JEE Main & Advanced Chemistry Analytical Chemistry Test For Different Gases

Test For Different Gases

Category : JEE Main & Advanced

(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,

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

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