Gas to solid

[sueemc]

I know that water vapour can turn to ice on v. cold windows in winter (hence the lovely patterns), are there any other examples of gases changing directly to solids?

[JudgeJ]

Under the circumstances you mention the water vapour does not turn directly to solid water. I does pass through the liquid state, albeit quite quickly.

There are a very few substances which turn directly from solid to gas. From my schoolboy chemistry I recall Iodine and Arsenic among them. Also, solid Carbon Dioxide (dry ice) converts directly to gas upon heating. (The fact that it does not liquefy explains why it is �dry�), This process is known as sublimation.

Sublimation depends on the fact that the boiling point of the solid substance is lower than its melting point at atmospheric pressure. Thus by increasing pressure, a substance that sublimes can be made to go through a liquid stage before passing into the vapour state.
Some substances that do not sublime at atmospheric pressure can be made to do so at low pressures. This is the principle of freeze-drying, during which ice sublimes at low pressure.

[theprof]

Place a bag of frozen food inside a plastic bag and seal the plastic bag tightly before putting it in the freezer. After the pack has been frozen for some time, you will notice that ice crytals have formed on the inside surface of the plastic bag.
In this case, water vapour (gaseous phase) from the food package has evaporated and sublimed on to the plastic as ice (solid phase).

[theprof]

If you heat a small quantity of ammonium chloride in a test tube, the solid decomposes into a mixture of two gases, ammonia and hydrogen chloride. After a couple of minutes, remove the heat from under the test tube. As the solid cools, you will see the colourless gases condensing back into white crystals of ammonium chloride around the cooler rim of the test tube. This is the reversal of the heat decomposition and is another example of sublimation.

[Gef]

Despite whay JudgeJ writes water vapour can, and does sometimes, turn directly into ice crystals without going through the liquid phase. This leads to the lovely patterns that sueemc mentions and also the beautiful, fine crystals formed on twigs and leaves.

It is to do with the triple point of water.

The reversible chemical reaction mentioned by the prof has nothing to do with sublimation.

[theprof]

Gef

So you think that the condensed ammonium chloride in the reaction I cited was not an example of sublimation. Perhaps you would care to tell us precisely why you do not consider this to be so as the experiment has been demonstrated in countless school laboratories for generations as an example of sublimation.
I conducted a quick "show of hands" vote on this matter this morning in a lecture room of undergraduate students. It seems that the ammonium chloride sublimation comes in second place behind the familiar iodine sublimation invariably demonstrated in school science laboratories. What have they been demonstrating all these years?

[theprof]

Gef:
For further confirmation that your understanding of sublimation is flawed, I would suggest that you review the following webpage, which was recommended by my twelve year son:

www.sciencefair-projects.org/chemistry-projects/preparation-of-crystals-by-sublimation.html

Although the cited webpage is necessarily simplistic due to school age-range of the intended readers, it should suffice for you to reconsider the final sentence of your last post.

[Hamish]

Sorry theprof, but I agree with gef. Sublimation if defined as: sublimation

n 1: (chemistry) a change directly from the solid to the gaseous state without becoming liquid

In the example outlined the materials does not change directly into the gas phase, it decomposes thereby ceasing to be said material. It has all the attributes of sublimation, but isn't really sublimation, but a reversable reaction. Just because 'everyone thinks it is' doesn't make it so, and it really goes to show how little UG think.

[theprof]

Thanks Hamish. The definition you provided sounds like it has come from a conventional dictionary.

Here's a definition from the Academic Press Dictionary of Science and Technology:

"Thermodynamics: A phase-transition phenomenon in which a solid is transformed into a gas while bypassing the intermediate liquid phase; the term applies to the reverse process as well."

Continued:

[theprof]

Ammonium chloride does not change into the liquid phase during the reaction cited - it decomposes directly into the two gases. The fact that it decomposes as far as the term sublimation is concerned is immaterial - it still sublimes. This is the gist of the final sentence in the above definition. The fact that it is a reversible reaction in the sense that you imply does not mean that it cannot be sublimation. Chemicals reactions can be of more than one type.

With regard to you comment that a phenomenon may not be true despite everyone thinking it is so, you are mistaken. The ammonium chloride sublimation reaction has been demontrated in countless laboratories by thousands of scientists for centuries. It is not the opinion of one man. Perhaps you have other ideas as to what precisely my undergraduate students were telling me when I conducted a poll on the most familiar type of sublimation they had seen. These are students who attend my university from all over the UK and have obviously all had different chemistry teachers in the sixth-form at school or lecturers at FE colleges. Have all these teachers/lecturers been mistaken? Are all the authors of their physical chemistry textbooks and others before them mistaken?
Finally, although I possess a Master's degree in chemistry, I am a professor of Biochemistry not a professor of Physical Chemistry. However, I have had the opportunity to speak to two of the many Physical Chemistry professors at my university this morning. Both have told me that I am correct in my interpretation of this matter.

