Launch practical
About 60 minutes
- Set up the bench with the 13 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.
How to read it. The positive electrode is the rod with the red lead.
What it shows
At the negative electrode: Cu²⁺ + 2e⁻ → Cu
At the positive electrode: 2Cl⁻ → Cl₂ + 2e⁻
In a solution the ions move, not electrons: positive ions to the negative electrode, negative ions to the positive. At the negative electrode the less reactive of the metal and hydrogen forms. At the positive electrode a halide gives the halogen; otherwise hydroxide ions from the water give oxygen.
The graph to expect. A steady current makes gas at a steady rate, so the volume of hydrogen rises in a straight line from (nearly) the origin; a little gas dissolves at first.
Key equation for the exam: at the negative electrode: Cu²⁺ + 2e⁻ → Cu; at the positive electrode: 2Cl⁻ → Cl₂ + 2e⁻.
Equation to write · Higher tier. Reduction (gain of electrons) at the negative electrode, oxidation at the positive.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Safety goggles | Protect your eyes from splashes of the solutions. | — |
| Low-voltage power pack (d.c.) | Set to 4 V: pushes the current through the solution. | — |
| Ammeter (digital multimeter, 10 A range) | Shows that a current is flowing, and how big it is. | 0.01 A |
| Leads with crocodile clips | Connect the rods to the power pack, red to positive. | — |
| 100 cm³ beaker, Petri-dish lid and two graphite rods | The cell: the lid holds the rods apart. | — |
| Copper(II) chloride, copper(II) sulfate, sodium chloride and sodium sulfate solutions, 0.5 mol/dm³ | The four electrolytes, 50 cm³ each. | — |
| Blue litmus paper, tweezers and a wash bottle | Tests for chlorine; the water dampens the paper and rinses the rods. | — |
| Gas-collection cell with graphite rods through its base | Part B: the gases collect over each rod. | — |
| Two 10 cm³ measuring cylinders, clamped upside down | Measure the volume of each gas. | 0.2 cm³ |
| Two test tubes | The other way to collect the gases (no scale). | — |
| Stopclock | Times the readings every minute. | 1 s |
| Wooden splints and a Bunsen burner | A lit splint tests for hydrogen; a glowing one for oxygen. | — |
| Copper residues bottle | Copper solutions go here, not down the sink. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical differently. Your course setting picks one; you can change it before you start.
Why: AQA looks at the products with graphite rods; Edexcel also uses copper electrodes and weighs them. The exam questions follow your board.
Test the leads
Before you use each lead, touch its two ends to the terminals of a continuity tester. Or put it in a circuit with a lamp.
Why: A lead that is broken inside its plug looks fine but carries no current. Testing every lead first saves you hunting for the fault later.
Keep the chlorine downSafety
Open a window. Keep the lid on the beaker. Set the power pack to 4 V, and run each solution for no more than 5 minutes.
Why: The chloride solutions give off chlorine, which is toxic. A low potential difference and a short time keep the amount small.
Clean the rods between solutions
After each solution, rinse the rods with water from the wash bottle and wipe off any coating with a paper towel.
Why: Anything left on a rod from one solution is still there in the next, and looks like a product.
Choose how to collect the gases
In part B, each gas collects in a tube full of sodium sulfate solution, upturned over its electrode.
Do: Two 10 cm³ measuring cylinders (0.2 cm³ divisions).
Why: To compare the amounts of hydrogen and oxygen, you need to measure their volumes.
Take the readings
Part A: four solutions, one at a time, for 5 minutes each. Part B: the volume of hydrogen and oxygen every minute for 5 minutes, then the gas tests.
Repeats: Each solution is electrolysed once, so the results are observations, not measurements. Repeat a solution if you got no clear result, such as no bubbles or an unclear litmus change. In part B the volumes are read every minute, which gives five points for a graph, then the run is repeated with the cylinders refilled and read again at 5 minutes, to check the volumes agree.
