The decomposition of mercury(II)oxide

This reaction needs heat. How do we call such a reaction?

The reaction is endothermic.

Why is water chased out of the tube?

Oxygen, less dense than water, replaces the water

.

Two elementary substances are produced. How do we call such a reaction?

It is an analysis.

Write down its equation!

$2HgO$ $\longrightarrow$ $2Hg$ $+$ $O_2$.