Launch practical
About 60 minutes
- Set up the bench with the 7 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results and plot the graph.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 18 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. The starting temperature of the acid. Record the display exactly as it shows.
What it shows
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Neutralisation is exothermic: it transfers energy to the surroundings, here the solution itself, so the temperature rises with each portion of alkali until all the acid has reacted. After the end point no more reaction happens, and the extra, cooler alkali and energy lost to the surroundings make the temperature fall. Where the two straight lines cross is the end point.
The graph to expect. Two straight lines that meet at a peak: the temperature rises while acid is left to react, then falls once the alkali is in excess. The lines are extended until they cross.
Key equation for the exam: acid + alkali → salt + water.
Equation to write. Temperature change = highest temperature − starting temperature.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Eye protection (goggles) | Protects the eyes from corrosive splashes. | — |
| Dilute hydrochloric acid, 2.0 mol/dm³ (irritant) | The acid: 30 cm³ for each run. | — |
| Sodium hydroxide solution, 2.0 mol/dm³ (corrosive) | The alkali, added 5 cm³ at a time. | — |
| Expanded polystyrene cup and lid with a hole | Insulated container for the reaction. | — |
| 250 cm³ glass beaker | Holds the cup steady (or is the container, if you choose). | — |
| Measuring cylinders: 50 cm³ and 10 cm³ | Measure the acid and the portions of alkali. | 1 cm³ and 0.2 cm³ |
| Thermometer: digital, or glass 0–110 °C | Measures the temperature of the mixture; also used to stir. | 0.1 °C (digital), 1 °C divisions (glass) |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Check the thermometer
Stand the thermometer beside the room thermometer for a minute and compare the two readings.
Why: A thermometer that reads high (or low) adds the same error to every temperature.
Let both solutions reach room temperature
Pour some of each solution into a labelled beaker and stand them side by side on the bench until both read the same temperature.
Why: The temperature change only comes from the reaction if the acid and the alkali start at the same temperature.
Put on eye protectionSafety
Wear goggles from the moment the bottles are opened until everything is cleared away.
Why: Sodium hydroxide solution is corrosive: a splash in the eye can cause serious, permanent damage.
Choose what to do the reaction in
The reaction mixture must lose as little energy to the surroundings as possible.
Do: Polystyrene cup with a lid.
Why: Energy that escapes through the container does not raise the temperature of the mixture.
Stand the cup in a beakerSafety
Put the polystyrene cup inside a 250 cm³ glass beaker.
Why: A polystyrene cup is light and tips over easily, spilling corrosive solution.
Choose a measuring cylinder for the 5 cm³ portions
Each portion of sodium hydroxide is 5 cm³. Measure the 30 cm³ of acid with the 50 cm³ cylinder.
Do: 10 cm³ measuring cylinder (0.2 cm³ divisions).
Why: The smaller the divisions, the more precisely you can measure a small volume.
Fit the lid after each addition
Put the lid on the cup straight after pouring in each portion, with the thermometer through the hole.
Why: Warm air and water vapour escape from an open top, carrying energy away.
Choose a thermometer
You will stir with it through the hole in the lid, with the bulb (or probe tip) fully under the solution.
Do: Digital thermometer (resolution 0.1 °C).
Why: The temperature changes by only a few degrees each time, so the resolution matters.
Take the readings
Starting temperature, then the highest temperature after each 5 cm³ of sodium hydroxide, up to 40 cm³. Two runs, then the mean.
Repeats: Repeat the whole investigation with fresh acid and calculate the mean highest temperature for each volume. A mean reduces the effect of random errors (stirring differently, reading at slightly different moments) but not systematic ones (energy transferred to the surroundings, a thermometer that reads high).
Make safe and clear upSafety
Pour the neutral salt solution down the sink with plenty of water. Rinse the thermometer and the measuring cylinders with water. Put the used cup in the bin and wipe up any spills. Put the lids back on the stock bottles, then take off your goggles last.
While you take readings
Stir after each addition
Gently stir the solution with the thermometer (or probe) as soon as each portion is in.
Why: Until it is stirred, the cold alkali and the warm mixture are in layers, and the thermometer reads only the part around it.
Wait for the highest temperature
Watch the reading after each addition and record it when it stops changing: the highest temperature.
