Q.iii. How is SO$_2$ prepared in laboratory from sodium sulfite? Give two physical properties of SO$_2$.

Sulfur dioxide, SO2, is prepared by burning sulfur in air, by treating sodium sulfite with dilute sulfuric acid (laboratory route), or industrially by roasting sulfide ores such as zinc sulfide and iron pyrites. It is a colourless, pungent, poisonous gas, highly soluble in water (giving sulfurous acid), liquefying under about 2 atm at room temperature. It reacts with chlorine (to sulfuryl chloride), with oxygen over V2O5 (to sulfur trioxide, the key Contact-process step), with NaOH (to sodium sulfite), and acts as a reducing agent in moist conditions (reducing Fe3+ to Fe2+, decolourising KMnO4, and reducing halogens to halogen acids). Structurally it is bent (angular), O-S-O angle about 119.5 degrees, its S-O bonds a resonance hybrid with dpi-ppi back-bonding c …