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Chemistry / Re: Does fehling's solution oxidize alcohols? Why not?

« on: 23/02/2016 16:47:02 »

Fehling's solution does not oxidize alcohols because the copper complex does not have enough oxidizing power to activate alcohols (maybe easily oxidized alcohols like benzyl alcohol or p-methoxylbenzyl alcohol or phenol might give false positives).

My suspicion is that under the alkaline aqueous conditions of the test, the aldehyde gets hydrated to an acetal, which is much more easily oxidized than an alcohol because there are two oxygens that can stabilize the carbocationic intermediate (vs just one with an alcohol). Ketones do not react because there is no hydrogen atom connected to the carbon at the center of a ketal. Note that formic acid and formate esters (possibly also formamides) will give a positive result as well, because they do have an oxidizeable carbon-hydrogen bond on the carbonyl center.

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Chemistry / Re: Why does a rough surface encourage water to form droplets?

« on: 25/01/2016 05:42:59 »

The following may be involved:  The more synclastically curved a surface is, the lesser area of it is required to accumulate a given force per unit length of periphery because the lay of the surface deviates from flatness over a shorter distance. Therefore, if a sphere of liquid encounters a hydrophobic surface that is flat, that deforms the surface the same as does placing a basketball on the floor, resulting in tighter curvature in certain areas that repels it from the floor enough to stabilize it there. However, if you take the same basketball and place it on a surface having irregularities that are substantial but smaller than the diameter of the ball, and try to force the ball into the same amount of contact that it had with the flat surface, it will resist more vigorously because the curvature required to make it  conform to the surface is greater.  That suggests that hydrophobic liquids will tend to have less contact with a rough surface than a smooth one at any given pressure, because the surface tension resists deforming so as to match the roughness.

As for hydrophilic surfaces: Because they tend to attract the liquid, the liquid will tend to cover the surface. It is energetically favorable for the liquid to contact the entire surface rather than just the high points of it, which it will try to do. But there is also the issue of the surface tension on the exposed side of the liquid. The liquid will try to keep that surface flat.   The volume of the available region is determined by the thickness of the region of roughness. If the amount of liquid is less than this volume, it will not be able to assume a fully flat surface on the outside, and will develop a negative pressure (relative to the environment) because of the conflict, and if additional liquid  becomes available, it will be pulled into the system as the system tries to flatten its outside surface. Thus, the rough surface will tend to adsorb liquid until its roughness is filled.  On a flat (smooth) hydrophilic surface, of course there is no roughness and therefore no roughness volume, so there is no energy advantage for a thin layer of liquid more than several molecules thick to become thicker.

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Chemistry / Why are metals shiny?

« on: 12/02/2007 19:54:29 »

Strictly it isn't a chemical reason. No reaction takes place. Anyway If I remember this rightly from long ago when I studied this sort of thing, the electrons in a metal are relatively free to move about (which is why metals conduct heat and electricity well). That means they have a lot of possible energy states and can absorb incoming radiation unusually well. Paradoxically, this is why they reflect the light. The absorbtion is very strong and the wavefunction of the photon is forced to fall to zero very abruptly. This rapid change in wavefunction is not easy to produce and the only way for the system to cope is to absorb the photon, then re-emit it immediately as a reflection. This isn't a very clear explanation and I hope someone comes up with a better one but, in short, it's the loose electrons that do it.