Why: The thermometer needs a few seconds to catch up with the mixture.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Sodium hydroxide solution, 2.0 mol/dm³ (corrosive) | Serious eye damage and skin burns if splashed | Wear goggles throughout; pour at bench level; wash any splash off at once with plenty of water. |
| Hydrochloric acid, 2.0 mol/dm³ (irritant) | Irritates the eyes and skin | Wear goggles; wash any splash off with plenty of water. |
| Polystyrene cup tipping over | Corrosive solution spilt onto skin and the bench | Stand the cup inside a 250 cm³ beaker so it cannot tip. |
| Glass thermometer used to stir | Cuts from broken glass | Stir gently, do not lean on the thermometer, and report any breakage to the teacher. |
Variables
- Independent: you change
- Volume of sodium hydroxide solution added in cm³
- Dependent: you measure
- Highest temperature of the mixture in °C
- Control: you keep the same
- Volume of hydrochloric acid. 30 cm³, measured with a 50 cm³ measuring cylinder.
- Concentration of the acid and the alkali. Use the same bottles (2.0 mol/dm³) for both runs.
- Starting temperature of the solutions. Both at room temperature.
- Container, lid and stirring. The same cup and lid, stirred the same way each time.
Results from a specimen run
| Volume of sodium hydroxide added in cm³ | Highest temperature, run 1 in °C | Highest temperature, run 2 in °C | Mean highest temperature in °C |
|---|---|---|---|
| 0 | 22.8 | 22.2 | 22.50 |
| 5 | 26.4 | 26.3 | 26.35 |
| 10 | 29.5 | 29.3 | 29.40 |
| 15 | 31.0 | 31.0 | 31.00 |
| 20 | 30.5 (anomalous, left out of the mean) | 32.9 | 32.90 |
| 25 | 33.9 | 33.8 | 33.85 |
| 30 | 35.0 | 34.8 | 34.90 |
| 35 | 34.1 | 34.4 | 34.25 |
| 40 | 33.1 | 33.5 | 33.30 |
Anomaly spotted: Highest temperature, 20 cm³ added, run 1, left out of the mean.
Processing the data
- Shape of the graph
- The temperature rises steadily, then falls once all the acid has reacted.
- Volume where the lines cross
- 26.4 cm³
- Temperature where the lines cross
- 35.5 °C
- Temperature rise
- 13.0 °C
| Quantity | Specimen run | Accepted |
|---|---|---|
| Volume that neutralises the acid (where your lines cross; reference from the 2.0 mol/dm³ labels) | 26.4 cm³ | 30.0 cm³ |
| Temperature rise if no energy were lost (reference for 2.0 mol/dm³ solutions) | 13.03 °C | 13.39 °C |
Across 120 simulated careful groups, volume of sodium hydroxide that neutralises 30 cm³ of acid (V) came out between 26.2 and 27.2 cm³ (median 26.8). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Use it without checking
If your thermometer reads high, every temperature is too high by the same amount (a systematic error). Temperature changes are still right, because the error cancels when you subtract.
The fix: Check the thermometer. A thermometer that reads high (or low) adds the same error to every temperature.
Method: Pour straight from the stock bottle
The sodium hydroxide comes from the cold stock bottle, 2.5–4 °C below room temperature. Every portion cools the mixture, so your highest temperatures are 1–2 °C too low, more in the first run than the second (the bottle warms up), so your two runs disagree.
The fix: Let both solutions reach room temperature. The temperature change only comes from the reaction if the acid and the alkali start at the same temperature.
Method: Skip it
No effect on your data, but a splash of corrosive alkali could blind you: flagged in the safety review.
The fix: Put on eye protection. Sodium hydroxide solution is corrosive: a splash in the eye can cause serious, permanent damage.
Exam answer: Leaving out: Volume of sodium hydroxide (solution) added; do not accept "amount"
The fix: The variable you changed: the volume of sodium hydroxide added. AQA does not accept "amount".
Exam answer: Leaving out: (Highest / maximum) temperature (of the mixture); allow change in temperature
The fix: The variable you measured: the highest temperature of the mixture.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Independent variable · Knowledge[1 mark]
A student added sodium hydroxide solution 5 cm³ at a time to 30 cm³ of hydrochloric acid in a polystyrene cup and recorded the highest temperature after each addition.
Give the independent variable in this investigation.
Worked answer and how the marks are given
A full-mark answer
The volume of sodium hydroxide solution added.
Mark scheme
- Volume of sodium hydroxide (solution) added; do not accept "amount" Do not accept “volume of the acid / hydrochloric / hcl”.
