Question

Reaction which produces a yellow precipitate is:
A. \[NaOH(aq) + HCl(aq) \to NaCl(s) + {H_2}O\]
B. \[NaOH(aq) + BaCl(aq) \to BaOH(s) + NaCl(aq)\]
C. \[Pb{(N{O_3})_2}(aq) + 2KI(aq) \to 2KN{O_3}(aq) + Pb{I_2}(s)\]
D. \[CuO(s) + Mg(s) \to Cu(s) + MgO(s)\]
E. \[4Fe + 3{O_2} \to 2F{e_2}{O_3}\]

Answer 1

Hint: This reaction is a double displacement reaction, also known as exchange reaction. The products formed are ionic in nature. It is also similar to iodoform reactions which usually give yellow precipitate as a product but along with this, one more product is formed which is white in colour.

Complete step-by-step answer:
Let us discuss each reaction one by one and see the colour of the product formed in the reaction.
First is \[NaOH(aq) + HCl(aq) \to NaCl(s) + {H_2}O\] .
In this reaction, white coloured solution of sodium hydroxide reacts with colourless solution of hydrochloric acid and produces white crystals of sodium chloride salt with water. We can say that it is a neutralisation reaction in which an acid reacts with base to form salt and water.
Second is \[NaOH(aq) + BaCl(aq) \to BaOH(s) + NaCl(aq)\] .
 In this reaction, white coloured solution of sodium hydroxide reacts with white coloured solution of barium chloride so as to form white crystals of barium hydroxide and sodium chloride salt which is also white coloured in solution as well as solid form.
Third is \[Pb{(N{O_3})_2}(aq) + 2KI(aq) \to 2KN{O_3}(aq) + Pb{I_2}(s)\] .
In this reaction, white crystalline solution of lead nitrate is prepared and reacted with white coloured solution of potassium iodide to form a yellow coloured solution of potassium iodide. Another product is also formed which is a white solution of potassium nitrate as it is a double displacement reaction.
Fourth is \[CuO(s) + Mg(s) \to Cu(s) + MgO(s)\] .
 In this reaction, a black solid of cupric oxide reacts with magnesium metal to form copper metal and magnesium oxide. This is due to the fact that magnesium is higher in the reactivity series than copper and thus displaces copper from its compound. Both the products formed are solid in state as we can see in the equation.
Fifth is \[4Fe + 3{O_2} \to 2F{e_2}{O_3}\] . In this reaction, we heat iron in presence of oxygen to form iron oxide which is brown in colour. We know that when a substance is burnt in presence of oxygen to form its oxide, it is a combustion reaction.

Hence the correct option is (C).

Note: Displacement reaction is a chemical reaction in which one element gets replaced by another element in a compound. Whereas double displacement reaction is a chemical reaction which involves exchange of bonds between two non-reacting chemical species which leads to the formation of products with similar bonding affiliations.