Question

Reaction which produces a yellow precipitate is:
(A) $NaO{H_{aq}}\,\, + \,\,HC{l_{aq\,}}\,\, \to \,\,NaC{l_s}\, + \,{H_2}O$
(B) $NaO{H_{aq}}\, + \,BaC{l_{aq}}\,\, \to \,\,BaO{H_s}\,\, + \,\,NaC{l_{aq}}$
(C) $Pb{\left( {N{O_3}} \right)_2}_{aq}\, + \,\,2K{I_{aq}}\, \to \,\,2KN{O_3}_{aq}\, + \,Pb{I_2}{}_s$
(D) $Cu{O_s}\, + \,M{g_s}\, \to \,C{u_s}\, + \,Mg{O_s}$
(E) $4Fe\, + \,3{O_2}\, \to \,2F{e_2}{O_3}$

Answer 1

Hint: In this question our aim is to identify the reaction which produces a yellow precipitate out of the given five reactions . A precipitate is defined as a substance which is deposited at the bottom of the solution in solid form.

Complete answer:
> Precipitation is a reaction in which a solid product known as precipitate is formed when the reaction takes place in a liquid medium. The chemical that helps in the formation of precipitate is known as a precipitant. The liquid left after the precipitate formation is known as supernatant .A precipitate is formed when the ionic product is more than the solubility product.
(Ionic product is the product of concentration of ions of electrolytes raised to the power of their coefficients in a balanced chemical equation while solubility product is just the concentration of ions of the electrolyte).
For example,
> Nickel compounds are green, cobalt gives red colour, copper gives blue or green coloured compounds, lead gives yellow compounds ,etc.
> The precipitation reaction is also used in the detection of group 1 to group 6 basic radicals with the help of qualitative analysis.
> Among the reactions given, when lead nitrate reacts with potassium iodide it gives lead nitrate and a precipitate of lead iodide is formed which is yellow in colour.
$Pb{\left( {N{O_3}} \right)_2}_{aq}\, + \,\,2K{I_{aq}}\, \to \,\,2KN{O_3}_{aq}\, + \,Pb{I_2}{}_s$
> In all the other given reactions no yellow precipitate is formed.

Hence, option C is correct.

Note: Many compounds containing metal ions, especially those of the transition elements form coloured compounds due to d-d transition, that is they absorb visible light, therefore they show the light which is complementary to the absorbed light.