Question

A mixture of salts \[\left( {N{a_2}S{O_3} + {\text{ }}{K_2}C{r_2}{O_7}} \right)\] in a test tube is treated with \[dil.{H_2}S{O_4}\] and resulting gas is passed through lime water. Which of the following observations is correct about this test?
A.Solution in test tube becomes green and lime-water turns milky
B.Solution in test tube is colourless and lime-water turns milky
C.Solution in test becomes green and lime-water remains clear
D.Solution in the test tube remains clear and lime-water also remains clear.

Answer 1

Hint: We must know that \[S{O_2}\] turns acidified potassium dichromate green and also lime water into milky. Sulphur dioxide gas has a few characteristic features such as the suffocating feeling of burnt sulphur.

Complete step by step answer:
In this reaction, the sodium sulfite reacts with the \[dil.HCl\] to produce the sulphur dioxide gas and sodium chloride. The reaction is given below,
\[N{a_2}S{O_3}{\text{ }} + {\text{ }}2HCl{\text{ }} \to {\text{ }}2NaCl{\text{ }} + {\text{ }}{H_2}O{\text{ }} + {\text{ }}S{O_2}\]
We know sulphur dioxide gas has a few characteristic features such as the suffocating feeling of burnt sulphur. There are a few confirmatory tests to prove presence of sulphur dioxide and the most common one is to pass it through acidified potassium dichromate which is originally an orange colour solution and turns green upon contact with Sulphur dioxide.
The reaction is the following,
\[{K_2}C{r_2}{O_7}{\text{ }} + {\text{ }}{H_2}S{O_4}{\text{ }} + {\text{ }}3S{O_2}{\text{ }} \to {\text{ }}{K_2}S{O_4}{\text{ }} + {\text{ }}C{r_2}{\left( {S{O_4}} \right)_3}\left( {green} \right){\text{ }} + {\text{ }}{H_2}O\]
This is an excellent example of a redox reaction and we can see that the reducing agent is sulphur dioxide while the oxidizing agent is the potassium dichromate.
Coming back to the question, the sodium sulfite reacts with the \[dil.HCl\] liberating the sulphur dioxide gas. This sulphur dioxide gas then reacts with the potassium dichromate, turning it green. Since the entire sulphur dioxide reacts with the potassium dichromate, there is nothing left to react with the lime water and thus it does not turn milky.
Hence option C is the correct answer.

Note:
We must know that the gases that turn the lime water milky include \[C{O_2}\] and \[S{O_2}\]. Both these gases form an insoluble compound when reacts with lime water which turns the lime water milky in colour. This method of turning lime water milky is used by many to determine the presence of \[C{O_2}\] and the \[S{O_2}\] gases during the reaction.