Make safe and clear upSafety
Switch off the power pack before you unclip the leads. Take out the rods, then wipe the copper off them and rinse them. Check both copper solutions went into the residues bottle, never down the sink, and put its lid on. Rinse the sodium solutions away with plenty of water. Put the splints out in water, then bin them with the litmus. Keep the room ventilated until any smell of chlorine has gone.
While you take readings
Test the gas at the positive electrode
Use tweezers to hold a piece of blue litmus paper in the gas just above the solution, next to the positive electrode. Keep it out of the solution.
Do: Damp litmus, held in the gas just above the solution.
Why: Chlorine turns damp blue litmus red, then bleaches it white. The paper must be wet, because the chlorine has to dissolve in the water on it first.
Read the upturned scale at eye level
Crouch so your eye is level with the water surface inside each measuring cylinder.
Why: Looking from above, the surface seems to line up with a mark higher up the cylinder. On an upturned scale that is a smaller number, so you read too low.
Keep the hydrogen tube upside down
Lift the tube off the negative electrode with its mouth still pointing down, and bring the lit splint up to the mouth.
Why: Hydrogen is much less dense than air, so it rises out of a tube that is turned the right way up.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Chlorine gas from the chloride solutions (toxic) | Irritates the eyes, nose and lungs; can bring on an asthma attack | Keep the lid on, ventilate the room, use 4 V for no more than 5 minutes, and never sniff the gas. |
| Copper(II) chloride and copper(II) sulfate solutions (irritant; toxic to water life) | Irritate the eyes and skin; harmful if swallowed; poison water life | Wear goggles, wash off splashes, and pour copper solutions into the residue bottle. |
| The rods touching inside the beaker | A short circuit: the leads and the power pack get hot (burns) | Hold the rods apart in the holes of the lid; switch off before moving them. |
| Hydrogen and oxygen collected together, and the lit splint | A small explosion or burns if a large volume of the mixture is lit | Collect only a few cm³, test each tube on its own, away from the cell, and put splints out in water. |
| Spilt solution near the mains-powered power pack | Electric shock | Keep the power pack away from the cell, wipe up spills, and switch off before touching the rods. |
Variables
- Independent: you change
- The salt in the solution (its positive and negative ions)
- Dependent: you measure
- The product at each electrode
- Control: you keep the same
- Potential difference in V. 4 V on the power pack for every solution.
- Time in min. No more than 5 minutes for each solution.
- Concentration of the solution in mol/dm³. 0.5 mol/dm³ for every salt.
- Volume of solution in cm³. 50 cm³ each time.
- The electrodes. The same graphite (inert) rods, the same distance apart.
Results from a specimen run
| Solution | Current in A | Positive electrode | Blue litmus | Negative electrode after 5 min |
|---|---|---|---|---|
| CuCl₂ | 0.35 | Bubbles of gas | Turns red, then white (bleached) | A pink-brown coating |
| CuSO₄ | 0.17 | Bubbles of gas | Stays blue: no change | A pink-brown coating |
| NaCl | 0.20 | Bubbles of gas | Turns red, then white (bleached) | Bubbles of gas |
| Na₂SO₄ | 0.21 | Bubbles of gas | Stays blue: no change | Bubbles of gas |
| Time in min | Volume of hydrogen in cm³ | Volume of oxygen in cm³ |
|---|---|---|
| 1 | 1.4 | 0.6 |
| 2 | 2.6 | 1.2 |
| 3 | 4.0 | 1.6 |
| 4 | 5.2 | 2.4 |
| 5 | 6.4 | 3.2 |
| 5 (repeat) | 6.6 | 3.2 |
| Test | Result |
|---|---|
| Lit splint, gas from the negative electrode | The gas burns with a squeaky pop |
| Glowing splint, gas from the positive electrode | The glowing splint relights |
- Hydrogen (negative electrode)
- Oxygen (positive electrode)
Processing the data
- The product at each electrode
- Cu / Cl₂, Cu / O₂, H₂ / Cl₂, H₂ / O₂
- Explaining the negative electrode
- Copper is less reactive than hydrogen, so copper ions gain electrons instead of hydrogen ions. Sodium is more reactive, so hydrogen ions from the water gain them.
