Expert: Tom Whiting - 1/22/2010

Question
QUESTION: What is the process by which water changes from one state to another?

ANSWER: Hi Charlie,
When water (or any other substance) changes from a solid to a liquid, that's called
melting.  The reverse process is called freezing, or solidifying.
When water changes from the liquid to the gaseous state, that's called evaporation
(generally other substances, it's called vaporization).
The reverse process is called condensation.
When water (or any other substance) changes from the solid to the gaseous state, bypassing
the liquid state, it's called sublimation.  (Dry ice or frozen carbon dioxide being the most
famous on this planet).
The reverse process is called deposition. (frost and snowflakes being the best examples of
deposition on this planet).
The actual processes are caused by either the release of energy, generally heat (exothermic)
or absorption of energy, generally heat. (endothermic).  So it all deals with heat exchanges.
If a gaseous substance loses energy, then it goes to first a liquid state (if there is enough pressure on it) then to the solid state. And of course, the reverse is true.  
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA

---------- FOLLOW-UP ----------

QUESTION: SO the heat is absorbed, not liberated?

Answer
It's both, depending upon the outside surroundings of the environment around the substance.

If icecubes are placed in 80 degree F. air temperature, heat is absorbed and the cube melts.
If liquid water finds itself in a 10 degree F air temperature, heat is liberated from the liquid
water and the water freezes to a solid. So, works both ways depending on what environment it finds itself.
That's how water vapor forms clouds (small liquid cloud droplets)...the air with humid water
vapor gas rises, expands (due to lower pressure)...expansion is a cooling process, eventually
the water vapor reaches the dew point temperature, heat is liberated and the water vapor
condenses to small liquid cloud droplets...a cloud (or fog if it's on or near, the ground).
(Water vapor gas always rises because it's lighter than air.... molecular weight 18 compared
to air at an average mole wt of 29.)
Clear skies,
Tom