Step 1: Understand what "lime water" is.
Lime water is a common name for a dilute aqueous solution of calcium hydroxide, Ca(OH) .
It is clear and colorless.
Step 2: Recall the reaction of common gases with lime water.
- Oxygen (O ): Does not react with lime water to cause milkiness.
- Nitrogen (N ): Generally unreactive, does not react with lime water.
- Benzene (C H ): An organic compound, a liquid at room temperature (or gas if heated).
It does not typically react with lime water to cause milkiness.
- Carbon dioxide (CO ): When carbon dioxide gas is passed through lime water, it reacts with calcium hydroxide to form calcium carbonate (CaCO ), which is a white, insoluble precipitate.
This precipitate makes the lime water appear milky or turbid.
Reaction: If excess CO is passed, the milkiness disappears due to the formation of soluble calcium bicarbonate:
Step 3: Identify the correct gas.
The gas that makes lime water milky is Carbon-di-oxide (CO ).
This matches option (4).