Overview
A bubble is a globule of a gas substance in a liquid. In the opposite case, a globule of a liquid in a gas, is called a drop.
Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance.
Common examples
Bubbles are seen in many places in everyday life, for example:
As spontaneous nucleation of supersaturated carbon dioxide in soft drinks
As vapor in boiling water
As air mixed into agitated water, such as below a waterfall
As sea foam
As a soap bubble
As given off in chemical reactions, e.g., baking soda + vinegar
As a gas trapped in glass during its manufacture
As the indicator in a spirit level
Physics and chemistry
Bubbles form and coalesce into globular shapes because those shapes are at a lower energy state. For the physics and chemistry behind it, see nucleation.
Appearance
From Wikipedia (CC BY-SA 4.0).