- Bung in at once
- Same length of ribbon
Launch practical
About 120 minutes
- Set up the bench with the 14 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 30 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.
Your eye does the reading. Each volume starts with your eye off level, so the scale lines up with the wrong mark. You move your eye, on the bench or in the close-up, until it is level. Nothing is right by default.
What it shows
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) and Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l)
Particles react only when they collide with enough energy. A more concentrated solution has more particles in the same volume, so collisions are more frequent and the rate is higher. Mean rate = quantity of product ÷ time; the rate at one moment is the gradient of a tangent to the curve.
The graph to expect. The volume of hydrogen rises steeply at first, then more and more slowly, and levels off when the magnesium is used up. The gradient at any point is the rate at that moment.
Key equation for the exam: mean rate of reaction = quantity of product formed ÷ time taken.
Recall: learn it. For the disappearing cross, use 1 ÷ time as a measure of rate.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| 100 cm³ gas syringe | Collects and measures the hydrogen. | 1 cm³ |
| Bung with delivery tube | Seals the flask and leads the gas to the syringe. | — |
| Clamp stand, boss and clamp | Holds the syringe level and safe. | — |
| 100 cm³ conical flask (gas) | Where the magnesium reacts with the acid. | — |
| Magnesium ribbon (4.0 cm pieces) and emery paper | The metal; emery paper cleans off the oxide. | — |
| Hydrochloric acid, 1.0 and 2.0 mol/dm³ | Reacts with the magnesium and with the thiosulfate. | — |
| Sodium thiosulfate solution, 40 g/dm³ | Reacts with acid to make the sulfur that hides the cross. | — |
| Distilled water (wash bottle) | Dilutes the thiosulfate; rinses the flask. | — |
| Measuring cylinders: 10 cm³ and 100 cm³ | Measure the acid, the thiosulfate and the water. | 0.2 cm³ and 1 cm³ |
| 100 cm³ conical flask (cross) | Where the sulfur forms. | — |
| Printed black cross on white paper | What you watch disappear. | — |
| Stopclock | Times each run. | 1 s (m:ss display) |
| Thermometer | Room temperature, a control variable. | 1 °C |
| Safety glasses | Protect your eyes from the acid. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Exam boards set this practical with different reactions. Your course setting picks one; you can change it before you start.
Why: The reaction, the variable you change and the exam questions follow your board.
Check the gas syringe
Push the plunger right in: it should read 0 cm³. Pull it out a little and let go: it should slide back smoothly.
Why: A plunger that sticks lags behind the gas, then jumps, so some readings come out too low.
Check the bung and tubing are airtight
Push the bung firmly into the empty flask. Pull the plunger out to 10 cm³ and let go: if it springs back to 0, no air is leaking in.
Why: Gas that leaks out round a loose bung or a split tube never reaches the syringe.
Put on eye protectionSafety
Wear safety glasses for the whole practical.
Why: 2.0 mol/dm³ hydrochloric acid irritates the eyes, and it can spit as the magnesium fizzes.
Ventilate the roomSafety
Open the windows (or work in a well-ventilated lab). Pour each cloudy mixture away as soon as you have timed it.
Why: The thiosulfate reaction gives off sulfur dioxide, which can bring on an asthma attack.
Clamp the gas syringe and connect it
Clamp the syringe level, plunger pushed in. Connect it to the bung’s delivery tube. Inside the flask, the end of the tube must sit above the acid, not in it.
Why: A clamped syringe cannot fall when the plunger slides out. A tube dipping into the acid lets the gas push acid into the syringe.
Measure out the acid
With the 100 cm³ measuring cylinder, pour 50 cm³ of 1.0 mol/dm³ hydrochloric acid into the 100 cm³ conical flask. Read the bottom of the meniscus at eye level.
Why: The same volume of acid every time; only its concentration changes.
Clean the magnesium ribbon
Rub each 4.0 cm piece of ribbon with emery paper until it is shiny all over.
Why: Magnesium is coated with a dull layer of magnesium oxide. It reacts with the acid without giving any hydrogen, so it slows the start.
Set up the cross
Put the printed black cross on the bench and stand the clean, dry 100 cm³ conical flask on it. Use the 100 cm³ cylinder for the thiosulfate and water, and the 10 cm³ cylinder for the acid.
Why: Everyone uses the same printed cross, so “I can’t see it” means the same amount of cloudiness for each group.
Keep the total volume the same
Make every concentration up to 50 cm³ with water: 10 cm³ of thiosulfate + 40 cm³ of water gives 8 g/dm³, up to 50 cm³ of thiosulfate and no water for 40 g/dm³.
Why: Only the concentration of the thiosulfate changes. The depth of liquid you look through and the concentration of the acid stay the same: a fair test.
