ANS:C - SO₂ in alkaline solution.

The absorption of sulfur dioxide (SO2) in alkaline solutions, such as sodium hydroxide (NaOH) or calcium hydroxide (Ca(OH)2) solutions, is an example of absorption accompanied by chemical reaction. Explanation:

1. Absorption Process:
   • Sulfur dioxide (SO2) is a gas often produced in various industrial processes, such as combustion of fossil fuels and metal smelting.
   • To mitigate environmental pollution and comply with regulatory standards, SO2 emissions need to be controlled. One effective method is to absorb SO2 into a suitable solvent.
2. Chemical Reaction:
   • When sulfur dioxide gas is absorbed into an alkaline solution, such as NaOH or Ca(OH)2, chemical reactions occur between SO2 and the alkaline species present in the solution.
   • In the presence of hydroxide ions (OH^-) from the alkaline solution, sulfur dioxide reacts to form sulfite ions (SO3^2-) or bisulfite ions (HSO3^-), depending on the specific conditions and concentration of reactants.
   • The general chemical equations representing these reactions are:
     • SO2 + 2OH^- → SO3^2- + H2O (formation of sulfite ions)
     • SO2 + OH^- → HSO3^- (formation of bisulfite ions)
3. Reaction Mechanism:
   • The absorption of SO2 in alkaline solutions involves a series of complex chemical reactions, where SO2 molecules interact with hydroxide ions to form sulfite or bisulfite species.
   • These chemical reactions effectively trap and convert SO2 into soluble sulfite or bisulfite ions, thereby removing it from the gas phase and reducing atmospheric pollution.
4. Environmental Application:
   • The absorption of SO2 in alkaline solutions is widely used in environmental control systems, such as flue gas desulfurization (FGD) processes in power plants and industrial facilities.
   • By employing alkaline solutions as absorbents, SO2 emissions can be captured and converted into less harmful sulfite or bisulfite compounds, contributing to cleaner air and environmental protection.