Launch practical
About 90 minutes
- Set up the bench with the 12 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 16 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 cylinder is upside down: 0 cm³ is at the closed top and the numbers go down. Each small division is 0.2 cm³. Read where the middle of the water surface is, to the nearest 0.1 cm³.
What it shows
carbon dioxide + water → glucose + oxygen (light energy needed); light intensity ∝ 1 ÷ distance²
Photosynthesis uses light energy to make glucose from carbon dioxide and water, giving off oxygen. The faster the pondweed gives off oxygen, the faster it is photosynthesising. Moving the lamp away spreads its light over a bigger area, so less light reaches the pondweed: light intensity is proportional to 1 ÷ distance² (Higher). When light is the limiting factor, less light means a slower rate.
The graph to expect. The rate falls as the distance increases: steeply between 10 and 20 cm, then less steeply, getting close to zero by 40 cm. Plotted against light intensity (Higher), the rate rises steeply at first and then less steeply: at high light intensity another factor (carbon dioxide, temperature) starts to limit the rate.
Key equation for the exam: light intensity ∝ 1 ÷ distance².
Recall: learn it · Higher tier. Double the distance and the light intensity falls to a quarter.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Pondweed (Cabomba or Elodea), about 10 cm sprigs | The plant that photosynthesises. | — |
| Scalpel and white tile | To cut a fresh end on each stem. | — |
| Boiling tube in a clamp, or a 400 cm³ beaker with a glass funnel | Holds the pondweed in the solution. | — |
| 10 cm³ measuring cylinder | Upturned over the funnel: collects the oxygen. | 0.2 cm³ |
| Glass rod | Pushes the pondweed down into the solution. | — |
| 0.2 % sodium hydrogencarbonate solution | Supplies the carbon dioxide the pondweed needs. | — |
| Lamp (LED, or filament) | The light source. | — |
| Glass tank of water | A heat shield between a filament lamp and the pondweed. | — |
| Metre rule | Sets the distance from the pondweed to the lamp. | 1 mm |
| Thermometer | Checks the temperature of the solution stays the same. | 0.5 °C |
| Stopclock | Times each count or collection. | 1 s |
| Hand tally counter | Counts the bubbles without losing count. | — |
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 slightly differently. Your course setting picks one; you can change it before you start.
Why: Both investigate how light intensity affects the rate of photosynthesis; the organism, what you measure and the exam questions follow your board.
Cut the end of each stem
On the white tile, cut a few millimetres off the end of each stem with the scalpel, at a slant. Cut down onto the tile, away from your fingers.
Why: An old cut end is squashed and sealed, so the oxygen cannot get out of it. A fresh cut lets the bubbles stream out.
Choose how to measure the rate
Oxygen is a product of photosynthesis, so the faster the pondweed makes oxygen, the faster it is photosynthesising.
Do: Collect the oxygen: a bunch of pondweed under an upturned funnel, with an upturned 10 cm³ measuring cylinder over it. Measure the volume collected in 5 minutes.
Why: You need a measurement of how much oxygen is made in a given time.
Fill with the solution
Fill the boiling tube (or the beaker) with 0.2 % sodium hydrogencarbonate solution.
Do: Use 0.2 % sodium hydrogencarbonate solution.
Why: Pondweed needs carbon dioxide for photosynthesis. In water alone it soon runs short.
Put the pondweed in
Boiling tube: push the sprig down into the solution with the glass rod, cut end up, below the surface. Funnel: lay the bunch on the bottom of the beaker with the cut ends pointing up the funnel, put the funnel over it and stand the upturned measuring cylinder, full of solution, over the funnel’s stem.
Why: The bubbles must come from the cut end where you can see them, or rise into the cylinder.
Put the thermometer in
Stand the thermometer in the solution beside the pondweed. You will read it at every distance.
Why: Temperature also affects the rate of photosynthesis. It is a control variable, so you must check that it stays about the same.
Choose which lamp to use
The lamp shines on the pondweed from the side, along the metre rule.
Do: An LED lamp.
Why: A filament lamp gives out a lot of heat as well as light, and the heat would warm the solution. Temperature is a control variable, so the lamp must not change it.
