Generally, when we heat the water in a bowl, initially we observe the bubbles near the base of the bowl or sides of the bowl. These bubbles are AIR; water has a lot of air dissolved in it. This is what helps the aquatic animals to breath. The solubility of gases decreases when the temperature is raised, and that is why the dissolved air bubbles go out from the water. The molecules of water gain enough energy at their boiling point to change their phase from liquid to vapor so they readily form bubbles inside the liquid.