Thread: Oil floats over water

Hi,

I just am unanswerable to the question that why oil floats over water. I know it has it's individual properties that does it.

But I require more info on it.. Any help will be thankful

because the density of oil and water is different...

Water is denser than oil. It's molecules are segregated by more spaces than that of water. It's due to the buoyant force which makes the oil float.

To add you might want to look up electronegativity and polarity as to why oil and water don't mix.

oil is a very complex molecule i assume.
What is the molecular formula for oil.

isn't the volume of oil less compared to water(at the same temperature)?
If what you are saying is true it would be possible for oil to float in water(not on the surface).

Well you can find the answer using the density equation - d=m/v. If you test that on the oil and then on the water you will find that water is more dense... and denser substances sink.

Well, it floats on water because it is less dense, but there are plenty of other less dense substances that don't float on water. Oil is nonpolar and is not soluble in water. If oil was polar, it wouldn't float on top but it would dissolve. Someone please correct me if I am wrong.

Originally Posted by Keith

but there are plenty of other less dense substances that don't float on water.

Can you give me an example?

Lithium ions that are dissolved that dissociate from LiCl for example. They dissolve into water.

Yeah, but that's solvation, something completely different.

Many things dissolve in water regardless of their densities. Oxygen dissolves in water, somewhat quickly, but it is obviously less dense that water. Lead is very dense and sinks in water, it also dissolves in it, but just takes much longer.

I can't think of any less dense substance sinking in a more dense one. Since there is more mass per volume of something, it will be pulled down more by gravity than something with less mass.

Originally Posted by Keith

Lithium ions that are dissolved that dissociate from LiCl for example. They dissolve into water.

Because they are reactive they have proprer electronegetivity and ionization energy which is required for the same and which substances do not react forms mixture. They can be either homogenous just as that formed after reraction and they can be heterogenous as discussed above.
Oil and water form mixture.

OK, no one has pointed out the obvious...OIL IS NOT A COMPOUND! It is more accurately the description of a viscous, water insoluable compound. Most if not all oils are less dense than water and therefore, float. It has absolutely nothing to do with polariy, electronegativity, or solubility. Only density. It was a very simple question, let's stick to the basics.

Originally Posted by Dantak

OK, no one has pointed out the obvious...OIL IS NOT A COMPOUND! It is more accurately the description of a viscous, water insoluable compound. Most if not all oils are less dense than water and therefore, float. It has absolutely nothing to do with polariy, electronegativity, or solubility. Only density. It was a very simple question, let's stick to the basics.

I thought it was a compound, a hydrocarbon as it is cracked to give petrol and other hydrocarbons

Nope. You look up oil in any chemical suppliers catalog and you will not find it. There is motor oil, vegtable oil, linseed oil. There are all kinds of oils. If you run into a chemist who is doing an experiment and you say "hey throw some oil in there", he will look at you like your crazy.

the reason an oil floats on water is because it is less dense than water therefore filters to the top. The reason it does not mix with the water is that oil is usually made of glycerine with three fatty acid chains ester linked on. This molecule is made mostly of carbon hydrogen and oxygen but there is no polar groups so creating a molecule devoid of any polar intermolecular forces but due to its high mass has a large Van der Waals attraction. It does not mx with the water as H2O has a very high polarity and very low Van der Waal attraction. Therefore the oil and water mol do not pull each other about and mix together as they have no polar attraction and insufficient Van der Waals to overcome the higher attraction they have for their own kind. That why even if you mix it up the oil will remain in large droplets and will join back together after a while.

Oil isnt just one thing, there are many types of oils just like there are many types of alcohols, glycols, ketones. The oil you're talking about floats because it is insoluble in water. And since they dont mix they separate and the densest always gets to be on bottom. DCM is an example of when water would be on top. Water has a specific gravity of 1. When relating to water any liquid with a specific gravity less than 1.0 goes to the top, liquids with specific gravity of more than 1.0 go to the bottom

the reason an oil floats on water is because it is less dense than water therefore filters to the top. The reason it does not mix with the water is that oil is usually made of glycerine with three fatty acid chains ester linked on. This molecule is made mostly of carbon hydrogen and oxygen but there is no polar groups so creating a molecule devoid of any polar intermolecular forces but due to its high mass has a large Van der Waals attraction. It does not mx with the water as H2O has a very high polarity and very low Van der Waal attraction. Therefore the oil and water mol do not pull each other about and mix together as they have no polar attraction and insufficient Van der Waals to overcome the higher attraction they have for their own kind. That why even if you mix it up the oil will remain in large droplets and will join back together after a while.

To add to that I believe water molecules will often form induced (i.e non-permanent) hydrogen bonds between molecules dependant on the actual kinetic energy of the molecules concerned. Given that hydrocarbons - that is alkanes - have no polarity at all they can only form van der waals induced bonds with water molecules which simply isn't a strong enough force to overcome any H-bonding present. Saying that though oil is to a very small degree dissolved in water although these are very temporary and last as long as it takes for another water molecule to get in the way.

On a slightly related note an interesting casestudy is that of the phospholipd molecule which makes up the majority of the cell membrane in cells. This is (basically) a fatty acid with the 3rd functional group being taken up by a phosphate group instead of a hydrocarbon. This has the interesting effect of it being hydrophobic at one end and hydrophilic at the other! Depending on how you placed this in water it could do some pretty interesting things!

[quote="Dave Singleton"]

This has the interesting effect of it being hydrophobic at one end and hydrophilic at the other! Depending on how you placed this in water it could do some pretty interesting things!

Would it not have the same effects and characteristics of a soap as its sodium head is hydrophillic with a hydrophobic carbon chain.

Riddle: where (near your home) can you make oil non floating on water?
(No hints for the moment).

Would it not have the same effects and characteristics of a soap as its sodium head is hydrophillic with a hydrophobic carbon chain.

I think it could... as far as I can see it would depend on the placement on the phospohlipid and the water - if you were to (theoretically) put water in a glass and then place the phospholipids in I expect it would behave somewhat like an alkane although I'm not sure of it's density? If you lined the glass with the phospohlipid then put in the water you maybe able to actually construct a cell-like membrane... However, I'm completely lost as to what would happen if you placed the phospohlipid's at the bottom then added water! I expect that this may depend on actual densities and then maybe little rising droplets would form?? It maybe a very interesting experiment.