Question

When air is blown from mouth into a test-tube containing lime water, the lime water turned 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.

The correct option is B.

Explanation for the correct option:

• Carbon dioxide gas is an end-product of respiration. It is expelled from the body during exhalation.
• When passed through lime water (calcium hydroxide solution), carbon dioxide forms an insoluble precipitate of calcium carbonate or CaCO3. This precipitate is white.
• Carbon dioxide is present in the exhaled air, so when air is blown from the mouth into a test tube containing lime water, it turns milky.

Explanation for the incorrect options:

Option A-

Oxygen is not exhaled during breathing; rather, it is present in the inhaled air.

Option C-

Nitrogen is not known to form a precipitate with Ca(OH)2 solution, so it does not turn lime water milky.

Option D-

Water vapour does not react with the Ca(OH)2 solution to form insoluble ppt, so it does not turn lime water milky.