EduGrown | Class 10 Science Practical
Experiment No. 8To show experimentally that carbon dioxide is given out during respiration
Biology · Life Processes · CBSE Class 10 Science Practical File
1Aim
To prove by experiment that germinating seeds give out carbon dioxide gas during respiration.
2Apparatus and materials required
| S.No. | Apparatus / Material | Quantity |
|---|---|---|
| 1 | Conical flask (250 mL) with a two holed cork | 1 |
| 2 | Test tube with stand | 1 |
| 3 | Bent delivery tube and a thistle funnel | 1 each |
| 4 | Germinating gram or moong seeds | About 50 g |
| 5 | Boiled and cooled seeds (for control) | About 50 g |
| 6 | Small test tube with KOH solution | 1 |
| 7 | Freshly prepared lime water | About 20 mL |
| 8 | Thread and vaseline | As required |
3Theory
Respiration is the process in which living cells break down glucose to release energy. Oxygen is used up and carbon dioxide and water are produced.
Germinating seeds are alive and respire very fast, so they release a good amount of carbon dioxide. Boiled seeds are dead, so they do not respire.
Carbon dioxide turns lime water milky. This milkiness is used as the test for carbon dioxide.
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
CO2 + Ca(OH)2 → CaCO3↓ + H2O
C6H12O6 = glucose · Ca(OH)2 = lime water · CaCO3 = white insoluble calcium carbonate
The KOH solution kept inside the flask absorbs the carbon dioxide released by the seeds. This creates a partial vacuum, so water rises up in the delivery tube and shows that a gas has been used up.
4Experiment setup
Fig. 1 — Carbon dioxide released by germinating seeds turns lime water milky
5Procedure
- Take about 50 g of germinating seeds and put them in a clean conical flask.
- Hang a small test tube containing KOH solution inside the flask with the help of a thread.
- Fit a two holed cork on the flask. Put a thistle funnel in one hole and a bent delivery tube in the other.
- Apply vaseline around the cork so that the flask becomes airtight.
- Dip the free end of the delivery tube into a test tube containing freshly prepared lime water.
- Keep the setup undisturbed for about 1 to 2 hours.
- Observe whether the lime water becomes milky.
- Repeat the whole experiment with boiled seeds as a control and compare the two results.
6Observation table
| S.No. | Setup | Condition of seeds | Lime water after 2 hours | Conclusion |
|---|---|---|---|---|
| 1 | Experimental flask | Germinating (alive) | Turns milky | CO2 is given out |
| 2 | Control flask | Boiled (dead) | Stays clear | No CO2 is given out |
7Chemical equations
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
CO2 + Ca(OH)2 → CaCO3↓ + H2O
2KOH + CO2 → K2CO3 + H2O
CO2 + Ca(OH)2 → CaCO3↓ + H2O
2KOH + CO2 → K2CO3 + H2O
The white milkiness in the lime water is calcium carbonate. The third equation shows how KOH inside the flask absorbs the carbon dioxide.
8Result
Lime water in the experimental setup turned milky, but in the control setup with boiled seeds it stayed clear.
This proves that germinating seeds give out carbon dioxide during respiration, and that respiration takes place only in living cells.
This proves that germinating seeds give out carbon dioxide during respiration, and that respiration takes place only in living cells.
9Precautions
- The flask must be completely airtight, otherwise the gas will escape into the air.
- Use freshly prepared lime water, as old lime water does not turn milky.
- The small test tube of KOH must not touch the seeds, or it will harm them.
- Use healthy germinating seeds; dry seeds respire very slowly.
- Keep the setup undisturbed so that the water level in the tube does not change.
- Always set up the control experiment with boiled seeds for a fair comparison.
10Key points — quick revision
Glucose + O2 → CO2 + H2O + energy
CO2 + lime water → milky
Living seeds respire · Boiled seeds are dead and do not respire
11Before writing the practical
☐ Write the correct aim
☐ Draw the labelled diagram
☐ Write apparatus list
☐ Write theory
☐ Write stepwise procedure
☐ Fill the observation table
☐ Write both equations
☐ Mention the control setup
☐ Write the result
☐ Write precautions
12Viva voce — 10 questions the examiner may ask
- Which gas is released during respiration?Carbon dioxide, along with water and energy.
- How do you test for carbon dioxide?By passing it through lime water. Carbon dioxide turns lime water milky.
- What is the white substance that makes the lime water milky?Calcium carbonate (CaCO3), which is insoluble in water.
- Why are germinating seeds used instead of dry seeds?Germinating seeds are actively growing and respire very fast, so they release enough carbon dioxide to show the result quickly.
- What is the use of KOH in this experiment?It absorbs the carbon dioxide released by the seeds, which creates a partial vacuum inside the flask.
- Why do we take boiled seeds in the control setup?Boiled seeds are dead, so they do not respire. This proves that the carbon dioxide really came from living seeds.
- Why must the flask be airtight?If air leaks in or out, the carbon dioxide will escape and outside air will enter, giving a wrong result.
- Write the equation for aerobic respiration.C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
- Where in the cell does aerobic respiration take place?In the mitochondria, which is called the powerhouse of the cell.
- What happens if excess carbon dioxide is passed through lime water?The milkiness disappears, because soluble calcium hydrogen carbonate is formed.
★ Practical exam tip
- Always draw and mention the control setup with boiled seeds; many students forget it and lose marks.
- Label the KOH test tube, thistle funnel, delivery tube and lime water in your diagram.
- Write the word milky clearly in the observation; that single word is the proof.
- Remember: milky lime water means carbon dioxide, pop sound means hydrogen.