid.
A student tests a gas with dry red litmus paper and sees no change, then concludes the gas is not ammonia. Explain why this conclusion may be wrong.
Answer key and marking notes
- Hydrogen: hold a lighted splint at the mouth of the tube; there is a squeaky pop.
- Oxygen: put a glowing splint into the gas; it relights.
- Hold damp litmus paper in the gas.
- The litmus is bleached (turns white).
- The gas is ammonia.
- The solid contains ammonium ions, NH4+.
- Carbon dioxide reacts with calcium hydroxide (limewater) to form insoluble calcium carbonate.
- Ca(OH)2 + CO2 → CaCO3 + H2O; the white precipitate makes it milky.
- (a) hydrogen: lighted splint gives a squeaky pop.
- (b) carbon dioxide: limewater turns milky.
- (c) sulfur dioxide: acidified aqueous potassium manganate(VII) turns from purple to colourless.
- The litmus paper was dry; ammonia must dissolve in water to form an alkaline solution.
- The test should be repeated with damp red litmus paper.
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 gas gives a squeaky pop with a lighted splint?
- Ahydrogen
- Boxygen
- Ccarbon dioxide
- Dchlorine
What is the test for oxygen?
- Ait relights a glowing splint
- Bit gives a squeaky pop with a lighted splint
- Cit turns limewater milky
- Dit bleaches damp litmus paper
A gas turns damp red litmus paper blue. Which gas is it?
- Aammonia
- Bchlorine
- Csulfur dioxide
- Dcarbon dioxide
Calcium carbonate is added to dilute hydrochloric acid. Which test identifies the gas produced?
- Abubble it through limewater: limewater turns milky
- Bhold a glowing splint in it: the splint relights
- Chold damp red litmus in it: the litmus turns blue
- Dadd acidified potassium manganate(VII): purple to colourless
Which gas bleaches damp litmus paper?
- Achlorine
- Bammonia
- Chydrogen
- Doxygen
Sodium sulfite is warmed with dilute hydrochloric acid. Which observation would confirm the gas given off?
- Aacidified aqueous potassium manganate(VII) turns from purple to colourless
- Blimewater turns milky
- Ca lighted splint gives a squeaky pop
- Ddamp red litmus paper turns blue
A student holds dry red litmus paper in a jar of ammonia and sees no change. Why?
- Aammonia must dissolve in water to form an alkaline solution before litmus can change
- Bammonia is an acidic gas
- Cammonia only changes blue litmus
- Dammonia is less dense than air so it does not touch the paper
Concentrated aqueous sodium chloride is electrolysed with inert electrodes. Which row gives the correct test results for the gases at the anode and the cathode?
- Aanode: bleaches damp litmus paper; cathode: squeaky pop with a lighted splint
- Banode: squeaky pop with a lighted splint; cathode: bleaches damp litmus paper
- Canode: relights a glowing splint; cathode: squeaky pop with a lighted splint
- Danode: bleaches damp litmus paper; cathode: relights a glowing splint
Quiz key
Hydrogen burns rapidly with oxygen in the air, giving a squeaky pop. Oxygen relights a glowing splint rather than popping. Carbon dioxide puts out a lighted splint, and chlorine is tested with damp litmus paper.
Oxygen makes a glowing splint burst back into flame. The squeaky pop is the test for hydrogen, limewater turning milky is the test for carbon dioxide, and bleaching damp litmus is the test for chlorine.
Ammonia dissolves in the water on the paper to give an alkaline solution, which turns red litmus blue. Chlorine bleaches litmus, and sulfur dioxide and carbon dioxide are acidic gases that would turn blue litmus red, not red litmus blue.
Carbonates react with acids to give carbon dioxide, which turns limewater milky. A glowing splint relighting would show oxygen. Damp red litmus turning blue shows ammonia. Acidified potassium manganate(VII) turning colourless shows sulfur dioxide.
Chlorine dissolves in the water on the paper and bleaches it white (blue litmus may turn red first). Ammonia turns red litmus blue without bleaching. Hydrogen and oxygen have no effect on litmus.
Sulfites and acids give sulfur dioxide, a reducing agent that turns acidified potassium manganate(VII) from purple to colourless. Limewater turning milky is the test for carbon dioxide (sulfur dioxide can also cloud limewater, so the manganate(VII) test is the one used to identify it). A squeaky pop shows hydrogen and damp red litmus turning blue shows ammonia.
Ammonia forms hydroxide ions only when it dissolves in water, so the litmus must be damp for the alkali to turn it blue. Ammonia is alkaline, not acidic. It turns red litmus blue; blue litmus would show no change. Ammonia is less dense than air, but a jar full of the gas still surrounds the paper.
Chloride ions are oxidised at the anode to chlorine, which bleaches damp litmus; hydrogen is formed at the cathode and gives a squeaky pop. Swapping the electrodes puts a positive-ion product at the positive electrode, which is wrong. Oxygen at the anode is the product from dilute sodium chloride, not concentrated. Oxygen is never formed at the cathode.
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.