Science is a strict discipline and your final sentence is one that is generalistic and innapropriate to science. All science facts are established by experimentation, interpretation and repitition by many. They do not become established because everyone "thinks" they are true.

[Hamish]

Well here we have some confusion then:

Hawleys condenced chemical dictionary defines it as:

sublimation
The direct passage of a substance from solid to vapor without appearing in the intermediate (liquid) state. An example is solid carbon dioxide which vaporizes at room temperature; the conversion may also be from vapor to solid under appropriate conditions of temperature

Similarly many of the other referances refer to the change of phase of a substance, In NH4Cl there is no change of phase of the substance, it is a decomposition. Atkins defines it as A(s)---->A(g), however here we have AB(s)-----> A(g)+B(g).

With respect to common oppinion affecting how we look at science, for a long time people (scentists and laymen) thought a nuclear bomb would be a hugh device wich would have to be dilivered on a boat, that atoms were solid lumps and that nothing was smaller than the atom.

Ask your students if things always get more soluble as you heat them in a solvent. The dont - polymers tend to precipitate.

Ask is gold is an inert element - it isn't it forms a whole hoast of compounds and will catalise some reactions in the elemental form.

Ask if any the nobel gasses form compounds - all but He and Ne do.

You are right public/general opinion should not affect our perception of science, but it does. Ignoring this will only get us into trouble.

[theprof]

Thanks for that Hamish. Sorry I didn�t reply yesterday but is has been hectic in the department.

I am familiar with the definition of sublimation provided by Hawley, which in essence is the same as the one I quoted. Many other chemical dictionaries define it along the same lines including Rose and the CCD. Similar definitions may be obtained by those without access to Hawley by merely �Googling� . You will note that at no point have I tried to define sublimation otherwise. Therefore, I cannot see where confusion comes into it.

I freely acknowledge that NH4Cl decomposes at around 338-340 degrees C into hydrogen chloride and ammonia. My previous posts should confirm this. However, the decomposition and subsequent redeposition by recombination does not make the classification of the reaction as a sublimation untenable.

Peter and I have known one another for a number of years. While the Atkins definition you provide is correct, I puzzled for a few moments regarding your equation AB(s)� A(g) + B(g). Your equation seems to suggest that that sublimation of an inorganic compound is not possible. Yet, surely you are aware that there are a fair number of inorganic compounds that do exactly that. Without looking at any reference sources, I can think of all the ammonium halides, arsenic trioxide, molybdic oxide, zinc chloride etc. Confirmation that these compounds will sublime may be readily obtained via Gmelin on CrossFire and other CrossFire resources.. Hawley also does so along with innumerable other sources. I specifically chose the ammonium chloride sublimation reaction as this is a relatively well-known compound that the public may well recall having been shown subliming in school laboratories up and down the country; on thinking about it, I even remember doing the sublimation at home with my �Merit� chemistry set many years ago!

(continued)

[theprof]

(continued)

In respect of this, I note that you have not commented on my remarks regarding the readiness of schools for generations to show their pupils a typical sublimation using ammonium chloride. I recall having it shown to me with a classroom of others and I�m sure many other present day scientists and the general public will stand up and say the same. Were these teachers mistaken in calling it sublimation? If so, were my lectures, readers, professors and demontrators at university also incorrect in quoting the same reaction as an example of sublimation? Are the present academic staff in my university incorrect in doing the same? What has been going on all these years?

With regard to the three questions you suggested I ask my students, I was not surprised by the response received to your questions. Public school and state school pupils alike were almost all correct in their answers and my faith in current education standards has been partially restored.

I am familiar myself with smart polmers and LCST values � we use PNIPAm amongst others on the campus ourselves.

I used gold compounds myself in a mitochondrial thioredoxin reductase inhibition demonstration to DPhil students a few days ago. They had been aware for years that gold is not inert. The work of Tsang et al at Reading in nanotechnology is also a current subject of study and chrysotherapy is a subject which my staff have contributed to in many papers. There�s no fooling my current students.

With regard to the Noble gases, all the students were aware of the reactivity issues, which did not really surprise me.

[theprof]

Sorry about the equation. I'll try again. It should read:
AB(s)-----> A(g) + B(g)