Keep the water away from the mainsSafety
Put the lamp’s plug and lead on the side away from the water. Dry your hands before you touch the switch, and mop up any drips at once.
Why: Water and mains electricity together can give an electric shock.
Measure the distance
The metre rule’s 0 cm mark is at the middle of the pondweed. Set the lamp at each distance along the rule.
Do: Measure from the middle of the pondweed to the front of the lamp, where the light comes out.
Why: The light intensity at the pondweed depends on the distance from the place the light comes out, not from the lamp’s stand.
Switch off the room lights
Switch off the lights and close the blinds, so the lamp is the only light on the pondweed.
Why: Light from the room also reaches the pondweed. It would add to the light from the lamp.
Take the readings
The lamp at 10, 20, 30, 40 cm from the pondweed; at each, 5 minutes to adjust, the temperature, then three 5-minute collections of oxygen (or three 1-minute counts of bubbles).
Repeats: The method repeats each reading: three readings at each distance, then a mean. A mean reduces the effect of random errors (bubbles of different sizes, the plant itself) and lets you spot an anomaly. It does not remove a systematic error, such as heat from a filament lamp or light from the room.
Make safe and clear upSafety
Switch the lamp off at the wall and unplug it. Let a filament lamp cool before you move it. Put the pondweed back in the class tank. Never put it down the sink or into a pond or stream. Pour the solution down the sink and rinse the glassware. Put the scalpel and tile away, blade pointing down. Wipe up any spilt water, then switch the room lights back on.
While you take readings
Let the pondweed adjust
Each time you move the lamp, wait 5 minutes before you start measuring.
Why: The pondweed takes a few minutes to settle to the rate that suits its new light intensity.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Water near the lamp’s mains plug and lead | Electric shock | Keep the lead and plug away from the water, dry your hands before touching the switch, and mop up spills at once. |
| Hot bulb of a filament lamp | Burns to the fingers | Use an LED lamp, or do not touch the bulb, and let the lamp cool before you move it. |
| Sharp scalpel | Cuts to the fingers | Cut on a white tile, pressing down and away from your fingers, and carry the scalpel with the blade pointing down. |
| Glass boiling tube, beaker and tank | Cuts if the glass breaks | Clamp the tube, stand glassware away from the edge of the bench, and report any breakage. |
| Pondweed is an invasive plant | Harm to wildlife if it gets into ponds and rivers | Put it back in the class tank after the practical; never put it down the sink or into a pond. |
| A darkened room | Tripping over bags and stools | Put bags under the benches and stools away before the lights go off. |
Variables
- Independent: you change
- Light intensity, changed by the distance from the lamp to the pondweed in cm
- Dependent: you measure
- Rate of photosynthesis: the volume of oxygen collected in a given time (or the number of bubbles in a given time) in cm³ in 5 minutes
- Control: you keep the same
- Temperature of the solution. LED lamp (or a tank of water as a heat shield); read the thermometer at each distance.
- Carbon dioxide concentration. The same 0.2 % sodium hydrogencarbonate solution throughout.
- The pondweed. The same piece (or bunch) of pondweed at every distance.
- Time to adjust. 5 minutes after each move before measuring.
- Time for each reading. 5 minutes (or 1 minute for a count).
- Other light. Room lights off, blinds closed.
Results from a specimen run
| Distance from the lamp in cm | Temperature of the solution in °C | Volume 1 in cm³ | Volume 2 in cm³ | Volume 3 in cm³ | Mean volume of oxygen in 5 minutes in cm³ | Light intensity in a.u. |
|---|---|---|---|---|---|---|
| 10 | 20.0 | 2.4 | 2.3 | 2.4 | 2.37 | 100.0 |
| 20 | 20.5 | 1.0 | 1.1 | 1.1 | 1.07 | 25.0 |
| 30 | 20.0 | 0.5 | 0.6 | 0.4 | 0.50 | 11.1 |
| 40 | 20.5 | 0.2 | 0.3 | 0.27 | 6.3 |
Anomaly spotted: Volume of oxygen in 5 minutes at 40 cm (third), retaken.
