I am studying surface chemistry in high school I have this doubt and I am unable to find a answer your help is appreciated I searched for the answer on google I asked my chemistry teacher he said the question is beyond high school's curriculum and I even asked some of my friends but all in vain.

  • $\begingroup$ If you could edit the question such that the question includes how you tried to solve it, please do so. In its current state, the question is likely to be closed. $\endgroup$ – JavaScriptCoder Oct 25 '17 at 16:52
  • $\begingroup$ I have edited the question is it ok now $\endgroup$ – Shub Oct 25 '17 at 16:57
  • $\begingroup$ Who said that everything must have a physical state? $\endgroup$ – Ivan Neretin Oct 25 '17 at 17:11
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    $\begingroup$ The old standard characterization was solid, liquid, gas, then plasma was discovered, and so on. So there all sorts of add characterizations. A "gas adsorbed on a solid adsorbent" is redundant. A chemist would normally just say an adsorbed gas. If the gas is in a liquid then it is dissolved. If the gas is in a gas, then it is a mixture. So the only way a gas can be absorbed is onto a solid. There isn't any better way to characterize an adsorbed gas than that specific phrase. $\endgroup$ – MaxW Oct 25 '17 at 18:34
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    $\begingroup$ @IvanNeretin Every kind of particular matter is in some physical state. Otherwise it'd have to be in some unphysical state, which sounds unnatural to me. ;-) $\endgroup$ – Karl Oct 25 '17 at 21:39

Well, it usually depends on the temperature: if temperature is too low, molecules cannot move so freely so they are localized on top of the solid without much movement.

However, in general cases the adsorbed gas behaves like a liquid in two dimensions. Near the surface, the gas molecules are 'trapped' in the force field of the solid, so they don't escape so easily. Then the freedom of movement occurs only in two dimensions (the molecules travel on the surface).

I made some time ago a movie calculating (using molecular dynamics) movement nitrogen molecules adsorbed on a graphene surface at 77 K. This can give you a clear image of what happens on the surface. As you can see, the molecules can travel on top of the graphene in two dimensions (we can call it a 2D phase). This movie is the case when still a monolayer of gas does not complete. There can be cases with less molecules, say, less pressure of nitrogen in the gas phase; or more molecules.

The physical states can be observed more clearly in phase diagrams of the adsorbed surfaces. Typical examples shows a plot of the adsorbed phase transitions in a P vs. 1/T plot. This topic is extensively covered in the nice monograph on Physical Adsorption by Bruch, Cole and Zaremba, for example.

  • $\begingroup$ Is this movie an animation or a simulation? $\endgroup$ – AtmosphericPrisonEscape Oct 26 '17 at 13:53
  • $\begingroup$ It is a molecular dynamics simulation. Simulation was done in HOOMD and visualization with VMD. $\endgroup$ – Nando Oct 27 '17 at 15:19
  • $\begingroup$ Does the adsorbed gas behave like a 2d gas.(See Wikipedia for what's 2d gas) $\endgroup$ – Shub Oct 28 '17 at 11:39
  • $\begingroup$ @Shub Yes! This is correct! Actually, it can behave as a 2D gas (molecules move and diffuse a lot in the surface) or like a 2D solid (just localized molecules). It generally depends on the temperature. You can even find phase diagrams for these 2D/3D transitions. (I will edit the answer to reflect this). Thanks! $\endgroup$ – Nando Oct 29 '17 at 3:21

Since solid means ordered structure and when gas gets adsorbed orders itself on the surface (at least partially) and is not free in volume, I would consider it a solid.

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    $\begingroup$ Gas molecules adsorbed on a surface are typically highly mobile in the plane parallel to the surface. $\endgroup$ – Karl Oct 25 '17 at 21:36

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