Question

Which of the following gives white precipitate, with ammoniacal silver nitrate solution ?
1. C₂H₆ (Ethane)
2. C₂H₄ (Ethylene)
3. C₂H₂ (Acetylene)
4. C₃H₆ (Propene)

Answer 1

Correct Answer - Option 3 : C₂H₂ (Acetylene)

Concept:

• In the laboratory, ethylene and acetylene can be distinguished by Tollens reagent.
• Tollens' reagent is a chemical reagent used to determine the presence of aldehyde, aromatic aldehyde, and alpha-hydroxy ketone functional groups.
• Tollens' reagent is basically an ammoniacal silver nitrate solution. 
• It can also be used to identify C2H2 (Acetylene)

Explanation:

if Ethyne has acidic hydrogen which when passed through ammonium silver nitrate solution we get a white precipitate of  disilver acetate and the reaction can be given as 

\(\rm \underset{ethyne}{HC\equiv CH} + \underset{Tollen's \ reagent}{2[Ag(NH_3)_2]OH}\xrightarrow{ \ \ \ \ \ \ \ \ \ \ \ } \rm \underset{diSilver \ acetylide}{Ag \ C \equiv CAg} + 4 \ NH_3+2H_2O\)