Water molecules during evaporation. Computer-generated model of several molecules of water, at an early stage of evaporation. Each water molecule (H2O) consists of an atom of oxygen (yellow) bonded to two atoms of hydrogen (white). When heated to a temperature of 100 Celsius, water molecules enter a gaseous phase to form water vapor. This process of gasification is known as evaporation. During early evaporation (as shown here) the water molecules are becoming less closely packed, and they begin to move around energetically at high speed. When in the liquid phase, water molecules are more closely packed together.