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A shoot of Hydrilla under water gives you something rare in school biology: a process you can watch and count. Each bubble rising from the cut stem is mostly oxygen released by photosynthesis, so bubbles per minute is a rate. The skill being tested is turning that into a fair test and an honest graph.
What does the Hydrilla bubble experiment measure?
It measures how fast the plant photosynthesises under one set of conditions, then lets you change one factor at a time. The usual factor is light, changed by moving the lamp. The other is carbon dioxide, which the water supplies when you dissolve sodium hydrogen carbonate (sodium bicarbonate) in it, as this GCSE practical guide explains.
The idea behind both versions is the law of limiting factors, which F. F. Blackman published in Annals of Botany in 1905. The NCERT Class 11 Biology chapter on photosynthesis puts it this way: when a process depends on several factors, its rate is set by the factor nearest its minimal value.
How do you set up the Hydrilla experiment?
This method follows PMT Education's practical notes, with Hydrilla in place of pondweed.
Fill a boiling tube with a fixed volume of 0.2% sodium hydrogen carbonate solution.
Put a freshly cut 10 cm shoot in, cut end at the top, and push it down gently with a glass rod.
Place the tube 10 cm from the lamp, measured with a ruler.
Leave it for 5 minutes so the plant settles at that light level.
Count bubbles for one minute. Repeat twice more and take the mean.
Repeat at 20, 30 and 40 cm.
The same notes list the hazards: the lamp gets hot, water must stay away from sockets and wiring, and some people are allergic to pondweed. To stop the lamp warming the water, use an LED bulb or put a clear Perspex block between lamp and tube, as the practical guide suggests.
Which variables must you control?
Independent variable: lamp distance, which sets light intensity.
Dependent variable: mean bubbles per minute.
Controlled: the same shoot, the same volume and strength of bicarbonate solution, the same lamp, the same one-minute count, and the room's temperature and background light.
Control set-up: the same tube in darkness. It should give no steady stream of bubbles, which shows that light, not something else, drives them.
For new experiments, our guide on checking your independent and dependent variables walks through it.
For the bicarbonate version, fix the lamp at one distance and change the concentration instead. NCERT notes that air holds only 0.03 to 0.04 per cent carbon dioxide, which is why carbon dioxide is so often the limiting factor. As the Royal Society of Chemistry's limiting-factors sheet explains, raising it speeds photosynthesis until another factor runs short.
How do you plot rate against light intensity?
Do not plot bubbles against distance and call it light intensity. Light from a small source obeys the inverse square law, as HyperPhysics states, so relative intensity is approximately 1/d². The PMT notes ask for exactly this graph.
Worked example, converting each distance:
10 cm: 1/100 = 0.0100
20 cm: 1/400 = 0.0025
30 cm: 1/900 = 0.0011
40 cm: 1/1600 = 0.000625
Doubling the distance from 10 to 20 cm cuts the light to a quarter, not a half. For the mean, suppose (made-up numbers) your three counts at 20 cm were 14, 16 and 15: the mean is 45 ÷ 3 = 15 bubbles per minute. Plot that against 0.0025.
What shape should you expect? NCERT describes a linear rise at low light, then a levelling off as other factors become limiting, and notes that light saturation occurs at about 10 per cent of full sunlight. The RSC sheet adds that very bright light can damage chlorophyll. To see the plateau before your practical, try our photosynthesis limiting factor simulation, and test your 1/d² predictions in the inverse square law for light simulation.
If the plateau idea is still hazy, the Fermi Concept Library gives each biology concept its own lesson and shows where it sits in the map.
Why are bubble counts unreliable, and how do you improve them?
Bubbles differ in size, so counts miss how much oxygen each holds. Collecting the gas in a gas syringe and reading the volume is more accurate, the practical guide notes.
Close to the lamp, bubbles may form too fast to count, and temperature can drift, PMT warns.
Glare from windows or overhead lights adds to the lamp, so dim the room.
For the inverse-square and graph calculations behind this practical, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer. Convert every distance, and let the graph show where light stops being the limit.
