# Can water vapor go directly into a solid

## Main Question or Discussion Point

If water vapor is pulled inwards and cooled at a fast enough rate could if be frozen back into a solid form? i understand that they would have to be froze together as soon as contact is made but if this is possible what would the temperature have to be? And could this be the only thing that can directly go from a gas to a solid?

## Answers and Replies

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This graph should help you out. Of course, for water to freeze to a solid state it has be at 0* Celsius but that’s just half the puzzle. The atmospheric pressure has to be below 0.006 atm for it to go directly to a solid state instead of it at its triple point (where its solid, liquid, and gas phase all exist!).

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phyzguy
So if the vapor pressure is low enough, water can go directly from the vapor phase to the solid phase. Isn't this what we call frost?

jbriggs444
Homework Helper
2019 Award
The atmospheric pressure has to be below 0.006 atm for it to go directly to a solid state instead of it at its triple point (where its solid, liquid, and gas phase all exist!).
If the partial pressure is below the line delineating the solid phase and the temperature is below the triple point then I read the diagram to indicate that the equilibrium state will be as vapor. The net activity at a hypothetical solid/vapor interface would be sublimation from solid to vapor, not the reverse.

On the other hand, if the partial pressure were above 0.006 atm and the temperature is low enough that the liquid phase is avoided then the equilibrium state will be as solid. The net activity at a hypothetical solid/vapor interface would be condensation from vapor directly to solid.

jrmichler
Mentor
Water vapor directly to a solid is snow. Also frost as discussed above. I just cleared 8 inches of the stuff off my driveway yesterday.

Solid water directly to vapor can also be seen as a thin layer of ice on the driveway that disappears without melting over several days of below freezing weather.

Google dry ice maker for ways to make dry ice (solid carbon dioxide) directly from a tank of liquid carbon dioxide.

Google phase diagram mercury to find a phase diagram that shows mercury also going directly from vapor to solid, and the reverse. I expect that (some, many, most - help me out here) elements are capable of condensing directly from vapor to solid, and sublimating from solid to vapor.

Khashishi