Question

What happens when $C{{O}_{2}}$ is passed through lime water:
(1)For short duration
(2)For a long duration

Answer 1

Hint: When carbon dioxide gas is passed for short duration a calcium compound is formed which makes the solution milky, however, on passing more gas, the reaction proceeds further and this time a soluble salt formation takes place which gets dissolved and makes the solution clear.

Complete answer:
In order to answer the question, we need to know about the properties of calcium compounds. Let us know about calcium hydroxide first. Calcium hydroxide is an inorganic compound and it is generally found as white powder, or as a colourless crystal. It has a molar mass of $74g\,mo{{l}^{-1}}$ and boils at ${{580}^{0}}C$.
Calcium carbonate is another important compound of calcium that is mainly found in rocks and as sediments, in the banks of rivers. It has the molecular formula $CaC{{O}_{3}}$ and can be prepared from calcium hydroxide itself. It has a molar mass of $100g\,mo{{l}^{-1}}$ and appears as a fine powder, having a chalk-like taste.
Now, let us come to the question. Suppose we take lime water and pass carbon dioxide through it for a short period of time, then an incomplete reaction takes place with the lime water and the gas, producing the first calcium compound, which is calcium hydroxide. As it is a white powder, the solution appears to be milky. Let us see the reaction that is involved:
\[Ca{{(OH)}_{2}}+C{{O}_{2}}\to CaC{{O}_{3}}+{{H}_{2}}O\]
Now, for the next part of the reaction we assume that the previous setup is taken. Now, we pass more carbon dioxide into the milky solution, and now the reaction gets completed. This time, a salt is formed whose name is calcium hydrogen carbonate($Ca{{(HC{{O}_{3}})}_{2}}$)and this dissolves the calcium carbonate present in the solution. As a result the milkiness of the solution disappears and the solution can be seen clear. The reaction involved is:
\[CaC{{O}_{3}}+C{{O}_{2}}+{{H}_{2}}O\to Ca{{(HC{{O}_{3}})}_{2}}\]

Note:
It is to be noted that calcium hydroxide was not soluble, causing the milkiness, whereas, calcium hydrogen carbonate is a soluble salt and since it dissolves, the solution becomes clear.