JEE Main & Advanced Chemistry Analytical Chemistry Test For Different Gases

(1) Colourless gases

(i)  Tests for \[\mathbf{C}{{\mathbf{O}}_{\mathbf{2}}}\] : It is colourless and odourless gas. It gives white ppt. with lime water which dissolves on passing excess of \[C{{O}_{2}}\]. \[\underset{Lime\,water}{\mathop{Ca{{(OH)}_{2}}}}\,+C{{O}_{2}}\to \underset{White\,ppt.}{\mathop{CaC{{O}_{3}}}}\,\downarrow +{{H}_{2}}O\]

  \[\underset{White\,ppt.}{\mathop{CaC{{O}_{3}}}}\,+\underset{Excess}{\mathop{C{{O}_{2}}}}\,+{{H}_{2}}O\to \underset{So\operatorname{lub}le}{\overset{{}}{\mathop{Ca{{(HC{{O}_{3}})}_{2}}}}}\,\]

(ii) Test for CO :  It is colourless and odourless gas. It burns with a blue flame. \[2CO+{{O}_{2}}\to 2C{{O}_{2}}\]     

(iii) Test for \[{{\mathbf{O}}_{\mathbf{2}}}\] : It is colourless and odourless gas. It rekindles a glowing splinter.

(iv) Tests for \[{{\mathbf{H}}_{\mathbf{2}}}\mathbf{S}\] : It is a colourless gas with a smell of rotten eggs. It turns moist lead acetate paper black.

\[{{(C{{H}_{3}}COO)}_{2}}Pb+{{H}_{2}}S\to 2C{{H}_{3}}COOH+\underset{Black}{\mathop{PbS}}\,\]

(v) Tests for \[\mathbf{S}{{\mathbf{O}}_{\mathbf{2}}}\] : It is a colourless gas with a suffocating odour of burning sulphur. It turns acidified \[{{K}_{2}}C{{r}_{2}}{{O}_{7}}\] solution green.  \[3S{{O}_{2}}+{{K}_{2}}C{{r}_{2}}{{O}_{7}}+{{H}_{2}}S{{O}_{4}}\to {{K}_{2}}S{{O}_{4}}+\underset{Green}{\mathop{C{{r}_{2}}{{(S{{O}_{4}})}_{3}}}}\,+{{H}_{2}}O\]

(vi) Tests for \[\mathbf{N}{{\mathbf{H}}_{\mathbf{3}}}\]: It is a colourless gas with a characteristic ammonical smell. It gives white fumes of \[N{{H}_{4}}Cl\] with \[HCl\], \[N{{H}_{3}}+HCl\to \underset{White\,fumes}{\mathop{N{{H}_{4}}Cl}}\,\]. With Nessler’s reagents, it gives brown ppt.

\[\underset{Nessler's\ \,reagent}{\mathop{2{{K}_{2}}\left[ Hg{{I}_{4}} \right]}}\,\ +N{{H}_{3}}+KOH\to \underset{\underset{(Brown\,ppt)}{\mathop{Iodine\,\text{of M}illon's\,base}}\,}{\mathop{N{{H}_{2}}HgOHgI}}\,+7KI+2{{H}_{2}}O\]

It gives deep blue colour with \[CuS{{O}_{4}}\] solution, \[CuS{{O}_{4}}+4N{{H}_{3}}\to \underset{Deep\,blue}{\mathop{\left[ Cu{{(N{{H}_{3}})}_{4}} \right]S{{O}_{4}}}}\,\]. \[N{{H}_{3}}\] dissolves in water to give \[N{{H}_{4}}OH,\] which being basic, turns red litmus blue, \[N{{H}_{3}}+{{H}_{2}}O\to N{{H}_{4}}OH\]\[\rightleftharpoons \]\[NH_{4}^{+}+O{{H}^{-}}\].

(vii) Tests for HCl gas : It is colourless gas with a pungent irritating smell.  It turns moist blue litmus paper red i.e., it is acidic in nature. It gives white ppt. with \[AgN{{O}_{3}}\] solution. This white ppt. is soluble in \[N{{H}_{4}}OH.\] \[HCl+AgN{{O}_{3}}\to \underset{White\,ppt.}{\mathop{AgCl}}\,+HN{{O}_{3}}\]; \[AgCl+2N{{H}_{4}}OH\to \underset{\text{Soluble}}{\mathop{\left[ Ag{{(N{{H}_{3}})}_{2}} \right]}}\,\ +2{{H}_{2}}O\].

(viii) Test for \[C{{H}_{3}}COOH\] vapours : These vapours are colourless with a vinegar like smell.

(2) Coloured gases

(i) Tests for \[\mathbf{C}{{\mathbf{l}}_{\mathbf{2}}}\]:  It is a greenish yellow gas with a pungent smell. In small quantity it appears almost colourless. It bleaches a moist litmus paper, \[C{{l}_{2}}+{{H}_{2}}O\to 2HCl+\left[ O \right]\]; \[Colour+\left[ O \right]\to Colourless.\] Blue litmus paper first turns red and then becomes colourless.

(ii)  Tests for \[\mathbf{B}{{\mathbf{r}}_{\mathbf{2}}}\] : Brown vapours with a pungent smell. It turns moist starch paper yellow.

(iii) Tests for \[{{\mathbf{I}}_{\mathbf{2}}}\] : Violet vapours with a pungent smell. It turns moist starch paper blue.

(iv) Tests for \[\mathbf{N}{{\mathbf{O}}_{\mathbf{2}}}\]: Brown coloured pungent smelling gas. It turns moist starch KI paper blue

\[2KI+2N{{O}_{2}}\to 2KN{{O}_{2}}+{{I}_{2}}\];  \[{{I}_{2}}+Starch\to Blue\,colour.\]

It turns ferrous sulphate solution black,

\[3FeS{{O}_{4}}+N{{O}_{2}}+{{H}_{2}}S{{O}_{4}}\to F{{e}_{2}}{{(S{{O}_{4}})}_{3}}+\underset{Black\,brown}{\mathop{FeS{{O}_{4}}.\,NO}}\,+{{H}_{2}}O\]