Science Buddies: "Ask an Expert"

Dear Chemist,

I am doing an independent study for science class and I am required to have a live expert for one of my sources. I would like to ask you some questions. Why do some chemicals sublimate instead of going through a liquid stage? How and why are evaporation and boiling different? Why do different chemicals have different melting, freezing, and boiling points? Are there any experiments I could do to show the answer to this? Thank you.

Sincerely,
Eli

Hi,

You are asking some very deep questions that are going to take a lot of study to understand very well.

The simple essence of an answer lies in how much the molecules of the substance want to stick to each other. If there is no attraction between them the material will only occur as a gas. If they stick together tightly the temperature must be raised quite high to get them to separate. The liquid stage is really an intermediate stage where the molecules are trying to stick together but have too much kinetic energy to lock in fixed orientations to each other.
Here is a page discussing sublimation vs. melting:

http://www.av8n.com/physics/melt-sublimate.htm

http://en.wikipedia.org/wiki/Sublimatio ... ransition)

Regarding the distinction between evaporation and boiling: If you have a gas phase coexisting with a solid or liquid phase, molecules of the gas are collideing and s sometimes sticking to the liquid or solid, and sometimes molecules are breaking off the liquid or solid and entering the gas phase. Again how fast each of these processes is depends on how much the molecules want to stick to each other. If the rate of molecules escaping the liquid or solid and entering the gas phase exceeds the rate of the gas phase molecules sticking to the gas or liquid, the material is said to be evaporating (or alternatively sublimeing in the case of a solid). If the temperature is so high that at equilibrium no liquid exists, the liquid boils.
Here is another explanation:

http://www.learner.org/courses/essentia ... oser2.html

To go deeper in understanding all this you need to investigate:

1) What are the forces attracting molecules together?
http://www.elmhurst.edu/~chm/vchembook/ ... molec.html

2) The ideas of Thermodynamics:
http://en.wikipedia.org/wiki/Thermodynamics

3) The basics of phase diagrams:
http://www.eng.usf.edu/~campbell/Thermo ... /tut1.html

http://en.wikipedia.org/wiki/Phase_diagram

http://www.soton.ac.uk/~pasr1/

As far as experiments go, there are lots and lots of great demonstrations, but most of them require more skill than students ar your level are expected to have. You might want to explore colligate properties (the raising of boiling points, or lowering of freezing points in solutions) - here is one Science Buddies experiment:

https://www.sciencebuddies.org/science- ... p049.shtml

You could explore the differences between evaporation and boiling of water by carefully observing what happens when you heat water on the stove. You might be able to observe sublimation by warming up iodine crystals. You can grow crystals from melts or from solutions, see this Science Buddies experiment:

https://www.sciencebuddies.org/science- ... p082.shtml

You can find some mixtures of compounds that form two liquid phases at lower temperatures and one phase at slightly higher temperatures, and a large number of compounds that dissolve in water at higher temperatures, but crystalize out of solution at lower temperatures. You can see solid dry ice sublime to gaseous carbon dioxide, or get a Carbon dioxide liquid fire extinguisher and make solid dry ice and gaseous carbon dioxide by releasing the pressure.

I hope this helps.

Best regards,

Barrett L Tomlinson

In a solid, the molecules are held together by intermolecular forces like Van der Waals forces, dipole moments, ionic bonding, and hydrogen bonding. Melting basically occurs as molecules pull farther away from each other. This can be caused by an increased temperature, which increases the kinetic energy of the molecules. In a gas, the molecules are even farther apart.

Different substances have different melting and boiling points because of the different strengths of the bonds between the molecules. For example, an ionic bond (between positive and negative ions) have some of the strongest bonds, and so they have high melting points--NaCl (Na+ and Cl-) has a melting point of 800 degrees Celsius.

I think, but I am not exactly sure, that the distinction between evaporation and boiling depends on how the substance naturally exists. Evaporation refers more to how molecules in a liquid (especially those close to the surface) naturally escape into the gas phase; while boiling refers more to how increased temperature causes the molecules to get excited, break their intermolecular forces with each other, becomes a vapor.