Take the readings
Gas: the volume of hydrogen every 10 s with 1.0 mol/dm³ acid (to 90 s) and 2.0 mol/dm³ acid (to 60 s). Cross: the time at 8, 16, 24, 32 and 40 g/dm³, the whole set three times.
Repeats: AQA’s method does the whole set of five concentrations three times and takes a mean for each. A mean reduces the effect of random errors (judging when the cross has gone, reaction time) and shows up anomalies. It does not remove systematic errors, such as starting the clock late every time.
Make safe and clear upSafety
Pour the last cloudy mixture down the sink with plenty of water, straight away (sulfur dioxide). Rinse the flasks and brush out the yellow sulfur. Take the bung out, push the syringe plunger back in and unclamp the syringe. Rinse the magnesium flask and wipe up any acid spills. Put the acid and thiosulfate bottles back and wash your hands.
While you take readings
Put the bung in straight away
Drop the magnesium in, put the bung in at once and start the clock.
Why: Any hydrogen made before the bung is in escapes into the room and is never measured.
When to start the stopclock
Tip the acid into the flask, swirl, and start the stopclock.
Do: Start it as the acid goes in, swirling at the same time.
Why: The reaction starts the moment the acid meets the thiosulfate.
Who watches the cross?
Look down through the top of the flask and stop the clock when you can no longer see the cross.
Do: The same person watches every run.
Why: “Can no longer see it” is a judgement: each person stops at a slightly different cloudiness.
Rinse the flask between runs
As soon as a run is timed, pour the mixture away, rinse the flask with water and dry it before the next run.
Why: Sulfur left on the glass makes the next mixture cloudy before the reaction has made its own sulfur.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Sulfur dioxide given off by the thiosulfate reaction (toxic) | Breathing difficulties; can trigger an asthma attack | Work in a well-ventilated room, do not lean over the flask, and pour each mixture away with plenty of water as soon as it is timed. |
| 2.0 mol/dm³ hydrochloric acid (irritant) | Irritates the eyes and skin | Wear safety glasses, measure it in a small cylinder and wipe up spills at once. |
| Gas syringe plunger pushed out by too much gas | The plunger falls out and the glass breaks: cuts | Use only 4.0 cm of magnesium (about 40 cm³ of gas) and hold the syringe in a clamp. |
| Hydrogen gas (extremely flammable) | A flame could ignite it: burns | No Bunsen burners or flames nearby; the volumes are small. |
Variables
- Independent: you change
- Concentration: of sodium thiosulfate in g/dm³ (cross method); of hydrochloric acid in mol/dm³ (gas method)
- Dependent: you measure
- Time for the cross to disappear in s (cross method); volume of hydrogen in cm³, every 10 s (gas method)
- Control: you keep the same
- Cross method: volume and concentration of the hydrochloric acid. 10 cm³ of 2.0 mol/dm³ each time.
- Cross method: total volume of the mixture. Always 50 cm³ of thiosulfate and water, plus the acid.
- Cross method: the cross and the person judging it. The same printed cross and the same eyes.
- Gas method: volume of hydrochloric acid. 50 cm³ each time (only its concentration changes).
- Gas method: length (mass) of magnesium. 4.0 cm of ribbon each time.
- Both methods: temperature. Room temperature, noted at the start.
Results from a specimen run
| Time in s | Volume of hydrogen with 1.0 mol/dm³ acid in cm³ | Volume of hydrogen with 2.0 mol/dm³ acid in cm³ |
|---|---|---|
| 0 | 0 | 0 |
| 10 | 8 | 14 |
| 20 | 15 | 26 |
| 30 | 21 | 34 |
| 40 | 26 | 38 |
| 50 | 31 | 40 |
| 60 | 34 | 40 |
| 70 | 36 | — |
| 80 | 38 | — |
| 90 | 39 | — |
| Volume of sodium thiosulfate in cm³ | Volume of water in cm³ | Concentration of sodium thiosulfate in g/dm³ | Time, trial 1 in s | Time, trial 2 in s | Time, trial 3 in s | Mean time in s |
|---|---|---|---|---|---|---|
| 10 | 40 | 8 | 119 | 118 | 119 | 118.7 |
| 20 | 30 | 16 | 60 | 55 | 54 | 56.3 |
| 30 | 20 | 24 | 36 | 34 | 37 | 35.7 |
| 40 | 10 | 32 | 26 | 28 | 28 | 27.3 |
| 50 | 0 | 40 | 23 | 23 | 22.0 |
Anomaly spotted: Time for the cross to disappear, 40 g/dm³, trial 2, retaken.
