[cubejunkies]

Question number one:
Predict which of the following liquids has greater surface tension: ethanol (C₂H₅OH) or dimethyl ether (CH₃OCH₃)

I said dimethyl ether, because the compound is symmetric, so in 3d space, the O atom in between two methyl groups could more easily form hydrogen bonds with other dimethyl ether species. I don't know if this one is right because it's an odd so the answer isn't in the back of my book.

Question number two:
Classify the solid state of the following substances as ionic crystals, covalent crystals, or mettallic crystals:
a. CO₂ b. B₁₂ c. S₈ d. KBr e. Mg f. SiO₂ g. LiCl h. Cr

a. Molecular b. Mettallic c. Molecular d. Ionic e. Molecular f. Covalent g. Ionic h. Molecular

Are these right? My biggest concerns are b and c. For E, I know that Mg is a mettallic element that does not form mettallic solids, however, what other type of solid would it form? Those are the few that I'm not feeling so hot on. Otherwise, I'm fairly certain I'm correct for ones like g where the species is obviously ionic. But for the rest, who knows, I could get them all wrong for all I know

Question number 3:
"A student hangs wet clothes outdoors on a winter day when the temperature is -15 degrees C. After a few hours, the clothes are found to be fairly dry. Describe the phase changes in the drying process"

My best guess is that the water in the clothes condenses in the cold air and drips off, but too me this doesnt seem likely, or that there is something deeper going on I suppose.

Thanks!

Anthony

[rabolisk]

For 1, ether does not hydrogen bond with each other. 2 looks correct to me. 3 is tough call.

[cth]

Question 1)
Rabolisk is correct, dimethyl ether molecules can't do hydrogen bonding with each others because they don't have hydrogen atoms attached to electronegative atoms.


Question 2)
I think b), e) and h) are incorrect.

B₁₂ has a bandgap of 2eV (see http://en.wikipedia.org/wiki/Boron), so it isn't a metal.
And it would simply be called B or B_metal if it was metallic: The fact that there is a number '12' indicates a discrete molecular structure, not a metal.

Why do you think Mg and Cr can't be metallic solids?  Remark: these are simply named Mg and Cr without numbers. You never find anything like Mg₁₂, Cr₁₀ or whatever number... This indicates extended structures, by opposition to molecular structures.


Question 3)
It makes me think about freeze-drying process http://en.wikipedia.org/wiki/Freeze_drying 

[cubejunkies]

Wait for question 2, Cr would be mettallic? I know mg cant because we have a chart that lists mg among Na and a few others to be mettallic elements that are exceptions which do not form mettallic solids

[Vidya]

Cr and Mg both are metals

[cth]

Wait for question 2, Cr would be mettallic? I know mg cant because we have a chart that lists mg among Na and a few others to be mettallic elements that are exceptions which do not form mettallic solids

 

Magnesium, sodium and chromium are definitely metals. You must be somehow confused when reading your chart.
Throwing a little piece of metallic sodium into water and seeing it fizzling is something most chemist have done 

Perhaps your chart makes a difference between alkaline, alkaline-earth and transition metals. But still they are all metals.

[tamim83]

I did some searching and apparently there is an "intermediate" between metallic and network covalent bonding in some metals (see http://en.wikipedia.org/wiki/Bonding_in_solids).  I don't think I'd throw this sort of thing at high schoolers, but it does exist.  This accounts for some of the very "brittle" metals like tungsten and probably magnesium as well. 

[vmelkon]

For question 3, it depends. I have placed wet cloths in the cold and it has frozen solid.
So it would be a matter of how much water is in the fibers and the humidity of the air.

The question is intended to make you think and use your imagination.