Cellular respiration runs in every living cell, but the naked eye sees none of it. This experiment uses three perfectly ordinary things to verify its three outcomes one by one: a thermometer for the energy released, a candle for whether oxygen has been used up, and clear limewater for whether carbon dioxide has built up. The materials are easy to gather at home or at school — but the testing stage involves an open flame and limewater, so it needs an adult to run it or stand watch.

Safety first (read this section before you start)

  • The candle test must be done by an adult: light the flame, and lower the burning candle or wooden splint into the jar, only at a table cleared of anything flammable, with a damp cloth within reach.
  • Clear limewater is an alkaline liquid — never taste it, never rub your eyes with it on your hands. Wear disposable gloves when mixing and pouring; if it splashes on skin, rinse with plenty of water.
  • When opening a jar that has held seeds overnight, point the mouth away from you and don't lean in for a sniff. The air inside is low in oxygen and high in carbon dioxide — a lungful will only make you feel unwell.
  • Boiling the seeds on a stove is an adult's job; let the boiled seeds cool completely before they go into the jar, or the "temperature rise" comparison is ruined from the start.
  • Do not eat any seeds used in the experiment; wash the jars and your hands thoroughly afterwards.

Materials and equipment

  • About 300 g of mung bean, wheat or soybean seeds (sprouted in advance — see Step 1);
  • Two identical thermos flasks (or wide-mouth jars with lids), labeled "A: germinating" and "B: boiled";
  • Two thermometers (any that fit through the jar mouth and cover 0–50 °C);
  • A candle or long wooden splint, a lighter, and a length of wire to hold the candle;
  • Clear limewater: stir a small spoonful of food-grade slaked lime (from a pharmacy or online) into water, let it settle, and use the clear liquid on top;
  • Gauze or paper towels, rubber bands, a data table and pen.

Step 1: sprout the seeds — and boil half of them as the control

  1. Two days ahead, soak the seeds in clean water for 8 hours, then drain and spread them on damp gauze, rinsing once or twice a day to keep them moist. When the embryonic roots poke out about half a centimeter, you have your germinating seeds.
  2. Take half of them and boil them for 5 minutes, then let them cool completely. The heat destroys the cells, so this half no longer has any life activity — while everything else about them matches the unboiled half. That makes them the perfect control.
  3. Put the germinating half into jar A and the boiled half into jar B, stand a thermometer in each, plug the mouths tightly (cotton or a stopper both work), and record both jars' starting temperatures.
Why the control is "boiled seeds," not "an empty jar": an empty jar differs from a jar of seeds in too many ways to prove anything. Boiled seeds match germinating seeds in appearance, number and water content almost exactly — the only difference is whether they are alive. Change just that one variable, and every difference you observe afterwards can be pinned on respiration. It is the same logic used in the seed germination control experiment.

Step 2: wait 24 hours, then run three tests

  1. Test 1 (temperature): after sealing, read both thermometers every few hours. Within 24 hours, jar A typically climbs 3–6 °C while jar B barely moves.
  2. Test 2 (oxygen): open jar A and immediately lower a burning candle slowly into the mouth — the flame goes out almost at once. Do the same in jar B, and the candle keeps burning normally.
  3. Test 3 (carbon dioxide): pour a little clear limewater into each jar, cap it, and swirl gently. Jar A's limewater turns visibly milky and cloudy; jar B's stays clear.
  4. Enter all three results in your data table, keeping jars A and B separate.

Note: run the candle test and the limewater test on separate pairs of jars, or do the candle first and the limewater second. Once a jar has been opened for the candle, inside and outside air have mixed, and a carbon dioxide reading taken afterwards will come out low.

Interactive simulation: this experiment has a drag-and-play simulator in our Interactive Lab. The lab interface is currently Chinese-only — the controls are simple sliders and switches, so it is still easy to explore.

What you will see and record

Typical results look like this (the numbers shift with seed quantity and jar size, but the A-versus-B difference should be consistent and unmistakable):

  • Temperature: jar A (germinating) rises from 20 °C to 24–26 °C; jar B (boiled) stays near room temperature throughout;
  • Candle: in jar A it goes out within 1–2 seconds; in jar B it burns as usual;
  • Limewater: in jar A it clouds milky white within seconds of swirling; jar B's clear liquid barely changes;
  • Opening jar A, you can often spot fine droplets on the inside of the glass — that is water produced by respiration;
  • The embryonic roots in jar A have grown a little longer than when it was sealed: the seeds have been steadily consuming their own stored food.

