Question

When air is blown from the mouth into a test tube containing lime water, the lime water turns milky due to the presence of

• A. Oxygen
• B. Carbon dioxide
• C. Nitrogen
• D. Water vapour

Answer 1

The correct option is B Carbon dioxide

Hint - $Ca(OH{)}_2$ (lime water) gets converted into $CaC{O}_3$ (Calcium carbonate) by removal of water and addition of another gas.

Part 1 - Definition: Respiration

• Respiration is the process by which a cell uses glucose in the presence of oxygen to produce energy in the form of ATP and carbon dioxide as a by-product.
• This carbon dioxide ($C{O}_2$) is formed during the respiration process as a by-product is released during the exhalation of air from the lungs.

Part 2 - Experiment to show the production of $C{O}_2$ during respiration process.

• The exhaled air has a high partial pressure of carbon dioxide gas.
• When this exhaled air is blown into a test tube containing lime water ($Ca(OH{)}_2$), it converts the lime water into limestone ($CaC{O}_3$).
• Chemical reaction: $Ca(OH{)}_2+C{O}_2\to CaC{O}_3+H_{2}O$

Part 3 - Conclusion

• From the above discussion, we can conclude that exhaled air contains a high amount of carbon dioxide gas.
• This high concentration of carbon dioxide converts the lime water $Ca(OH{)}_2$ into limestone ($CaC{O}_3$).

Final step - In the presence of carbon dioxide, the limewater turns milky.