JEE MAIN - Chemistry (2003 - No. 28)
A red solid is insolvable in water. However it becomes soluble if some $$K{\rm I}$$ is added to water. Heating the red solid in a test tube results in liberation of some violet colored fumes and droplets of a metal appear on the cooler parts of the test tube. The red solid is :
$$Hg{{\rm I}_2}$$
$$HgO$$
$$P{b_3}{O_4}$$
$${\left( {N{H_4}} \right)_2}C{r_2}{O_7}$$
Explanation
When $$K{\rm I}$$ is added to mercuric iodide it dissolve in it and form complex.
$$\mathop {Hg{{\rm I}_2}}\limits_{red,solid(inso{\mathop{\rm lub}} le)} \,\, + \,\,\,K{\rm I} \to \mathop {{K_2}\left[ {Hg{{\rm I}_4}} \right]}\limits_{(soluble)} $$
On heating $$Hg{{\rm I}_2}$$ decomposes as
$$Hg{{\rm I}_2}\,\,\,\,Hg\,\,\rightleftharpoons\, + \mathop {{{\rm I}_2}}\limits_{(violetvapours)} $$
$$\mathop {Hg{{\rm I}_2}}\limits_{red,solid(inso{\mathop{\rm lub}} le)} \,\, + \,\,\,K{\rm I} \to \mathop {{K_2}\left[ {Hg{{\rm I}_4}} \right]}\limits_{(soluble)} $$
On heating $$Hg{{\rm I}_2}$$ decomposes as
$$Hg{{\rm I}_2}\,\,\,\,Hg\,\,\rightleftharpoons\, + \mathop {{{\rm I}_2}}\limits_{(violetvapours)} $$
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