chapter 7.  ELEMENTS OF GROUPS 16, 17 AND 18 class 12 chemistry textbook solution

4. Answer the following.

iii. How is SO2 prepared in laboratory from sodium sulfite? Give two physical properties of SO2.


Sulfur dioxide (SO2) can be prepared in the laboratory from sodium sulfite (Na2SO3) through the reaction of sodium sulfite with a strong acid, typically hydrochloric acid (HCl). Here’s the reaction and the process for preparing SO2 from sodium sulfite:

Reaction: Na2SO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g)


  1. Start with a small amount of sodium sulfite (Na2SO3) powder in a reaction vessel or flask.

  2. Carefully add concentrated hydrochloric acid (HCl) to the flask containing the sodium sulfite. The reaction will produce sulfur dioxide gas (SO2), which evolves as a gas.

  3. Collect the sulfur dioxide gas by using a suitable collection apparatus, such as a gas syringe or an inverted graduated cylinder in a water bath. Sulfur dioxide is denser than air, so it can be easily collected by displacement of air.

  4. The collected sulfur dioxide gas can be used for various chemical applications or experiments.

Physical Properties of SO2:

Two physical properties of sulfur dioxide (SO2) are as follows:

  1. Color and Odor: Sulfur dioxide is a colorless gas with a pungent, suffocating odor. It has a distinctive smell often described as similar to that of burnt matches. Even in low concentrations, the odor of sulfur dioxide is readily detectable.

  2. Density: Sulfur dioxide is denser than air, with a density of approximately 2.9 grams per liter (g/L) at standard temperature and pressure (STP). This property is often utilized in the collection of sulfur dioxide gas by displacement of air, as mentioned in the preparation process.

Sulfur dioxide is a chemically and physically active gas with important applications in various industries, including as a precursor in the synthesis of sulfuric acid and in food preservation.

chapter 7.  ELEMENTS OF GROUPS 16, 17 AND 18 Textbook Solution page 164