Question

How \[{\rm{S}}{{\rm{O}}_2}\] turns lime water milky?

Answer 1

Hint: Lime water is calcium hydroxide. It is a basic solution which is sparingly soluble in water. On passing gases such as carbon dioxide, there is a formation of compounds which turns the solution milky.

Complete step by step answer:
Lime water contains calcium hydroxide. It is dilute aqueous calcium hydroxide solution. It is a clear and colourless solution. At room temperature, calcium hydroxide is sparingly soluble in water.

 Sulfur dioxide reacts with calcium hydroxide to form white precipitate of calcium sulfite.
\[\begin{array}{l}
{\rm{S}}{{\rm{O}}_2}{\rm{ + Ca}}{\left( {{\rm{OH}}} \right)_2}{\rm{ }} \to {\rm{ CaS}}{{\rm{O}}_3} \downarrow {\rm{ + }}{{\rm{H}}_2}{\rm{O }}\\
{\rm{ white}}\\
{\rm{ precipitate }}
\end{array}\]
Calcium sulphite is insoluble in water. Due to white precipitate of calcium sulfite, the lime water turns milky.

Carbon dioxide gas also turns lime water milky. Carbon dioxide gas reacts with calcium hydroxide to produce calcium carbonate.
\[\begin{array}{l}
{\rm{C}}{{\rm{O}}_2}{\rm{ + Ca}}{\left( {{\rm{OH}}} \right)_2}{\rm{ }} \to {\rm{ CaC}}{{\rm{O}}_3} \downarrow {\rm{ + }}{{\rm{H}}_2}{\rm{O }}\\
{\rm{ white}}\\
{\rm{ precipitate }}
\end{array}\]
Calcium carbonate is insoluble in water. Due to white precipitate of calcium carbonate, the lime water turns milky.
The rate of the reaction of sulfur dioxide gas with lime water is less than that of carbon dioxide.

Note:
It is not true that only carbon dioxide gas turns lime water milky. Sulphur dioxide gas also turns lime water milky.