Sign up ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Yesterday I asked a question at Chemistry SE, which is if the molten NaCl molecules have enough energy to break into free moving ions, won't be the Na Cl vapor contain free moving ions? So, I've got this answer telling me that that's what we call plasma. But after reading about plasma, I've read that "plasma is a gas which received enough energy to be ionized so ions and electrons can coexist ". In my question there will be ionization, but this ionization will take place when the ionic compound is melting breaking down to positive and negative ions not positive ions and electrons. So basically I need to know if Na Cl vapor is ionized "into positive and negative ions" or it will turn to plasma.

share|cite|improve this question

2 Answers 2

up vote 7 down vote accepted

You're presumably thinking that vaporised NaCl would consist of Na$^+$ and Cl$^-$ ions, but this isn't what you get. At temperatures just above the boiling point (1413c) you get neutral NaCl molecules and Na$_2$Cl$_2$ dimers, and possibly bigger polymers. Amazingly someone has measured this: see this paper for details (it's behind a paywall I'm afraid but you can see the abstract).

So the answer is no, the vapour is not a plasma. You'd have to heat it to much higher than the boiling point to get a plasma.

share|cite|improve this answer
Do other ionic compounds like MgO for example also vaporize into neutral MgO molecules ? Or is this something exclusive for NaCl ? – Abanob Ebrahim Nov 20 at 12:56
@AbanobEbrahim: my guess is that any ionic solid will form neutral molecules when it vaporises. That's because the energy required to dissociate the molecules is comparable to the energy needed to make a plasma, and that requires very high temperatures. But this is just a guess. You would have to search the literature for experimental measurements to be sure. – John Rennie Nov 20 at 14:01

It may sound strange, but Sodium Chloride exists as a molecule in vapourosed form. It is the only ionic compound in this world that exhibits this strange property.

share|cite|improve this answer
Wouldn't most salts behave this way? – Kevin Kostlan Aug 30 '13 at 12:09
You need to provide some references or a more detailed justification to this assertion because it is not common scientific knowledge like F=ma or the Coulomb force. – Bill N Sep 30 at 13:52

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.