Heat from Germinating Moong: Reading Three Flasks
Rohan, Class 8, is investigating whether germinating seeds release heat. He takes three identical 250 mL conical flasks. In flask A he puts 50 g of moong beans soaked overnight and just beginning to sprout. In flask B he puts 50 g of the same soaked beans, boiled for five minutes and then cooled to room temperature. In flask C he puts 50 g of dry, unsprouted moong beans. Each flask is sealed with a one-hole cork carrying a thermometer whose bulb hangs in the air above the seeds, and all three are kept together in a dark cupboard at a steady 28 °C. After four hours the readings are: A = 33.0 °C, B = 28.4 °C, C = 28.5 °C. Rohan now wants to show that carbon dioxide is also produced, using lime water. His partner argues that the temperature rise proves nothing, because the sprouting beans had absorbed water and 'water soaking into a seed releases heat as it swells'.
1. Interpret the three readings. Why are both flask B and flask C needed instead of just one control?
Read the worked answer
Flask A rose about 5 °C above the cupboard temperature; B and C stayed at cupboard temperature.
The only flask containing living, actively dividing and respiring tissue is A, so the heat released is best explained by respiration in germinating seeds.
Flask B and flask C answer two different objections, so neither is superfluous:
- B controls for the material and the water. It has the same mass, the same kind of seed and the same absorbed water, but the tissue is dead. If heat came from wet seed material alone, B would have warmed too. It did not.
- C controls for the seed itself. It has living seeds that are not germinating. If seeds always gave off heat, C would have warmed. It did not.
Together they isolate the one variable that matters: living, germinating tissue.
2. Describe how Rohan should carry out the lime-water test so that his conclusion is trustworthy, and state the result he should expect and the result that would refute his idea.
Read the worked answer
Setup: Fit flask A with a two-hole cork. Through one hole pass a tube dipping into a test tube of freshly prepared lime water; through the other, connect a syringe or a rubber bulb so that air can be pushed out of the flask without Rohan blowing into it (his own breath is rich in CO₂ and would ruin the test). Keep the flask in the dark as before. Bubble air slowly for a few minutes.
Expected result: the lime water turns milky, showing carbon dioxide.
Controls that make it trustworthy:
- Bubble air from flask C (living but non-germinating seeds) through a fresh sample of lime water - it should stay clear.
- Bubble ordinary room air through a third sample - it should stay clear, proving the milkiness did not come from the atmosphere or the tubing.
Refutation: if lime water from A stayed clear while the controls behaved as expected, the CO₂ claim would be false.
3. Evaluate the partner's claim that the heat came from water soaking into and swelling the seeds.
Read the worked answer
The claim is contradicted by the experiment itself.
Flask B held the same soaked, fully swollen beans, containing the same amount of absorbed water, and it stayed at 28.4 °C - essentially the cupboard temperature. If water uptake were the source of the heat, B would have warmed just as much as A.
The single difference between A and B is whether the tissue was alive and germinating. That points to a metabolic process, respiration, breaking down stored food in the cotyledons and releasing energy, part of which appears as heat.
A fair caveat: the claim would need a separate test if B had not been run, and the 0.4–0.5 °C differences in B and C are within thermometer reading error, so Rohan should not treat them as real effects.