- Explaining the positive electrode
- Halide ions lose electrons to form the halogen. Sulfate ions are not discharged, so hydroxide ions from the water lose electrons and oxygen forms.
- Going further: the concentration of chloride
- In very dilute solutions, oxygen forms as well as chlorine: there are too few chloride ions near the electrode.
- Half equations
- Cu²⁺ + 2e⁻ → Cu; 2Cl⁻ → Cl₂ + 2e⁻; 2H⁺ + 2e⁻ → H₂
- The two gases
- There is about twice as much hydrogen as oxygen.
- Shape of the graph
- A straight line (almost through the origin): hydrogen forms at a steady rate, because the current is steady.
- Gradient of the hydrogen line (rate of hydrogen production)
- 1.26 cm³/min
- The ratio of the gases
- 1.92
- Why two to one?
- Each H₂ needs 2 electrons but each O₂ releases 4: the same current makes twice as many H₂ molecules. (In effect water, H₂O, is split.)
- Moles of hydrogen (Chemistry only, Higher)
- 0.00027 mol
| Quantity | Specimen run | Accepted |
|---|---|---|
| Hydrogen : oxygen, collected from 1 to 5 minutes | 1.92 | 2.00 |
| Rate of hydrogen production (from the current, 24 dm³/mol) | 1.26 cm³/min | 1.32 cm³/min |
Across 120 simulated careful groups, volume of hydrogen ÷ volume of oxygen (ratio) came out between 1.89 and 2.28 (median 2.06). 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 them as they are
If one of your leads is broken, which happens in about one kit in seven, nothing happens when you switch on. The ammeter reads 0.00 A, and you have to find the fault before you can start.
The fix: Test the leads. A lead that is broken inside its plug looks fine but carries no current. Testing every lead first saves you hunting for the fault later.
Method: Leave the lid off
Your observations are the same, but more chlorine reaches the room. This is flagged in the safety review.
The fix: Keep the chlorine down. The chloride solutions give off chlorine, which is toxic. A low potential difference and a short time keep the amount small.
Method: Just move them to the next solution
The copper from the copper solutions stays on the negative rod, so the sodium solutions seem to give copper too. That is a wrong product in your results, and a systematic error.
The fix: Clean the rods between solutions. Anything left on a rod from one solution is still there in the next, and looks like a product.
Current through copper(II) chloride: Typed 0 A
The fix: The ammeter reads 0.00 A before you switch on. The current is flowing now, so read what it shows.
Positive electrode, copper(II) chloride: Recorded “A pink-brown coating”
The fix: That is the negative electrode. Look at the positive one (red lead).
Exam answer: Leaving out: Damp (blue) litmus paper
The fix: Say the litmus paper must be damp: chlorine does not bleach dry litmus.
Exam answer: Leaving out: Is bleached (turns white)
The fix: Give the result: the litmus paper is bleached (turns white). "Turns red" alone is not the test.
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 tests · Knowledge[2 marks]
Describe the test for chlorine. Give the result of the test.
Worked answer and how the marks are given
A full-mark answer
Hold damp blue litmus paper in the gas. Chlorine bleaches it: it turns white.
Mark scheme
- Damp (blue) litmus paper
- Is bleached (turns white)
Where marks are usually lost
- Missing: Damp (blue) litmus paper. Say the litmus paper must be damp: chlorine does not bleach dry litmus.
- Missing: Is bleached (turns white). Give the result: the litmus paper is bleached (turns white). "Turns red" alone is not the test.
- Question 2Moles of gas · Applying knowledge[3 marks]
You collected 6.4 cm³ of hydrogen in 5 minutes.
The volume of one mole of any gas at room temperature and pressure is 24 dm³. (Chemistry only, Higher.)
Calculate the amount of hydrogen, in moles, collected.
Give your answer in standard form to 2 significant figures.
Amount =
Worked answer and how the marks are given
A full-mark answer
6.4 cm³ = 0.0064 dm³
moles = 0.0064 ÷ 24
= 2.7 × 10⁻⁴ molMark scheme
- 6.4 cm³ = 0.0064 dm³ (or ÷ 24 000 cm³) (method)
- 2.7 × 10⁻⁴ mol
- Given to 2 significant figures (precision)
- Allow answers from 0.000264 to 0.000269 (the marker's tolerance; correct rounding of the student's own working is also accepted).