Processing the data
- Mean rate, 1.0 mol/dm³ acid
- 0.75 cm³/s
- Mean rate, 2.0 mol/dm³ acid
- 1.3 cm³/s
- Shape of the graph
- Steep at first, then less steep, then level: the rate falls as the magnesium is used up.
- Rate of reaction at 30 s
- 0.550 cm³/s
- Why the stronger acid is faster
- There are more acid particles in the same volume, so they collide with the magnesium more frequently.
- Shape of the graph
- It falls steeply at first, then more slowly: a curve.
- Rate as 1 ÷ time
- 0.045 /s
- Shape of the graph
- A straight line through (or very close to) the origin: the rate is directly proportional to the concentration.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Rate at 30 s, 1.0 mol/dm³ acid (careful technique, true curve) | 0.55 cm³/s | 0.59 cm³/s |
| Final volume of hydrogen, 1.0 mol/dm³ (all 4.0 cm of ribbon reacted) | 39 cm³ | 41 cm³ |
| Mean time at 40 g/dm³ (a typical observer, careful technique, 18 °C) | 22 s | 23 s |
Across 120 simulated careful groups, rate of reaction at 30 s (1.0 mol/dm³ acid) (rate) came out between 0.550 and 0.700 cm³/s (median 0.600). 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: Skip the check
The plunger sticks now and then: about one reading in three is 1–3 cm³ too low before it jumps back (a random error that scatters your curve).
The fix: Check the gas syringe. A plunger that sticks lags behind the gas, then jumps, so some readings come out too low.
Method: Skip the check
The bung does not quite seal: 8–15 % of the hydrogen escapes, so every volume, the final volume and the rate come out too low (a systematic error).
The fix: Check the bung and tubing are airtight. Gas that leaks out round a loose bung or a split tube never reaches the syringe.
Method: Skip it
No effect on your data, but sulfur dioxide builds up around the bench: flagged in the safety review (it can trigger asthma).
The fix: Ventilate the room. The thiosulfate reaction gives off sulfur dioxide, which can bring on an asthma attack.
Volume of hydrogen at 10 s, 1.0 mol/dm³ acid: Typed 0 cm³
The fix: 0 cm³ is where the plunger started. By 10 s it has already moved: read where its front edge is now.
Volume of hydrogen at 20 s, 1.0 mol/dm³ acid: Typed 7 cm³
The fix: That is how far the plunger moved since the last reading. The table wants the total volume on the scale at 20 s, so read the front edge from the zero mark.
Exam answer: Leaving out: Some hydrogen (gas) escaped before the bung (stopper) was put in
The fix: Say what happened to the gas made before the bung went in: it escaped.
Exam answer: Leaving out: So not all the hydrogen made was collected in the syringe (allow: the bung cannot be put in instantly)
The fix: Link it to the measurement: that gas never reached the syringe, so less was measured.
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 1Gas lost before the bung goes in · Analysing and evaluating[2 marks]
You put the bung in as soon as the magnesium was in the acid.
Another student dropped the magnesium into the acid, watched it fizz, and then put the bung in. Their final volume of hydrogen was less than expected.
Explain why the final volume of hydrogen was less than expected.
Worked answer and how the marks are given
A full-mark answer
Some of the hydrogen escaped from the flask before the bung was put in, so not all the hydrogen made was collected in the gas syringe.
Mark scheme
- Some hydrogen (gas) escaped before the bung (stopper) was put in Do not accept “air escape…”.
- So not all the hydrogen made was collected in the syringe (allow: the bung cannot be put in instantly)
Where marks are usually lost
- Missing: Some hydrogen (gas) escaped before the bung (stopper) was put in. Say what happened to the gas made before the bung went in: it escaped.
- Missing: So not all the hydrogen made was collected in the syringe (allow: the bung cannot be put in instantly). Link it to the measurement: that gas never reached the syringe, so less was measured.
- Question 2Mean rate of reaction · Applying knowledge[3 marks]
Table 1 shows your volumes of hydrogen with 1.0 mol/dm³ hydrochloric acid.
Table 1 Time in s Volume of hydrogen in cm³ 0 0 10 8 20 15 30 21 40 26 50 31 60 34 70 36 80 38 90 39 Calculate the mean rate of the reaction in the first 20 seconds.
Give your answer to 2 significant figures.
Mean rate = cm³/s
Worked answer and how the marks are given
A full-mark answer
mean rate = 15 ÷ 20
= 0.75 cm³/sMark scheme
- 15 (cm³) and 20 (s) used (method)
- mean rate = 15 ÷ 20 = 0.75 (cm³/s)
- 0.75 cm³/s to 2 significant figures (precision)
- Allow answers from 0.743 to 0.758 cm³/s (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as cm3/s, cm^3/s, cm3 s-1, cm³ s⁻¹, cm3s-1, cubic centimetres per second.
