Question

$S{{O}_{2}}$ turns lime water milky due to formation of -----------.

Answer 1

Calcium hydroxide is also called lime water. The molecular formula of lime water is$Ca{{(OH)}_{2}}$. Generally lime water is used to detect the presence of$C{{O}_{2}}$. Carbon dioxide reacts with lime water and forms an insoluble milky product called calcium carbonate.
$C{{O}_{2}}+Ca{{(OH)}_{2}}\to CaC{{O}_{3}}+{{H}_{2}}O$

Complete answer:
Sulphur dioxide reacts with lime water ($Ca{{(OH)}_{2}}$) and forms calcium sulphite as a product.
$S{{O}_{2}}+Ca{{(OH)}_{2}}\to CaS{{O}_{3}}+{{H}_{2}}O$
Calcium sulfite is milky white in color.
The milkiness disappears due to the formation of calcium hydrogen sulphite. Calcium hydrogen sulphide is soluble in water.
$CaS{{O}_{3}}+S{{O}_{2}}+{{H}_{2}}O\to Ca{{(HS{{O}_{3}})}_{2}}$
Therefore $S{{O}_{2}}$(Sulphur dioxide) turns lime water milky due to the formation of calcium sulfite ($CaS{{O}_{3}}$).

Additional information:
Calcium hydroxide ($Ca{{(OH)}_{2}}$) is also called slaked lime.
Calcium hydroxide is obtained by the reaction of water on calcium oxide.
$CaO+{{H}_{2}}O\to Ca{{(OH)}_{2}}$
When calcium oxide mixed with water, a minor proportion of calcium oxide dissolves, and forms a solution known as limewater.
By using lime water we can identify both carbon dioxide and sulphur dioxide.
Carbon dioxide is odourless, whereas sulphur dioxide is pungent in smell.
Sulphur dioxide turns potassium dichromate paper into green color, whereas carbon dioxide does not undergo such change with potassium dichromate paper.

Note:
Carbon dioxide is known for making the lime water into milk. This happens because insoluble calcium carbonate is formed. Sulphur dioxide also forms calcium sulphite by the formation of milky precipitate with calcium hydroxide but forms slowly when compared to calcium carbonate.