# Could a gas go directly to a solid without becoming a liquid?

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?

• en.wikipedia.org/wiki/Sublimation_(phase_transition) – valerio Jan 24 '18 at 15:35
• Since it seems you have a practical reason for asking, not just out of curiosity, I won't post this as an answer, but note that in the interstellar medium, everything heavier than hydrogen and helium ("metals") tends to deplete directly from a gaseous phase to a solid phase (dust grains), if the temperature is low enough and the density is high enough. Roughly 1/3 of the metals are locked up in dust this way in the ISM. – pela Jan 24 '18 at 17:51
• @NathanielBarnhill It's wise to use healthy skepticism when considering Wikipedia in general, but frankly, that page is likely better sourced and more complete than any answer you'll get here. (no offense to any answer here...it's just that a short/quick q&a site is a different entity than a Wikipedia page) – Beska Jan 24 '18 at 18:19
• @NathanielBarnhill you should start doing that then. – A. C. A. C. Jan 24 '18 at 19:25
• Part of our expectation for the level of research you should undertake before asking a question is that you should look up relevant topics on Wikipedia, as well as performing a Google search which in this case would probably turn up a relevant Wikipedia page. If you don't trust the Wikipedia result you find, that's fine, but you do need to find it and explain in the question why you don't consider it to provide a valid answer. – David Z Jan 25 '18 at 3:14

Changing a substance from its physical state of a gas to the physical state of a solid requires the removal of thermal energy. A gas has particles that have larger amount of kinetic or moving energy, they are vibrating very rapidly. A solid has particles with lower amounts of kinetic energy and they are vibrating slower without changing position. This change of state from a gas to a solid is not a very common phase change but is referred to as deposition. It is called deposition because the particles in the gas form are depositing into a solid form.

Examples of Gas to Solid:

1. Making dry ice or solid carbon dioxide involves the removal of gaseous carbon dioxide from air and using cold temperatures and higher pressure causes the gas particles to skip the liquid phase and deposit into a solid to form a chunk of dry ice.

2. A carbon dioxide fire extinguisher has been filled with gaseous carbon dioxide but inside the canister the higher pressure causes this to turn into solid carbon dioxide which later is released as a white powder when putting out a fire.

3. In severely cold temperatures frost will form on windows because the water vapor in the air comes into contact with a window and immediately forms ice without ever forming liquid water.

4. Deposition has become a manufacturing technology application where solid alloys are heated to a gaseous state and then sprayed onto things like semiconductors. When the spray is released onto the semiconductor the heat energy is lost and the gaseous substance becomes a solid metal alloy.

• No, it doesn't always occur like that. Both temperature and pressure have to be ideal. The conditions for deposition vary from gas to gas. I don't know whether someone has tried out what you said, but I've never heard about it. – Wrichik Basu Jan 24 '18 at 16:05
• Thermal evaporation of metals (PVD) could be considered such a transition, too. The solid collected upon sublimation (for purification purposes, iodune for instance) is another example. You could add that to your answer (which I upvoted as it is). – Alchimista Jan 24 '18 at 17:20
• @Alchimista Here it says: in most instances of Thermal Evaporation processes the material is heated to its melting point and is liquid. – Wrichik Basu Jan 24 '18 at 18:16
• Nitpick: a dry powder fire extinguisher is different from a $\mathrm{CO}_2$ fire extinguisher. In particular, a $\mathrm{CO}_2$ extinguisher doesn't leave any powder behind. – Michael Seifert Jan 24 '18 at 23:00
• "hydrogen's freezing point is -434.5°F if that is doubled" What do you mean? Are you asking what would happen to hydrogen gas if it were exposed to -869 F? That's impossible. – PM 2Ring Jan 25 '18 at 3:48

Yes. The Phase Diagram of a substance shows when that substance is in which state. Here is a phase diagram of water (source):

You can see that there are phase boundaries between each pair of states. Where all three states meet is the Triple Point. The boundary where solid and gas (vapor) are adjacent is generally below the triple point in both temperature and pressure; for water it starts at the familiar freezing temperature, but at less than 1% of standard atmospheric pressure. If you want to get water to desublimate, you'll have to do it in near-vacuum.

• Shouldn't it be enough that the partial pressure of water in the air is below the triple-point pressure? Then you can cool it down past 0 °C without the water condensing out, and if you cool the air down even further it will already be too cold for liquid water to exist even at atmospheric pressure, so it will have to deposit directly as ice instead. – Henning Makholm Jan 25 '18 at 2:09
• I was thinking of an isolated pure substance, not a mixture, so I didn't consider that. Unfortunately I don't remember enough on the subject to be sure, and my search didn't yield a direct answer. It sounds plausible - and might be testable in an ordinary freezer. – ShadSterling Jan 25 '18 at 5:21

It's called deposition. It occurs fairly frequently with water vapor, resulting in snow (rather than hail which usually results from freezing liquid water) or frost.

It is possible to go from the solid state to a liquid/vapor state. What I mean by liquid/vapor state is that there is a certain point called the critical point at which liquid and vapor are indistinguishable.

The critical point you are interested in is the end of the liquid-vapor equilibrium curve on which under strictly defined circumstances the phases coexist without any flows. To do that you need to have very high pressure and then heat up the solid as shown in this graph: https://upload.wikimedia.org/wikipedia/commons/thumb/3/34/Phase-diag2.svg/530px-Phase-diag2.svg.png

• OK so a lot of pressure would be required to turn from the gas to a solid but either way there is always going to be a liquid state no matter how fast it can be transformed? – Nathaniel Barnhill Jan 24 '18 at 15:51
• No. Not always. after the critical point on the graph you can see an area called supercritical fluid. In this area weird stuff happen: you won't be able to tell the difference between water and vapor because the properties of water change towards those of vapor and vice-versa. For example, water is incompressible with high dielectric constant and low thermal expansion coefficient. After the critical point the water becomes compressable with high thermal expansion coefficient (like vapor). – Ognyan Petkov Jan 24 '18 at 17:29
• According to the phase diagram you provided, all you need to do is lower the pressure and temperature to get a gas-solid phase transition. Any pressure below the triple point (on that diagram) has a temperature at which sublimation/deposition will occur. – fyrepenguin Jan 24 '18 at 22:35

## protected by Qmechanic♦Jan 24 '18 at 21:25

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