When water is heated, the steam at first forms in very small bubbles at the bottom of the vessel, and as these bubbles rise through the colder water, the steam in them is condensed to water, so that they break before reaching the surface, thus producing what is called “simmering.”
When does water boil?
Water boils when it becomes heated so that the bubbles rise to the surface of the water before breaking.
Why will a vessel full of water run over when heated?
Because the water is expanded by heat, and some of it must run over.
Why will the water boil over when the vessel is not full?
Because the steam, in trying to escape, throws some of the water over the edge of the vessel.
Vaporization.
When water is boiled, what change does it undergo?
It is changed into a vapor, called steam.