The specification says: Investigate and describe the effect of light and dark conditions on gas exchange in an aquatic plant using hydrogencarbonate indicator solution
Aim
To investigate how light and dark conditions affect the carbon dioxide concentration around an aquatic plant, using hydrogencarbonate indicator solution.
Background
An aquatic plant such as Elodea carries out photosynthesis and respiration at the same time. Photosynthesis takes in carbon dioxide and needs light. Respiration releases carbon dioxide and happens all the time, in the light and in the dark.
Hydrogencarbonate indicator solution changes colour with the concentration of carbon dioxide dissolved in it. It is red in equilibrium with the carbon dioxide in air, yellow when the carbon dioxide concentration is higher, and purple when it is lower.
In bright light the rate of photosynthesis is greater than the rate of respiration, so there is a net uptake of carbon dioxide. In the dark only respiration happens, so there is a net release of carbon dioxide.
Hypothesis
The indicator around Elodea in the light will turn purple and the indicator around Elodea in the dark will turn yellow, because photosynthesis needs light but respiration does not.
Variables
| Independent | Light or dark conditions (tube wrapped in foil or not) |
|---|---|
| Dependent | The colour of the hydrogencarbonate indicator after 2 hours |
| Control |
|
Equipment
- 4 boiling tubes with bungs, in a rack
- Hydrogencarbonate indicator solution (red), about 100 cm³
- Fresh Elodea, two identical pieces about 4 cm long
- 25 cm³ measuring cylinder
- Forceps
- Aluminium foil
- Lamp, with a clamp stand
- Rectangular tank or large beaker of water as a heat shield
- Stop-clock
- Marker pen
- Eye protection
Risk assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Mains-powered lamp | The bulb gets hot and can burn skin. Water spilt near the lamp can cause a shock. | Do not touch the lamp. Keep water away from sockets and electrical equipment. |
| Hydrogencarbonate indicator solution | Low hazard but may irritate the eyes. | Wear eye protection and wash splashes off at once. |
| Glass boiling tubes | Cuts if broken. | Stand the tubes in a rack and push in bungs gently. Report any breakages to the teacher. |
| Pond plants | Pond water can contain microbes. | Wash your hands after handling the plants. Do not put the plants near your mouth. |
Method
- Put on eye protection. Label four boiling tubes 1 to 4 with a marker pen.
- Use a measuring cylinder to put 20 cm³ of hydrogencarbonate indicator in each tube. Check that the indicator is red in all the tubes and record the colour.
- Use forceps to put one piece of Elodea in tube 1 and an identical piece in tube 2. Push each piece down so that it is completely covered by the indicator.
- Leave tubes 3 and 4 with no plant.
- Put a bung in each tube at once, so that the tubes are airtight. Do not breathe into the tubes.
- Wrap tubes 2 and 4 completely in aluminium foil so that no light can reach them.
- Stand the four tubes in the rack at the same distance from the lamp, with the tank of water between the lamp and the tubes.
- Switch on the lamp and start the stop-clock.
- Leave the tubes for 2 hours.
- After 2 hours, unwrap tubes 2 and 4 and immediately record the colour of the indicator in each tube.
- Switch off the lamp, empty the tubes and wash your hands.
Results
Fill this table in as you go. Print the PDF for a copy to write on.
| tube | conditions | colour at start | colour after 2 hours | change in carbon dioxide concentration |
|---|---|---|---|---|
| 1 | Elodea, light | |||
| 2 | Elodea, dark | |||
| 3 | No plant, light | |||
| 4 | No plant, dark |
Example results and answersPractice data, conclusion, errors and 10 exam questions (26 marks) with mark schemes
Example results
| tube | conditions | colour at start | colour after 2 hours | change in carbon dioxide concentration |
|---|---|---|---|---|
| 1 | Elodea, light | Red | Purple | Decrease |
| 2 | Elodea, dark | Red | Yellow | Increase |
| 3 | No plant, light | Red | Red | None |
| 4 | No plant, dark | Red | Red | None |
Conclusion
In the light the indicator turned purple, so the carbon dioxide concentration decreased: photosynthesis was faster than respiration and the plant took up more carbon dioxide than it released. In the dark the indicator turned yellow, so the carbon dioxide concentration increased: there was no photosynthesis but respiration continued and released carbon dioxide. The tubes without a plant stayed red in both the light and the dark, which shows that the colour changes were caused by the plant and not by the light, the foil or the air in the tube.
