# Liquid to gas transition - not vapour

Is there a name for a direct liquid to gas transition which is not vapour in an intermediate stage? It seems the name 'vapourisation' is not suitable in this circumstance. Yet boiling is usually regarded as a type of vapourisation (along with evaporation).

• It is not really clear to me, Is the question inquiring in the existence of a liquid-gas phase transition that does not involve latent heat?
– user137661
Sep 27, 2019 at 17:02
• The direct transition from a solid to a gas, is called sublimation. It applies, for example, to dry ice which is the name given to frozen carbon dioxide. Sep 27, 2019 at 17:04
• user50229, you really need to add more details in your question. What particular form of vaporization or boiling is different from the standard form? What characteristic of that vaporization needs an explanation? Sep 27, 2019 at 17:35
• a vapor is a gas Sep 27, 2019 at 18:45
• @Adrian Howard That is why I said a gas which is not vapour Sep 30, 2019 at 10:13

By definition, a vapor is a type of gas, namely, a gas which is below its critical temperature (source: https://en.wikipedia.org/wiki/Vapor). A gas which is above its critical temperature can no longer be called a vapor.

A "pure" liquid state (i.e. one in which there is an insignificant* amount of vapor present) only exists below the boiling point of the substance. Crucially, the boiling point is always lower than than the critical temperature, except at the substance's critical pressure. If the temperature of the substance is between the boiling point and the critical temperature, any gaseous matter present is by definition a vapor. In order to go directly from the liquid phase to a non-vapor gas phase, you have to have a way to make the temperature change discontinuously, jumping instantaneously from below the boiling point to above the critical point. Since this is impossible, no such transition exists.

If the substance happens to be at its critical pressure, then the boiling point and the critical temperature are the same. But at the critical point, the properties of a liquid and the properties of a gas are identical, so the substance isn't really undergoing a phase change in the same sense.

*There is always some amount of vapor that exists above a liquid (how much vapor exists under given conditions is determined by the vapor pressure), and if you included that vapor, then your question would immediately be rendered moot, because even a liquid is always a liquid-vapor mixture. That said, for the purposes of the rest of the discussion we'll only count the vapor that exists in excess of the vapor pressure under the starting conditions of the liquid, because there are some valuable points to be made if we don't prematurely stop the argument.

• Is it never possible to have a discontinuous jump in temperature? You can get discontinuities in other variables such as entropy. Sep 30, 2019 at 10:19
• @user50229 Probably because of the relationship between the rate of temperature increase as a function of time and heat transfer rate: $\frac{dQ}{dt}=C\frac{dT}{dt}$, where $C$ is the appropriate heat capacity of the object. In order to have a discontinuous jump in temperature, your heat transfer rate into or out of the system would have to be infinite, which isn't possible. Sep 30, 2019 at 14:39

The liquid-to-gas transition process is called vaporization. Evaporation and boiling are two mechanisms for how vaporization occurs.

• All of this is mentioned in the question! Sep 30, 2019 at 10:19