Question

What is observed when sulfur dioxide is passed through water and lime water? Also, write the chemical equation for the reaction that takes place.

Answer 1

Hint: Calcium hydroxide \[Ca{{(OH)}_{2}}\] is sparingly soluble at room temperature. Mixing calcium hydroxide with water and removing excess of undissolved solute (e.g by filtration or by heating) gives limewater.

Complete step by step answer:
-The dilute aqueous solution of calcium hydroxide is known as lime water.
-Evolved carbon dioxide when passed through lime water turns limewater milky. Hence, limewater is used for testing the presence of carbon dioxide in gaseous samples in laboratories, and the process of refining sugar known as carbonatation.
-Sulfur dioxide is a chemical compound with chemical formula $S{{O}_{2}}$ and is a reducing gas that turns moist litmus first pink (due to its acidic nature) and then white (due to its bleaching effect).
-When sulfur dioxide is passed through water, it forms sulfurous acid $({{H}_{2}}S{{O}_{3}})$. Due to the formation of acid, the blue litmus is turned red.
$S{{O}_{2}}+{{H}_{2}}O\rightleftharpoons {{H}_{2}}S{{O}_{3}}$

-When sulfur dioxide is passed through lime water, a white precipitate of calcium sulfite $(CaS{{O}_{3}})$is formed which further reacts with sulfur dioxide in excess to form calcium hydrogen sulfite.

$\begin{align}
 & Ca{{(OH)}_{2}}+S{{O}_{2}}\to CaS{{O}_{3}}+{{H}_{2}}O \\
 & CaS{{O}_{3}}+S{{O}_{2}}+{{H}_{2}}O\to Ca{{(HS{{O}_{3}})}_{2}} \\
\end{align}$

Note: Sulfur dioxide has so many uses in different fields. In the production of sulfuric acid, sulfur dioxide is an intermediate formed which is then converted to sulfur trioxide and then to oleum which is further made into sulfuric acid. Here sulfur dioxide acts as a precursor to sulfuric acid. Sulfur dioxide owing to its microbial properties is used as a preservative in the preserves of dried apricots, dried figs, and other dried fruits. Sulfur dioxide being a good reductant is used especially in reducing bleach for papers and delicate materials such as clothes. Sulfur dioxide is mostly known as a versatile solvent which is used for dissolving highly oxidizing salts.