Question

For \[H{{g}_{2}}C{{l}_{2}}\] on treatment with alkali gives red solid which is:
A.\[H{{g}_{2}}O\]
B.\[HgO\]
C.\[H{{g}_{2}}{{\left( OH \right)}_{2}}\]
D.\[Hg{{\left( OH \right)}_{2}}\]

Answer 1

Hint: We know that the mercuric oxide in solid form appears as red or orange-red odorless, dense crystalline powder or scales, yellow when finely powdered. Mercuric oxide \[HgO\] decomposes to mercury on being heated which condenses on the cool parts of the test tube forming a silver mirror.

Complete answer: We know that thermal decomposition is a chemical reaction where a single substance breaks into two or more simple substances on being heated. The Castner-Kellner process is a method of electrolysis of an alkali chloride solution in aqueous medium and it produces the corresponding alkali hydroxide at the end of electrolysis.
In this process, the apparatus has two kinds of cells. In the first cell, we use sodium chloride solution as our electrolyte with a graphite anode and a mercury cathode. This cell surrounds the second cell from both sides. In the second cell, we use an electrolyte of sodium hydroxide solution and a mercury anode and iron cathode. There is a wall which separates both the cells but mercury is allowed to flow beneath them. This is the central cell.
\[H{{g}_{2}}C{{l}_{2}}+\underset{~Alkali~}{\mathop{2NaOH}}\,\to \underset{~Red\text{ }Solid~}{\mathop{HgO}}\,+2NaCl+Hg+{{H}_{2}}O.\]
Therefore, the correct answer is option B.

Note:
Remember that to test for mercury, we can simply darken the room and use a mercury-based UV lamp along with a white background. If we shine this on the glass tube, we can clearly see the vapor’s silhouette on the paper. A gas is also emitted whose nature we can ascertain by conducting the following experiments.