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Topic: Hexane dissolving in substance X  (Read 6019 times)

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Offline Rutherford

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Hexane dissolving in substance X
« on: April 12, 2013, 12:53:32 PM »
In a test tube, substance X and water were added. The test tube was shaken. Two layers were formed (the upper one was water). As hexane was slowly added, it formed a third layer above water. The test tube was shaken again forming two layers (the upper one was water). What substance could be substance X:
a)CCl4;
b)oil?

There were more options but these two I can't choose of. Why is is a), but not b) as both are non-polar and should dissolve hexane?

Offline Dan

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Re: Hexane dissolving in substance X
« Reply #1 on: April 12, 2013, 01:18:38 PM »
Two layers were formed (the upper one was water).

If you mix oil and water, which layer is on top?
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Offline Rutherford

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Re: Hexane dissolving in substance X
« Reply #2 on: April 12, 2013, 01:31:42 PM »
I think I have to say oil  :-\. So, is there a list of common organic solvents where it is said whether they have a smaller or bigger density than water? I know only for ether and CCl4 that their density is lower than that of water.

Offline Big-Daddy

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Re: Hexane dissolving in substance X
« Reply #3 on: April 12, 2013, 01:47:44 PM »
I think I have to say oil  :-\. So, is there a list of common organic solvents where it is said whether they have a smaller or bigger density than water? I know only for ether and CCl4 that their density is lower than that of water.

CCl4 has density higher than water ... which is the precise point of this problem. I also did not know this before but I knew oil was less dense than water which means that oil would have been on top.

Hexane would dissolve in both CCl4 and oil, but it can't, because it can't reach substance X - there is a water layer in between, and as we know hexane does not dissolve in water. But that's not even relevant - the point is that water is less dense, and hence on top, of X. So it can't be oil. So it must be CCl4.

Mind saying what the other options were originally?

Offline Rutherford

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Re: Hexane dissolving in substance X
« Reply #4 on: April 12, 2013, 02:10:43 PM »
C2H5OH and CH3COOH, but those would mix with water.
All in all, when mixed with water:
CCl4-bigger density than water (bottom layer);
(C2H5)2O-lower density than water (top layer);
oil-lower density than water (top layer);

If instead of oil, ether was written, it would be the same. Thanks for the help.

Offline curiouscat

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Re: Hexane dissolving in substance X
« Reply #5 on: April 12, 2013, 02:38:42 PM »
As a tip I think most Chlorinated solvents are high density. But you can check a few to verify.

Offline Big-Daddy

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Re: Hexane dissolving in substance X
« Reply #6 on: April 12, 2013, 02:39:26 PM »
C2H5OH and CH3COOH, but those would mix with water.
All in all, when mixed with water:
CCl4-bigger density than water (bottom layer);
(C2H5)2O-lower density than water (top layer);
oil-lower density than water (top layer);

If instead of oil, ether was written, it would be the same. Thanks for the help.

:)

Shouldn't ethoxyethane (what you are calling ether) be polar and thus dissolve in the water anyway? It should have a dipole moment ... then if it were ether you wouldn't get layers (as with anything containing -C=O or -OH groups).

Offline Big-Daddy

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Re: Hexane dissolving in substance X
« Reply #7 on: April 12, 2013, 02:43:54 PM »
As a tip I think most Chlorinated solvents are high density. But you can check a few to verify.

I've checked just a couple and there are some trends I'm seeing (all alkanes though ...): yeah they all appear to have higher than 1 gcm-3 density, density is on the whole greater when you have more of the halogen atoms (there were a couple of exceptions though), the density contribution of iodine appears to be greater than that of bromine, which is greater than that of chlorine, etc.

Anything you know of to suggest these don't work?

