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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?
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For more on marking an answer as the "Best Answer", please visit our FAQ.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.
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.
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.