The shape of the lesson
By the end of the lesson, learners can
- The pH scale: Use the colour chart to read a pH.
- Following neutralisation: Run the titration: add acid from the burette to the alkali in the flask and watch the indicator change at the end point.
- pH 7 neutral; below acidic; above alkaline.
- Universal indicator: red, orange, yellow, green, blue, purple.
- Lower pH = higher H+ concentration.
- Neutralisation moves the pH towards 7.
One picture every idea returns to
The pH scale runs from 1 to 14. pH 7 is neutral; below 7 is acidic and above 7 is alkaline. Universal indicator gives a different colour for each pH, from red (strongly acidic) through green (neutral) to purple (strongly alkaline). The lower the pH, the higher the concentration of H+ ions. In neutralisation, H+ and OH− ions react to form water, so the pH moves towards 7.
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, How acidic is acidic?. Indicators like litmus only say acid or alkali. 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 pH scale | Use the colour chart to read a pH. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Following neutralisation | Run the titration: add acid from the burette to the alkali in the flask and watch the indicator change at the end point. Project tab 6 of the lab (Stoichiometry lab, titration) and let a student drive it while the class predicts. | Rally Robin |
| 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 Following neutralisation, 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.
Rally Robin
Pairs take turns to say one item each, back and forth: a definition, a unit, a step of the method. Keep it fast and oral; the point is retrieval, not writing.
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 lower pH means a higher concentration of H + ions, not a lower one. | And pH 7 means neutral only if the solution contains no excess acid or alkali; "more acidic" means a smaller pH number. |
pH, indicators and neutralisation: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Give the approximate pH and universal indicator colour of (a) pure water, (b) dilute hydrochloric acid, (c) aqueous sodium hydroxide.
Four solutions have pH values of 1, 6, 9 and 13. Which is (a) a weak alkali and (b) the solution with the highest concentration of hydrogen ions?
Describe and explain the changes in pH as dilute nitric acid is added a little at a time to aqueous potassium hydroxide until the acid is in excess.
Describe how you would find the pH of a sample of soil water using universal indicator paper.
A lake has become acidic because of acid rain. Suggest how the pH could be raised and explain why it works.
State the name of the process in which an acid reacts with a base to form a salt and water.
Answer key and marking notes
- (a) pH 7, green.
- (b) pH 1 to 2, red.
- (c) pH 13 to 14, purple.
- (a) pH 9.
- (b) pH 1.
- The pH starts high (about 13 to 14) because OH− ions are present.
- It falls as H+ ions from the acid react with OH− ions to form water, reaching 7 when neutralisation is complete.
- It then falls below 7 as excess H+ ions are added.
- Dip (or spot) the soil water onto universal indicator paper.
- Compare the colour with the pH colour chart and read off the pH.
- Add powdered limestone or lime (calcium carbonate or calcium hydroxide).
- It neutralises the acid, removing H+ ions, so the pH rises towards 7.
Neutralisation.
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.
Four solutions, W, X, Y and Z, are tested with universal indicator. Their pH values are marked on the colour chart. Which solution has the highest concentration of hydroxide ions?
- AZ
- BW
- CX
- DY
A sample of rainwater turns universal indicator yellow, which matches pH 6. How is the rainwater best described?
- Aweakly acidic
- Bstrongly acidic
- Cneutral
- Dweakly alkaline
In a titration, dilute hydrochloric acid is run from a burette into aqueous sodium hydroxide containing thymolphthalein. What colour change shows the end point?
- Ablue to colourless
- Bcolourless to blue
- Cred to yellow
- Dyellow to red
10 cm3 of dilute hydrochloric acid of pH 1 is diluted by adding 90 cm3 of pure water. What happens to the pH?
- AIt rises, but stays below 7.
- BIt falls below 1.
- CIt becomes exactly 7.
- DIt rises above 7.
A wasp sting injects an alkaline liquid into the skin. Which substance would be most suitable to dab on the sting to neutralise it?
- Avinegar, pH 3
- Bsoap solution, pH 9
- Cpure water, pH 7
- Doven cleaner, pH 13
Dilute hydrochloric acid is exactly neutralised by aqueous sodium hydroxide. Which ions are present in large numbers in the final solution?
- ANa+ and Cl−
- BH+ and OH−
- CNa+ and OH−
- DH+ and Cl−
Which aqueous solution has a pH closest to 7?
- Asodium chloride solution
- Bsodium hydroxide solution
- Cdilute sulfuric acid
- Daqueous ammonia
Why is universal indicator not normally used to find the end point of an acid and alkali titration?
- AIts colour changes gradually through many colours, so the exact end point is hard to see.
- BIt does not change colour in alkaline solutions.
- CIt reacts with the acid and uses some of it up.
- DIt only works in solutions with a pH above 7.
Quiz key
The more hydroxide ions a solution contains, the more alkaline it is and the higher its pH. Z is at pH 13, the most alkaline, so it has the highest OH− concentration. Y at pH 9 is only weakly alkaline. X at pH 7 is neutral. W at pH 3 is acidic: it has the highest concentration of hydrogen ions, not hydroxide ions.
Any pH below 7 is acidic, and a pH just below 7 (4 to 6) is weakly acidic, so the rainwater is weakly acidic. Strongly acidic solutions have pH 1 to 3 and turn universal indicator red. Neutral is exactly pH 7 (green). Alkaline solutions have pH above 7 and give blue or purple colours.
The flask starts alkaline, so thymolphthalein is blue. At the end point the alkali has just been neutralised and the indicator turns colourless. The reverse change, colourless to blue, would happen if alkali were added to acid. Red and yellow are the colours of methyl orange, not thymolphthalein.
Diluting the acid lowers the concentration of H+ ions, so the solution becomes less acidic and the pH rises. Water does not react with the H+ ions, so the acid is not neutralised and the pH stays below 7 (in fact it rises to about 2). A fall below 1 would need a higher H+ concentration. Only a base or alkali could bring the pH to 7 or above.
An alkali is neutralised by an acid, and vinegar is a mild acid that is safe on skin, so it is the best choice. Soap solution and oven cleaner are alkaline, so they would make the sting more alkaline, and oven cleaner is also corrosive. Pure water is neutral: it only dilutes the alkali and does not neutralise it.
In neutralisation, H+ ions from the acid combine with OH− ions from the alkali to form water, so neither is left in large numbers when the reaction is exactly complete. The Na+ and Cl− ions do not react; they remain dissolved as aqueous sodium chloride. Excess OH− would mean too much alkali was added, and excess H+ would mean too little.
Sodium chloride is the salt formed when hydrochloric acid neutralises sodium hydroxide, and its solution is neutral, pH 7. Sodium hydroxide is a strong alkali (pH about 13 to 14) and aqueous ammonia is also alkaline (about pH 11). Dilute sulfuric acid is strongly acidic, around pH 1.
Universal indicator is a mixture that shows a different colour at each pH, so near the end point it drifts through green and neighbouring shades and there is no single sharp change. Indicators such as thymolphthalein or methyl orange have just two colours and change sharply, which is why they are used. Universal indicator does change colour in alkalis (blue to purple), works across the whole pH range, and is added in such small amounts that it does not use up the acid.
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.