Processing the data
- Shape of the graph
- It falls steeply at first, then less steeply as the distance increases.
- Reading your curve
- 0.72 cm³
- Relative light intensity (Higher)
- 11.1 arbitrary units
- Shape of the graph
- It rises steeply at low light intensity, then less steeply: the rate starts to level off.
- Four times the light, four times the rate?
- 2.2
- Why not four times?
- Close to the lamp, light is not the only limiting factor: carbon dioxide or temperature limits the rate, so more light has less effect.
| Quantity | Specimen run | Accepted |
|---|---|---|
| Rate at 10 cm ÷ rate at 20 cm (your pondweed, careful technique) | 2.2 | 2.3 |
| Mean at 10 cm, cm³ in 5 minutes (your pondweed, careful technique) | 2.4 cm³ | 2.4 cm³ |
Across 120 simulated careful groups, rate at 10 cm ÷ rate at 20 cm (4 if the rate were proportional to light intensity) (ratio) came out between 2.01 and 2.48 (median 2.24). 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
The squashed cut end lets only about half the gas out; the rest stays in the leaves. Every count is about half what it should be (a systematic error). Under a funnel some of the gas still escapes from the leaves and is collected, so volumes are about a quarter too low.
The fix: Cut the end of each stem. An old cut end is squashed and sealed, so the oxygen cannot get out of it. A fresh cut lets the bubbles stream out.
Method: Count the bubbles: one sprig in a boiling tube. Count the bubbles leaving the cut end in 1 minute.
Quick and simple, but bubbles are not all the same size, so the same volume of oxygen can give a different number of bubbles. Repeats differ by about ±2 bubbles at 10 cm and ±1 at 40 cm (up to 30 % of the count): a random error.
The fix: Choose how to measure the rate. You need a measurement of how much oxygen is made in a given time.
Method: Use tap water from the sink
Carbon dioxide limits the rate. Close to the lamp the rate is less than half what it is in the solution, and moving the lamp from 20 cm to 10 cm raises the rate much less than in the solution (typically about 1.5 times, against about 2 times): the graph levels off close to the lamp, because light is no longer the limiting factor (a systematic error). Your own ratio is worked out from your means in Analyse.
The fix: Fill with the solution. Pondweed needs carbon dioxide for photosynthesis. In water alone it soon runs short.
Exam answer: Leaving out: Distance (of the pondweed) from the lamp / light intensity (ignore "light" on its own)
The fix: Name what the student changed: the distance from the lamp (the light intensity).
Exam answer: Leaving out: Measure the volume of oxygen (or count the bubbles) given off in a given time (ignore "measure the amount" without a time)
The fix: Say what to measure and over what: the volume of oxygen (or the number of bubbles) in a given time.
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 · Applying knowledge[1 mark]
A student investigated how the distance of a lamp from some pondweed affects the rate of photosynthesis. The pondweed was in sodium hydrogencarbonate solution, and the rate was found from the oxygen it gave off.
What was the independent variable in this investigation?
Worked answer and how the marks are given
A full-mark answer
The distance of the pondweed from the lamp (the light intensity).
Mark scheme
- Distance (of the pondweed) from the lamp / light intensity (ignore "light" on its own) Do not accept “rate / bubble… / oxygen / volume / colour / wavelength / temperature”.
Where marks are usually lost
- Missing: Distance (of the pondweed) from the lamp / light intensity (ignore "light" on its own). Name what the student changed: the distance from the lamp (the light intensity).
- Question 2Mean without the anomaly · Applying knowledge[2 marks]
Look again at Table 1: your first three readings at 40 cm, before any retake.
Calculate the mean volume of oxygen collected in 5 minutes.
Do not use the anomalous result. Give your answer to 2 decimal places.
Mean = cm³
Worked answer and how the marks are given
A full-mark answer
mean = (0.2 + 0.3) ÷ 2
= 0.25 cm³Mark scheme
- (0.2 + 0.3) ÷ 2 (the anomaly left out) (method)
- 0.25
- Allow answers from 0.2449 to 0.2551 cm³ (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as cm3, cm^3, ml.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 0.50 cm³ (no marks). You included the anomalous reading. Leave reading 3 out: (0.2 + 0.3) ÷ 2 = 0.25.
