Question

In the experimental set-up shown above in diagram (I) atmospheric air is being passed into lime water with a syringe while in diagram (II) air is being exhaled into lime water. The time taken for the lime water to turn milky in both the test tubes is different. Give reason.

Answer 1

Explanation:

• In diagram (I), atmospheric air contains approximately 0.03–0.04% carbon dioxide (CO₂). When it is bubbled through lime water, the reaction is slower as the concentration of CO₂ is low.
• In diagram (II), exhaled air contains a much higher concentration of CO₂ (around 4%). This causes the lime water to turn milky faster.
• The milkiness of lime water is due to the formation of calcium carbonate (CaCO₃):

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Observation:

• Lime water in test tube (II) turns milky faster than in test tube (I).

Conclusion: The difference in the time taken for lime water to turn milky is due to the higher concentration of CO₂ in exhaled air compared to atmospheric air.