Question

A solution when diluted with ${ H }_{ 2 }{ O }$ and boiled, gives a white precipitate. On the addition of excess ${ NH }_{ 4 }OH{ /NH }_{ 4 }{ Cl }$, the volume of the precipitate decreases leaving behind a white gelatinous precipitate. Identify the precipitate which dissolves in ${ NH }_{ 4 }Cl{ /NH }_{ 4 }{ OH }$:
(A) ${ Zn(OH) }_{ 2 }$
(B) ${ Ar(OH) }_{ 3 }$
(C) ${ Mg(OH) }_{ 2 }$
(D) ${ Ca(OH) }_{ 2 }$

Answer 1

Hint: Precipitate: It is defined as the ions or molecules in two solutions that react to form a solid. A solid formed from two solutions is called a precipitate. The precipitate formed here is white in colour and the cation present in it has 30 protons and 28 electrons in it.

Complete step-by-step answer:
A solution of ${ Zn }^{ +2 }$ the particle reacts with water which on bubbling gives white ppt. of ${ Zn(OH) }_{ 2 }$. This precipitate dissolve in an abundance of ${ NH }_{ 4 }{ OH }$ in the presence of ${ NH }_{ 4 }{ Cl }$ in light of the fact that ${ Zn(OH) }_{ 2 }$ on reaction with ${ NH }_{ 4 }{ OH }$ in the presence of ${ NH }_{ 4 }{ Cl }$ structures tetrammine zinc (II) complex (soluble).
The following reactions will take place:
${ Zn }^{ +2 }{ +2H }_{ 2 }{ O\rightarrow Zn(OH) }_{ 2 }{ +2H }^{ + }$
${ Zn(OH) }_{ 2 }{ +NH }_{ 4 }{ OH+NH }_{ 4 }{ Cl\rightarrow \left[ Zn\left( { NH }_{ 3 } \right) _{ 4 } \right] ^{ 2+ } }{ +4H }_{ 2 }{ O+2OH }^{ - }$
The precipitate of ${ Zn(OH) }_{ 2 }$ forms dissolves in excess of ${ NH }_{ 4 }{ OH }$ to form a tetrammine complex which is soluble.
Hence, the correct option is A.

Additional Information:
Properties of precipitate reaction:
1. The precipitation reaction experiences in aqueous solutions or mediums in an ionic state.
2. The reaction happens between particles present in the aqueous solution, and the product is formed.
3. The products formed at the end of precipitation reaction are the hastens which are insoluble in aqueous solutions.
4. Precipitation responses are known as ionic responses since the particles effectively participate in the reaction and structure of the product.
5. These reactions rely upon the temperature, convergence of the arrangement, support arrangement, and so on.

Note: The possibility to make a mistake is that you may choose option D. But when calcium hydroxide reacts with ${ NH }_{ 4 }OH{ /NH }_{ 4 }{ Cl }$ then calcium chloride will be formed but the product formed is orangish-red, not white.