- Answer 0.50 cm³ (gains 1 mark). That is the total of the two. Divide by 2: 0.25.
- Missing: (0.2 + 0.3) ÷ 2 (the anomaly left out). Add the two readings that agree, then divide by 2.
- Question 3Planning: the colour of light · Knowledge[6 marks]
A student wants to find out how the colour of light affects the rate of photosynthesis in pondweed.
The student has: pondweed, sodium hydrogencarbonate solution, a boiling tube, a lamp, red, blue, green and yellow filters, a gas syringe, a metre rule and a stopwatch.
Plan an investigation to find the effect of the colour of light on the rate of photosynthesis.
Worked answer and how the marks are given
A full-mark answer
1. Put a piece of pondweed in a boiling tube of sodium hydrogencarbonate solution, with an LED lamp 10 cm away, in a dark room.
2. Put a red filter in front of the lamp. Keep the lamp at the same distance for every colour.
3. Leave the pondweed for 5 minutes to adjust to the light.
4. Collect the oxygen in a gas syringe for 5 minutes, timed with a stopwatch, and record the volume.
5. Repeat three times and calculate the mean.
6. Repeat with blue, green and yellow filters, using the same pondweed and keeping the temperature the same.Mark scheme
Indicative content:
- Put a coloured filter in front of the lamp (or use coloured LEDs) and change the colour
- Keep the lamp the same distance from the pondweed (same light intensity)
- Leave the pondweed to adjust (about 5 minutes) for each colour
- Measure the volume of oxygen collected (or count the bubbles)
- In a set time (e.g. 5 minutes), timed with a stopwatch
- Repeat for each colour and calculate a mean
- Control the temperature (LED, heat shield or water bath) and carbon dioxide (same solution); use the same pondweed
- Darken the room so the lamp is the only light
- Test at least three colours
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 |
|---|---|---|---|
| Counting bubbles that are different sizes | Random | The same volume of oxygen can come out as more small bubbles or fewer big ones, so the counts scatter. | Collect the gas in an upturned measuring cylinder (or a gas syringe) and measure its volume. |
| Heat from a filament lamp | Systematic | The solution warms up most when the lamp is close, so the rates close to the lamp are too high: temperature is not controlled. | Use an LED lamp, or put a tank of water between the lamp and the pondweed, and check the temperature at each distance. |
| Light from the room | Systematic | Room light adds to the lamp’s light at every distance, so the rates far from the lamp are too high. | Switch off the room lights and close the blinds. |
| Not waiting after moving the lamp | Systematic | The first reading at each distance is still partly at the old rate, so it does not agree with the other two. | Wait 5 minutes at each new distance before measuring. |
| Measuring the distance to the lamp’s stand | Systematic | The light is really 3 cm closer than recorded, so every rate is too high for its distance, most of all close up. | Measure from the middle of the pondweed to the front of the lamp. |
| Not enough carbon dioxide (tap water) | Systematic | Carbon dioxide limits the rate, so the rates are low and level off close to the lamp. | Use 0.2 % sodium hydrogencarbonate solution, the same for every distance. |
| A squashed cut end | Systematic | Less gas leaves the cut end, so every reading is too low. | Cut a few millimetres off the stem before you start. |
| Only four distances, 10 cm apart | Random | The curve between 10 and 20 cm, where it changes most, is drawn from two points, so its shape there is a guess. | Add more distances close to the lamp (for example 15 and 25 cm), and repeat with other pieces of pondweed. |
Repeatable. Your three readings at each distance, taken by you with the same method and pondweed, are close to each other.
Reproducible. Another group, with a different piece of pondweed or a different method (counting instead of collecting), also finds the rate falls as the lamp moves away, steeply at first.
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 6 | This page | — |
| Edexcel | 6.5 | Similar, with differences | Algal balls in hydrogen carbonate indicator rather than pondweed bubbles; spec point 6.6 asks for the inverse square law. |
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.