- One mark for giving the answer to 2 significant figures.
- Error carried forward: method and answer marks follow correctly from the student's own earlier value.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 0.27 (no marks). That is 1000 times too big: 6.4 cm³ is 0.0064 dm³. Change cm³ to dm³ (÷ 1000) before dividing by 24.
- Answer 150 (no marks). Divide by 24, don’t multiply: moles = volume in dm³ ÷ 24 = 0.0064 ÷ 24.
- Answer 0.15 (gains 1 mark). You changed to dm³ correctly, but multiplied by 24. Moles = volume ÷ 24 = 0.0064 ÷ 24.
- Question 3Describing the method · Knowledge[6 marks]
A student wants to find out what is produced at each electrode when sodium chloride solution is electrolysed.
Describe a method the student could use. Include how the student could identify the products.
Worked answer and how the marks are given
A full-mark answer
1. Pour about 50 cm³ of sodium chloride solution into a beaker.
2. Put two graphite (inert) electrodes into the solution, held apart so they do not touch.
3. Connect them to a low-voltage d.c. power supply (4 V) and switch on.
4. Look for bubbles at each electrode.
5. Collect the gas from each electrode in an upturned test tube full of the solution.
6. Test the gas from the positive electrode with damp blue litmus paper: it is bleached, so it is chlorine.
7. Test the gas from the negative electrode with a lit splint: a squeaky pop shows it is hydrogen.
8. Do it in a well-ventilated room, wear goggles, and switch off after a few minutes so little chlorine forms.Mark scheme
Indicative content:
- Put sodium chloride solution in a beaker (or electrolysis cell)
- Two inert (graphite / carbon) electrodes in the solution, not touching
- Connect to a low-voltage d.c. power supply and switch on
- Look for bubbles (gas) at each electrode
- Collect the gas from each electrode in an upturned test tube (or measuring cylinder)
- Damp (blue) litmus paper is bleached by the gas from the positive electrode: chlorine
- A lit splint gives a squeaky pop with the gas from the negative electrode: hydrogen
- Safety: well-ventilated room or fume cupboard, goggles, low voltage, short time
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 |
|---|---|---|---|
| Copper left on the negative rod from an earlier solution | Systematic | The sodium solutions seem to make copper too, so the wrong product is recorded. | Rinse and wipe the rods clean between solutions. |
| Dry litmus, or litmus held away from the electrode | Systematic | Chlorine does not bleach dry litmus, so it is recorded as oxygen. | Use damp blue litmus held right next to the positive electrode. |
| Some of each gas dissolves in the solution at the start (oxygen more than hydrogen) | Systematic | Early volumes are too small, oxygen most, so the hydrogen : oxygen ratio comes out above 2. | Use the gradients of the lines, or let the cell run for a few minutes before you start measuring. |
| Collecting the gases in test tubes, which have no scale | Random | The volumes can only be estimated by eye, so they are very uncertain. | Collect the gases in measuring cylinders (or inverted burettes) so the volume can be measured. |
| Reading the upturned scale from above (parallax) | Systematic | Every volume reads a little low. | Read with your eye level with the water surface. |
| A loose crocodile clip | Random | For one solution only a tiny current flowed: one anomalous current and very few bubbles. | Test the leads first and push every clip on firmly; watch the ammeter while it runs. |
| Bubbles sticking to the rods and the cylinder walls | Random | Each volume is a little different from the smooth line. | Tap the cell gently before each reading so the bubbles rise. |
Repeatable. When you electrolyse the same solution again with the same apparatus, you see the same products and similar currents.
Reproducible. Another group, or a different method (a Hofmann voltameter), gets the same products and about twice as much hydrogen as oxygen.
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 3 | This page | — |
| Edexcel | 3.31 | Similar, with differences | Adds copper electrodes with mass change against current, weighed and analysed quantitatively. |
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.