Answer 3:
The process you are describing is called
sublimation, in which a substance passes from the solid to the gas state without first becoming a liquid. Actually, there are a large number of materials that will sublime; it is just a question of the temperature and pressure at which this will occur.
For example, water will sublimate if you
lower the pressure and temperature enough. This
phenomenon is often used to freeze-dry things.
Iodine will sublime at room temperature if you
lower the pressure just a bit.
If you want to be more quantitative about it, you can look at the triple point of a material.
On a pressure-temperature phase diagram, this is one pressure and temperature point. Below either the triple point temperature OR the triple point pressure, the material is capable of sublimation.
For example, the triple point of mercury, found in
older thermometers, is roughly -39 o C and a
pressure of 0.2 mPa, so, if you have solid mercury
at -50 o C and 100 mPa, it should sublimate.
Alternatively, if it were at 10 o C and 0.1 mPa,
it should also sublimate. |