The gas which is used to make chloroform is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Methane
Chloroform is trichloromethane, \(\ce{CHCl3}\): a methane molecule in which three of the four hydrogen atoms have been replaced by chlorine atoms. It is made by the chlorination of methane. Methane and chlorine react in the presence of sunlight or ultraviolet light, and the hydrogen atoms are replaced one by one (\(\ce{CH3Cl}\), \(\ce{CH2Cl2}\), then \(\ce{CHCl3}\)).
\(\ce{CH4 + 3Cl2 -> CHCl3 + 3HCl}\)
Why the other options are incorrect
Option (1) Cyanogen — Incorrect. Cyanogen, \(\ce{(CN)2}\), has no hydrogen atoms for chlorine to replace.
Option (2) Radon — Incorrect. Radon is a radioactive noble gas, an element and not a carbon compound.
Option (3) Propane — Incorrect. Propane, \(\ce{C3H8}\), has three carbon atoms; chloroform has only one.