Where marks are usually lost
- Missing: Volume of sodium hydroxide (solution) added; do not accept "amount". The variable you changed: the volume of sodium hydroxide added. AQA does not accept "amount".
- Question 2Mean of repeats · Applying knowledge[2 marks]
Four groups measured the highest temperature after adding 30 cm³ of sodium hydroxide.
Table 2 Group Highest temperature in °C A 35.2 B 34.8 C 35.0 D 34.6 Calculate the mean highest temperature.
Mean = °C
Worked answer and how the marks are given
A full-mark answer
(35.2 + 34.8 + 35.0 + 34.6) ÷ 4 = 139.6 ÷ 4
mean = 34.9 °CMark scheme
- (35.2 + 34.8 + 35.0 + 34.6) ÷ 4 (method)
- = 34.9 (°C)
- Allow answers from 34.85 to 34.95 °C (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as c, oc, degrees c, degrees, deg c.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 46.5 °C (no marks). There are four values, so divide by 4: 139.6 ÷ 4 = 34.9 °C.
- Missing: (35.2 + 34.8 + 35.0 + 34.6) ÷ 4. Add the four temperatures and divide by 4.
- Question 3Planning a temperature-change investigation · Knowledge[6 marks]
A student investigated how the highest temperature reached depends on the mass of sodium carbonate added to dilute hydrochloric acid.
Describe a method the student could use.
Worked answer and how the marks are given
A full-mark answer
1. Measure 25 cm³ of dilute hydrochloric acid with a measuring cylinder.
2. Pour the acid into a polystyrene cup standing in a beaker.
3. Measure the starting temperature of the acid with a thermometer.
4. Weigh 0.5 g of sodium carbonate on a balance.
5. Add the sodium carbonate to the acid, put on the lid and stir.
6. Record the highest temperature reached.
7. Repeat with different masses of sodium carbonate (1.0 g, 1.5 g, 2.0 g, 2.5 g), using fresh acid at the same starting temperature each time.
8. Keep the volume and concentration of the acid the same, and repeat each mass to calculate a mean.Mark scheme
Indicative content:
- Measure a volume of (hydrochloric) acid with a measuring cylinder
- Pour it into a suitable container, eg a polystyrene cup (with a lid)
- Measure the initial temperature
- Add a known mass of sodium carbonate measured with a balance
- Stir
- Measure the highest temperature reached
- Repeat with different masses
- Use the same volume / concentration of acid and the same starting temperature
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Energy transferred to the surroundings through the sides and top of the container | Systematic | Every highest temperature is lower than the reaction could give, and the temperature falls faster after the end point. | Use a polystyrene cup with a lid, and insulate it (stand it in a beaker of cotton wool). |
| Not stirring, or reading before the temperature stopped rising | Random | Some readings are lower than they should be, by different amounts, so the points scatter about the lines. | Stir after every addition and wait until the reading stops changing before recording it. |
| The solutions starting at different temperatures | Random | A cold alkali lowers the temperature of the mixture, so the two runs give different highest temperatures. | Stand both solutions side by side until they are at the same (room) temperature before starting. |
| The rising points curve over (each 5 cm³ warms a bigger volume of solution, and a hotter mixture loses energy faster) | Systematic | The two straight lines cross a little before the true end point (about 27 cm³ instead of 30 cm³) and a little above the highest reading. | Add smaller portions (1 or 2 cm³) close to the end point, so more points lie near it. |
| Resolution of the thermometer | Random | A glass thermometer with 1 °C divisions can only be read to the nearest 0.5 °C, so small changes are lost. | Use a digital thermometer that reads to 0.1 °C. |
| Measuring the 5 cm³ portions in a large measuring cylinder | Random | The portions are not all 5 cm³, so the volumes on the graph are wrong and the end point moves. | Measure each portion with a 10 cm³ measuring cylinder (or a burette). |
| A thermometer that reads high (a zero error) | Systematic | Every temperature is too high by the same amount; temperature changes are not affected, because the error cancels. | Check the thermometer against a reference thermometer before starting. |
Repeatable. When you repeat the whole run with the same method and equipment, your highest temperatures at each volume agree closely (within about ±0.3 °C).
Reproducible. Another group, or a different method (a temperature probe and data logger), finds the end point at about the same volume.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 4 | This page | — |
| Edexcel | — | Not on its list | Edexcel does not name it. |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.