Question

Mark the compound which turns black with \[\text{N}{{\text{H}}_{\text{4}}}\text{OH}\] ?
(A) Lead Chloride
(B) Mercurous Chloride
(C) Mercuric Chloride
(D) Silver Chloride

Answer 1

Hint: Ammonia is dissolved in water to form ammonium hydroxide. You can represent it with the symbols\[N{{H}_{3}}_{\left( aq \right)}\]. It is actually impossible to isolate samples of \[\text{N}{{\text{H}}_{\text{4}}}\text{OH}\], despite the name ammonium hydroxide's suggestion that it is an alkali with the chemical formula \[[N{{H}_{4}}^{+}][O{{H}^{-}}]\]. Except in extremely diluted solutions, the ions do not contribute significantly to the total quantity of ammonia.

Complete Step by Step Solution:
\[\text{H}{{\text{g}}_{\text{2}}}\text{C}{{\text{l}}_{\text{2}}}\], is a mineral that contains both mercury and chloride. It has the extremely peculiar trait of turning black when ammonia, or \[N{{H}_{3}}_{\left( aq \right)}\], is introduced into the reaction. Calomel now serves as an electrode between metallic mercury and a chloride solution that is added to a calomel electrode, allowing it to be used to measure the pH of substances. It is thought to create a half cell with a calomel-coated mercury core.

It also includes a platinum wire, which is employed since platinum is thought to respond slowly because it generally does not surrender its electron, allowing it to function in the electrode for a very long time. The calomel also contains potassium chloride, which is a saturated solution, and ammonium hydroxide, which has the chemical formula \[\text{N}{{\text{H}}_{\text{4}}}\text{OH}\], along with the platinum wire. Below is a list of the calomel's reaction.

\[H{{g}_{2}}C{{l}_{2}}+2N{{H}_{4}}OH\to Hg+Hg(N{{H}_{2}})Cl+N{{H}_{4}}Cl+2{{H}_{2}}O\]
Mercury, ammonium chloride, and a water molecule are produced when calomel and ammonium hydroxide react. \[\text{Hg}\left( \text{N}{{\text{H}}_{\text{2}}} \right)\text{Cl}\] is a chemical that is dark in colour as a result of the calomel-ammonia reaction that results in black mass.
Hence option (B) is correct.

Note: Traditional qualitative inorganic analysis employs aqueous ammonia as a base and complexant. With copper(II) solutions, it exhibits a rich blue colouring similar to that of many amines. Silver oxide residues, such as those produced by the Tollens reagent, can be dissolved in an ammonia solution. It is frequently present in cleaning solutions for gold, silver, and platinum jewellery, but it may be harmful to permeable gems like opals and pearls.