When air is blown from mouth into a test tube containing lime water, the lime water turns milky. The milkiness is indicator of the presence of which gas ?

Question

When air is blown from mouth into a  test tube containing lime water, the  lime water turns milky. The  milkiness is indicator of the presence  of which gas ?

Accepted 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. This reaction occurs because carbon dioxide (CO2) reacts with calcium hydroxide (Ca(OH)2) present in the lime water to form calcium carbonate (CaCO3), which is insoluble in water and appears as a milky suspension. This is a well-known chemical reaction used to test for the presence of carbon dioxide.

Explanation for Option 4 (Carbon dioxide):
- When carbon dioxide is introduced into lime water, it reacts with calcium hydroxide to form calcium carbonate, which causes the lime water to turn milky. This is a classic test for the presence of carbon dioxide and is widely used in educational and laboratory settings to demonstrate this gas's properties.

Explanation for why other options are incorrect:
1. Nitrogen: Nitrogen is an inert gas and does not react with lime water. Blowing air, which contains a significant amount of nitrogen, into lime water would not cause any change in its appearance. Therefore, nitrogen cannot be responsible for the milkiness.

2. Oxygen: Oxygen is also present in the air but does not react with lime water to produce a milky appearance. Oxygen is not responsible for the formation of calcium carbonate, and thus, it cannot be the correct answer.

3. Water vapour: While water vapour is present in the breath, it does not cause lime water to turn milky. Water vapour does not react with calcium hydroxide to form an insoluble compound like calcium carbonate. Therefore, water vapour is not the correct answer.

In conclusion, the milkiness observed in lime water when air is blown into it is due to the presence of carbon dioxide, making Option 4 the correct answer.