Offline curiouscat

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Re: Hexane dissolving in substance X
« Reply #8 on: April 12, 2013, 02:48:50 PM »
Shouldn't ethoxyethane (what you are calling ether) be polar and thus dissolve in the water anyway? It should have a dipole moment ... then if it were ether you wouldn't get layers (as with anything containing -C=O or -OH groups).

Nope. You are wrong (I think). 

At certain  ratios diethylether and water will form two layers. They are not miscible in all proportions.

So far as I recall ether does not have more than 10% solubility in water. I could be wrong.

Offline curiouscat

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Re: Hexane dissolving in substance X
« Reply #9 on: April 12, 2013, 02:49:58 PM »

Anything you know of to suggest these don't work?

I didn't understand your question. To suggest what won't work?

Offline Big-Daddy

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Re: Hexane dissolving in substance X
« Reply #10 on: April 12, 2013, 02:55:00 PM »
Shouldn't ethoxyethane (what you are calling ether) be polar and thus dissolve in the water anyway? It should have a dipole moment ... then if it were ether you wouldn't get layers (as with anything containing -C=O or -OH groups).

Nope. You are wrong (I think). 

At certain  ratios diethylether and water will form two layers. They are not miscible in all proportions.

So far as I recall ether does not have more than 10% solubility in water. I could be wrong.

Yeah I found this conclusion reached online a lot - so you're not wrong - but why? Ethoxyethane should have a dipole moment, it's shape is not too different from water's itself (which has a large dipole moment) - why doesn't it dissolve?


I didn't understand your question. To suggest what won't work?

Well I obtained these principles on the basis of pretty rapid-fire googling, so I was wondering if you could confirm their accuracy or give some reason/experience suggesting they aren't reliable?

Offline curiouscat

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Re: Hexane dissolving in substance X
« Reply #11 on: April 12, 2013, 03:03:52 PM »

So far as I recall ether does not have more than 10% solubility in water. I could be wrong.
Yeah I found this conclusion reached online a lot - so you're not wrong - but why? Ethoxyethane should have a dipole moment, it's shape is not too different from water's itself (which has a large dipole moment) - why doesn't it dissolve?

It does dissolve. Just not at all proportions.

I can't give you a phenomenological explanation. My Chemistry isn't good enough.  :)

Quote
Well I obtained these principles on the basis of pretty rapid-fire googling, so I was wondering if you could confirm their accuracy or give some reason/experience suggesting they aren't reliable?

I think they are good as thumb rules or heuristics. Just don't treat them at par with Newtons Laws.

If you hunt hard enough you can probably find exceptions. 
« Last Edit: April 12, 2013, 03:16:55 PM by curiouscat »

Offline Rutherford

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Re: Hexane dissolving in substance X
« Reply #12 on: April 12, 2013, 03:14:35 PM »
Ether has a dipole moment, but maybe it is not much soluble because it can afford only one oxygen atom for hydrogen bonding. No hydrogen atoms (still they would be a little positive, but the interactions wouldn't be so strong).

Offline Big-Daddy

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Re: Hexane dissolving in substance X
« Reply #13 on: April 12, 2013, 03:30:49 PM »
Ether has a dipole moment, but maybe it is not much soluble because it can afford only one oxygen atom for hydrogen bonding. No hydrogen atoms (still they would be a little positive, but the interactions wouldn't be so strong).

That's an OK basic reason - the C-O-C won't produce as big of a dipole as -OH (obviously because C is much more electronegative than H). But I think C-O-C should be comparable to -C=O, and we know aldehydes and ketones of small sizes do dissolve. I would hypothesize a rough order is carboxylic acid > alcohol > ether ≈ aldehyde ≈ ketone > ester > non-polar substances. But Wikipedia says ethoxyethane dissolves 69 gdm-3 which is not that bad ...

I will just accept that we get layers.

Offline Rutherford

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Re: Hexane dissolving in substance X
« Reply #14 on: April 13, 2013, 02:58:26 AM »
I don't think that's too high. When you convert it to mass share, it is 6.45%.

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