Question

A metal hydroxide is precipitated as a white gelatinous substance when $N{H_4}OH$ is added to the solution (after previously removing acid insoluble sulphides with ${H_2}S$). The metal in metal hydroxide is-
(A) Cr
(B) Fe
(C) Al
(D) Zn

Answer 1

Hint: Precipitation is described as the formation of solid from a solution. The solid formed is called a "precipitate" if the reaction occurs in a liquid solution. The substance which forms the solid is called the precipitant. The precipitate stays in suspension without adequate gravitational force (settling) to mix the solid particles.

Complete step by step solution:
A reagent is a compound that is added to or added to a system to cause a chemical reaction. Reactant and reagent are commonly used interchangeably, but a reactant is a product that is more precisely used during a chemical reaction. The term "reagents" refers to an organic substance which is a chemical ingredient (a compound or mixture, usually made of inorganic or organic molecules). Examples include the Collins reagent, Grignard reagent and the Fenton's reagent.
Let’s consider our first reaction with Aluminum-
First, it will from a white precipitate-
$ \Rightarrow A{l^{3 + }} + 3O{H^ - } \to Al{\left( {OH} \right)_3} \downarrow $
Then, when excess of reagent was added to the precipitate, it dissolves the precipitate which in turn forms tetrahydroxoaluminate-
$ \Rightarrow Al{\left( {OH} \right)_3} + 3O{H^ - } \to {\left[ {Al{{\left( {OH} \right)}_4}} \right]^ - }$
As per given in the question, when $N{H_4}OH$ was added to this mixture, it results into gelatinous white precipitate of Aluminum hydroxide-
$ \Rightarrow {\left[ {Al{{\left( {OH} \right)}_4}} \right]^ - } + NH_4^ + \to Al{\left( {OH} \right)_3} \downarrow + N{H_3} \uparrow + {H_2}O$

Hence, it is clear that option C is the correct option.

Note: Aluminum hydroxide is considered as an antacid. It's used to treat heartburn, acid indigestion, stomach disturbance or acidity. For people with other diseases of the kidneys, aluminum hydroxide is often used to minimize levels of phosphate.