|MadSci Network: Physics
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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