When you apply heat to a metal hydrogen carbonate (frequently known as a metal bicarbonate as HCO3- is a bicarbonate ion), it will decompose into the corresponding metal oxide, carbon dioxide gas, and water vapor. The balanced reaction for the decomposition of zinc bicarbonate is as follows:
Zn(HCO3)2(s) --> ZnO(s) + 2CO2(g) + H2O(g)
(When you write out the reaction, heat is written as a delta or triangle above the arrow in the equation.)
The zinc bicarbonate decomposes to form solid zinc oxide, gaseous carbon dioxide, and water vapor. The water vapor is gaseous as it is formed but may condense at the opening of the test tube.
This is a general rule for metal bicarbonates. However, sodium bicarbonate (baking soda) will decompose to sodium carbonate, water vapor, and carbon dioxide when heated instead of going to the metal oxide. The carbon dioxide formed is what causes baked goods to rise in the oven (in the presence of heat).