Question

Lime water turns milky when ______ gas is passed through it.
A. ${{H}_{2}}$
B. $CO$
C. $C{{O}_{2}}$
D. $S{{O}_{2}}$

Answer 1

Hint: Think about what the composition of lime water is. Consider the reactions that will take place with all the given options and what will be the nature of the products obtained.

Complete step by step answer:
Lime water is the common name for the aqueous solution of calcium hydroxide. So, the chemical formula of lime water is $Ca{{(OH)}_{2}}$. Now, we will look at how calcium hydroxide reacts with each of the given options:
- When calcium hydroxide reacts with hydrogen.
Calcium hydroxide does not react with hydrogen. On the other hand, they are products of the reaction of calcium with water, so they are pretty stable on their own and do not need to react with each other for stability.
\[Ca+2{{H}_{2}}O\to Ca{{(OH)}_{2}}+{{H}_{2}}\]

- When calcium hydroxide reacts with carbon monoxide
Calcium hydroxide and carbon monoxide react to form calcium formate. This calcium formate is soluble in water and forms a whitish solution. This reaction takes place at extremely high temperatures and pressures, it is not possible for this reaction to be carried out just by bubbling carbon monoxide in lime water.
\[Ca{{(OH)}_{2}}+2CO\xrightarrow[\text{high pressure}]{\text{high temperature}}Ca{{(HCOO)}_{2}}\]

- When calcium hydroxide reacts with carbon dioxide
Calcium dioxide reacts with calcium hydroxide to form calcium carbonate. This product is soluble in water and forms a white solution. Just passing carbon dioxide through lime water is enough for the reaction to occur. No special conditions are required. The reaction is as follows:
\[Ca{{(OH)}_{2}}+C{{O}_{2}}\to CaC{{O}_{3}}+{{H}_{2}}O\]

- When calcium hydroxide reacts with sulphur dioxide
This reaction is similar to the previous reaction where calcium hydroxide combines with sulphur dioxide to give calcium sulfite and water. But the difference is that for this reaction to occur, the solution needs to be in boiling water, room temperature is not enough for this reaction to occur. The reaction is as follows: \[Ca{{(OH)}_{2}}+S{{O}_{2}}\to CaS{{O}_{3}}+{{H}_{2}}O\]

Considering how the reactants react with calcium hydroxide and what are the conditions that are required for this reaction to occur. We see that the only reaction that can occur by bubbling the gas at room temperature is the reaction with carbon dioxide, and causes the lime water to turn milky.
Hence, the answer to this question is ‘ $C{{O}_{2}}$’
So, the correct answer is “Option C”.

Note: Remember that lime water is the common name for the aqueous solution of calcium hydroxide. The powdered form of calcium hydroxide is also known as slaked lime and is obtained by adding water to quicklime. Quicklime is the common name for calcium oxide.