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