Question

Zinc chloride on treating with sodium sulphide forms a ……. colored \[Zn\] precipitate.
A. pink
B. dense white
C. black
D. orange

Answer 1

Hint: Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions. In double replacement reactions, the positive ions exchange negative ion partners. A double replacement reaction is represented by the general equation. \[AB + CD \to AD + CB\]
In a double displacement reaction, atoms from two different compounds switch places. The reactants are two compounds and the products are two different compounds. Double displacement reactions can also be called double replacement reactions.

Complete step by step answer:
Dense white precipitate of \[ZnS\] (Zinc sulphide) is formed when \[Z{n^{2 + }}\] ions meet \[{S^{2 - }}\] ions.
Assuming that the reaction takes place in aqueous solution because solid substances cannot react because no ions are present in the solid state. Both zinc chloride and sodium sulfide are soluble in water. So, there are four types of ions in the solution, viz., \[Z{n^{2 + }}\] , \[C{l^ - }\] , \[N{a^ + }\] , and \[{S^{2 - }}\].
However, when, in aqueous solution, zinc and sulfide ions meet, they react together to form the insoluble zinc sulfide (\[ZnS\]), a white coloured salt that forms dense white precipitate which finally settles down. It cannot be sodium chloride as it is highly soluble in water and does not normally form a precipitate under normal conditions.
The chemical equation for the above reaction is:
\[ZnC{l_{2\left( {aq} \right)}} + \;N{a_2}{S_{\left( {aq} \right)}} \to {\text{Zn}}{{\text{S}}_{\left( {\text{s}} \right)}}\left( {white\;ppt} \right) + 2\;{\text{NaC}}{{\text{l}}_{\left( {{\text{aq}}} \right)}}\]
It is to be noted that as happens in all double-displacement reactions, the cations and anions get interchanged and the compound which is insoluble forms a precipitate.

So the correct answer is (B).

Note: No attempt should be made in such reactions to form new ions in the products. The ions retain their identity and just get interchanged, i.e., change their places. For example, in this reaction \[{S^{2 - }}\] gets attached to \[Zn\].