MadSci Network: Physics

Re: At what temperature does water turn to plasma?

Date: Mon Feb 26 12:51:34 2001
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 981068867.Ph

Dear Nate Morry:
You ask "at what temperature does water turn to plasma?"  The way you ask 
this question I assume that you wish to make the plasma by only heating of 
the water thermally.  In actual fact a plasma in water vapor can be made at 
many different temperatures depending upon the pressure of the water vapor 
gas if you use an electrical source to initiate the plasma.  However I am 
taking your question to mean only by heating as the source-no electrical 
excitation.  Remember that a plasma of water means that you will have free 
electrons, water vapor positive ions (H2O+), and also other positive ions 
like OH+, O+, and H+ as well as neutral fragments like , , and  
because the water vapor molecules will be broken up in the plasma.

It has been measured that to create any degree of ionization in a water 
vapor atmosphere you need to have electrons with energy of at least around 
12 electron volts. (See Ref. below) That means that if you want to ionize 
the water (make a plasma) by thermal energy only you must impart enough 
thermal energy to get a lot of electrons of energy about 12 electron volts 
of higher.  That means the water molecules will have to be heated up to an 
excitation level of about this magnitude.   For a water molecule to be 
heated to this excitation energy we are talking about a temperature of 
about 12,000 degrees K.  So if you could heat the water to this temperature 
you would begin to make such a plasma.  Needless to say that is very 
difficult to heat the water purely thermally to this temperature with no 
electrical excitation.  

Using electrical means by applying a voltage across the space in the water 
vapor you could initiate a water plasma with a high voltage transfortmer at 
a pressure of about 1 torr of water vapor, but in this plasma only the 
electrons would be heated to this temperature, and the water molecules 
would much cooler, probable more like 400 degrees K.  This is called a 
non-equilibrium plasma, where the electrons have high energy (e.g. 12 
electron volts) but the water molecules are relatively cool;  and the ions 
are made by the high energy bombardment by the electrons while the water 
molecules are cool!  [REF:W.C. Price and T.M. Sugden, Trans. Faraday 
Society 44, 108 (1948).]

I hope this is all clear to you.  Ask more questions if you wish.

R. Bersin......

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