Errors and improvements
| Error | Effect on the results | Improvement |
|---|---|---|
| Heat from the lamp warms the tubes. | Temperature affects the rates of photosynthesis and respiration, so the colour change is not due to light alone. | Place a water tank between the lamp and the tubes, or use an LED lamp. |
| Breathing into the tubes or leaving the bungs out adds carbon dioxide from the breath or air. | The indicator turns yellow when it should not. | Put the bungs in quickly and keep breath away from the tubes. |
| Pieces of Elodea of different size are used in the two tubes. | The rates of photosynthesis and respiration differ, so the tubes cannot be compared fairly. | Use pieces of the same mass and length from the same plant. |
| The colours are judged by eye. | Different people may record different colours, especially for small changes. | Compare each tube with standard colours or use a colorimeter. |
| The indicator is not red at the start. | A colour change could be wrongly interpreted. | Check the colour at the start and bubble air through the indicator to bring it to red before use. |
Exam questions
10 questions, 26 marks. Write your answers on paper, then open each mark scheme.
Question 1
State the colour of hydrogencarbonate indicator solution when the carbon dioxide concentration is (a) the same as in air, (b) higher than in air and (c) lower than in air.
Show mark scheme for question 1
- (a) red (1)
- (b) yellow (1)
- (c) purple (1)
Question 2
Tube 3 contained indicator but no plant and was left in the light. Explain why this tube was included.
Show mark scheme for question 2
- it is a control (1)
- to show that the colour change in the other tubes is caused by the plant, not by the light or the air (1)
Question 3
The indicator in the tube with Elodea in the light turned purple. Explain this result.
Show mark scheme for question 3
- purple shows that the carbon dioxide concentration decreased (1)
- photosynthesis used carbon dioxide (1)
- rate of photosynthesis was greater than rate of respiration (1)
Question 4
The indicator in the tube with Elodea in the dark turned yellow. Explain this result.
Show mark scheme for question 4
- no light so no photosynthesis (1)
- respiration continued and released carbon dioxide (1)
- so carbon dioxide concentration increased (1)
Question 5
Give two variables that must be kept the same in tubes 1 and 2.
Show mark scheme for question 5
- size / mass of Elodea (1)
- volume of indicator (1)
- temperature / time / distance from lamp (1)
- Max 2
Question 6
Explain why bungs were put in the tubes.
Show mark scheme for question 6
- so that carbon dioxide from the air could not enter the tubes / so that carbon dioxide could not escape (1)
- so that the colour changes are caused only by the plant (1)
Question 7
A student put a third piece of Elodea in dim light. The indicator stayed red. Explain this result.
Show mark scheme for question 7
- no net change in carbon dioxide concentration (1)
- rate of photosynthesis equals rate of respiration (1)
- carbon dioxide released by respiration is used in photosynthesis (1)
Question 8
A student says, ‘Plants respire only in the dark.’ Use the results to explain why this is not correct.
Show mark scheme for question 8
- plants respire all the time / in the light and dark (1)
- in the light, carbon dioxide from respiration is used for photosynthesis / photosynthesis is faster so net uptake (1)
Question 9
Suggest three ways in which the investigation could be improved.
Show mark scheme for question 9
- use a colorimeter / colour standards to measure the colour (1)
- repeat and compare results (1)
- use a heat shield / LED lamp (1)
- use more pieces of Elodea / test different light intensities (1)
- Max 3
Question 10
A student wants to find out whether the colour change in the dark tube is caused by temperature. Describe how the student could show that it is not.
Show mark scheme for question 10
- place a thermometer in each tube / measure temperature of the tubes at start and end (1)
- use a water tank / keep tubes at the same temperature (1)
- result: temperature did not change / same in all tubes, or tubes with no plant (control) stay red (1)
Exam tips
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026