The science behind it

Everything a living cell does costs energy, and that energy comes from cellular respiration: with the help of oxygen, cells break organic matter down into carbon dioxide and water, releasing energy as they go. As an equation:

organic matter + oxygen → carbon dioxide + water + energy

The three tests map onto three terms of that equation. The candle going out shows that oxygen was consumed. The limewater turning cloudy shows that carbon dioxide was produced (carbon dioxide reacts with clear limewater to form insoluble calcium carbonate). The temperature rise shows that a good share of the released energy appears as heat. And the droplets on the glass account for the water among the products.

Germinating seeds are chosen as the material because germination is the most metabolically active period in a seed's life: cells are dividing and growing fast, demanding lots of energy, so respiration runs strongly enough to change the air in the jar measurably in a single night. The boiled seeds' cells have been destroyed by heat and all life activity has stopped — which is why nothing happens in jar B. Only with the two jars side by side can you show the changes really were caused by living seeds.

One more distinction that trips people up: cellular respiration is not the same thing as breathing. The rise and fall of your chest merely moves air into your lungs; the oxygen is actually "spent" by every cell in your body. Plants have no lungs, yet respiration runs in them day and night without pause. In daylight, photosynthesis outpaces respiration, so the plant's net behavior is to absorb carbon dioxide and release oxygen; at night, only respiration remains. This also picks up the thread left hanging in the leaves-make-starch experiment: the organic matter photosynthesis builds is exactly what respiration breaks down for energy — one process manufactures, the other consumes.

Common misconceptions

  • "Respiration only happens at night" — wrong. It runs around the clock; in daytime it is simply masked by the stronger photosynthesis, so the net effect hides it.
  • "The candle going out proves carbon dioxide was produced" — not rigorous. The direct cause of the flame dying is lack of oxygen; a rise in carbon dioxide must be proved separately with limewater. Each of the three tests establishes exactly one thing, and none can substitute for another.
  • "Dry seeds aren't alive, so there's no need to boil the control" — dry seeds are alive; their respiration is just extremely faint. The control must be boiled seeds, so that "everything is identical except life activity."
  • "Maybe the temperature rose because the room got warmer" — which is exactly why jars A and B sit in the same spot and are read at the same times: jar B exists to answer that very doubt.

Tips for teachers and parents

  • Time and grouping: assign the sprouting two days ahead; split sealing and testing across two lessons (or two days at home). Groups of four handle readings and records only, while an adult demonstrates the tests for everyone.
  • Safety points to state before hands go on equipment: proper open-flame procedure, limewater never near mouths or eyes, jar mouths pointed away when opening.

The step most often skipped. Before testing, have students predict: what will the candle and the limewater do in jar A, and in jar B? Writing the predictions down first makes the comparison with reality stick far better. Once the three results are in, let students connect each one to its term in the equation themselves — the teacher only confirms.

How to know they really understand. A student has the core of this experiment when they can state which part of the equation each of the three observations proves, explain what jar B is for, and answer "why do granaries dry grain before storing it?"

Take it further

  • Plot a warming curve: record jar A's temperature every 3 hours and draw a temperature–time line graph. Is the warming steady, or fast at first and slower later?
  • Repeat with different material: run it again with fresh store-bought bean sprouts instead of your own, or compare the temperature rise when you double the amount of seeds — do more seeds make more noticeable heat?
  • Where the miner's candle rule comes from: before entering a cellar or a dry well, people would lower a burning candle in first — and if the flame died, nobody went in. Have students use this experiment's principle to explain what lives that old rule has saved.
  • Loop back to germination: in the seed germination control experiment, the group fully submerged in water never sprouted. Now you can explain it: without oxygen, respiration cannot run, and the energy germination needs is never supplied.

When a student can place the candle, limewater and thermometer results back into their spots in "organic matter + oxygen → carbon dioxide + water + energy," and can say plainly what the boiled jar was for, this experiment has done everything it set out to do.