Question

Lime water turns milky when exhaled air is expelled in the test tube because of the presence of.
A) Carbon dioxide
B) Oxygen
C) Nitrogen
D) None of these

Answer 1

We need to know that the calcium hydroxide $Ca{\left( {OH} \right)_2}$, is sparsely soluble at room temperature in. "Pure" (i.e. less than or fully saturated) limewater is clear and colorless, with a slight earthy smell and an astringent/bitter taste. It is basic in nature with a pH of 12.4.

Complete answer:
When air is blown from the mouth into a test- tube containing lime water, the lime water turns milky due to the presence of carbon dioxide. When we exhale, carbon dioxide is produced in the process. Lime water is calcium hydroxide. Limewater is created with calcium hydroxide, or $Ca{\left( {OH} \right)_2}$. Named for the mineral, not the fruit, lime reacts with $C{O_2}$ in water to form calcium carbonate, which is white and does not dissolve in water, causing the water to turn cloudy.
Similarly, when we exhale we are removing $C{O_2}$ from our bodies, so breathing $C{O_2}$ into the lime water will produce the same reaction, though probably not as quickly due to the smaller amount of $C{O_2}$ in your breath. Limewater is widely used by marine aquarists as a primary supplement of calcium and alkalinity for reef aquariums.

Note:
We need to remember that the limewater may be prepared by mixing calcium hydroxide ($Ca{\left( {OH} \right)_2}$) with water and removing excess undissolved solute (for example by filtration). When excess calcium hydroxide is added (or when environmental conditions are altered, like; when its temperature is raised sufficiently), a milky solution results due to the homogeneous suspension of excess calcium hydroxide. This liquid has been known traditionally as milk of lime.