Question

What happens when mercuric oxide is heated in a clean and dry test tube?

Answer 1

Hint: We know that application of heat to certain substances causes only breakdown of a compound in which no new substance or substances are formed. In this case too only decomposition of an oxide takes place.

Complete step by step answer:
We know that thermal decomposition is a chemical reaction where a single substance breaks into two or more simple substances on being heated.
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. 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.
If we introduce a glowing splint into the gas we see that it is rekindled.
If we pass the gas evolved through an alkaline pyrogallol solution, it turns brown.
Hence, we can conclude that a silver mirror is formed on the cooler parts of the test tube and the gas evolved is oxygen.
The reaction stated above can be further illustrated by the following equation:
$2HgO \to 2Hg + {O_2}$

Additional information:
While testing for Oxygen there are sometimes confusions as the splint can create a pop very slightly on re-ignition, which sometimes is mistaken for hydrogen. But we must remember a hydrogen pop is much more violent, sometimes enough to completely extinguish the splint.

Note: We must not attempt this without a proper risk assessment and protection from mercury vapor. Also be sure to follow the procedures for dealing with the mercury produced.