- One mark for giving the answer to 2 significant figures.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 1.3 cm³/s (gains 1 mark). Upside down: mean rate = volume ÷ time = 15 ÷ 20 = 0.75 cm³/s.
- Answer 0.80 cm³/s (no marks). That uses the volume at 10 s. For the first 20 s use the volume at 20 s: 15 ÷ 20 = 0.75 cm³/s.
- Answer 0.70 cm³/s (no marks). That uses the volume at 30 s. For the first 20 s use the volume at 20 s: 15 ÷ 20 = 0.75 cm³/s.
- Question 3Planning the disappearing cross investigation · Knowledge[6 marks]
A student wants to investigate how the concentration of sodium thiosulfate solution affects the rate of its reaction with hydrochloric acid.
The student has: sodium thiosulfate solution, water, hydrochloric acid, measuring cylinders, a conical flask, a piece of paper with a black cross on it and a stopclock.
Plan an investigation to find out how the concentration of sodium thiosulfate affects the rate of this reaction.
Worked answer and how the marks are given
A full-mark answer
1. Measure 10 cm³ of sodium thiosulfate solution and 40 cm³ of water with a measuring cylinder and pour them into a conical flask.
2. Put the flask on a piece of paper with a black cross drawn on it.
3. Measure 10 cm³ of hydrochloric acid in a 10 cm³ measuring cylinder.
4. Add the acid to the flask, swirl, and start the stopclock at the same time.
5. Look down through the top of the flask and stop the clock when the cross can no longer be seen. Record the time.
6. Repeat with different concentrations of sodium thiosulfate: 20 cm³ thiosulfate + 30 cm³ water, and so on, keeping the total volume at 50 cm³.
7. Keep the volume and concentration of the acid and the temperature the same.
8. Repeat each concentration three times and calculate a mean time; rate = 1 ÷ mean time.Mark scheme
Indicative content:
- Measure a (known) volume of sodium thiosulfate solution with a measuring cylinder
- Put the thiosulfate in a conical flask
- Put the flask on the paper with the black cross
- Add (a measured volume of) hydrochloric acid and start the stopclock
- Measure the time for the cross to become no longer visible (look down through the top)
- Repeat for different concentrations of sodium thiosulfate (dilute with water, the same total volume)
- Repeat each concentration and find a mean
- Control variables: the volume and concentration of the acid; the total volume; the temperature; the same cross
- Rate = 1 ÷ time, or plot a graph of time (or 1 ÷ time) against concentration
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 |
|---|---|---|---|
| Judging the moment the cross can no longer be seen | Random | Each run stops at a slightly different cloudiness, so repeats scatter (more if people take turns). | Use a light sensor and data logger under the flask instead of eyes, or the same person every time. |
| Reaction time starting and stopping the stopclock | Random | About 0.2–0.4 s either way: under 2 % of a 20 s time, but enough that recording to 0.01 s means nothing. | Use a light sensor and data logger, which starts and stops automatically. |
| Starting the stopclock after pouring and swirling | Systematic | Every time is too short by 1–3 s, which matters most for the fast, concentrated runs. | Start the clock at the moment the acid goes in, while swirling. |
| The room warming up during the lesson | Systematic | Later runs are a little faster than earlier ones at the same concentration (the rate roughly doubles for every 10 °C). | Stand the solutions in a water bath at a constant temperature, and record the temperature of each run. |
| Hydrogen escaping before the bung goes in | Systematic | Every volume is too low by the gas made in the first seconds. | Put the bung in straight away, or hang the magnesium above the acid and tip it in with the bung already in place. |
| A leaking bung or tube | Systematic | A fraction of the gas escapes, so the volumes, the final volume and the rate are all too low. | Check the bung and tubing are airtight before you start. |
| A sticking syringe plunger | Random | Some readings lag behind and then jump, so points scatter either side of the curve. | Check the plunger moves freely before starting; twist it gently as it moves. |
| Oxide coating on the magnesium | Systematic | Little gas at first, so early volumes and the initial rate are too low. | Clean the ribbon with emery paper until it is shiny. |
| Sulfur left in the flask (not rinsed between runs) | Systematic | Each later mixture starts a little cloudy, so the cross disappears sooner: those times are 6–18 % too short, by a different amount each run. | Rinse the flask with water and dry it between runs. |
Repeatable. When you repeat each concentration three times yourself with the same method and equipment, your times agree closely.
Reproducible. Another group, or a different method such as a light sensor, gets similar times and the same pattern (handbook step 10: compare with others in the class).
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 5 | This page | — |
| Edexcel | 7.1 | Similar, with differences | The gas part uses marble chips rather than magnesium; the disappearing cross is the same. |
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.