Diffusion

[brownpanda]

Hi, I'm a Secondary School student, which would be the American equivalent of a High School student, I think. I have a question about diffusion that's been bugging me for a long time:

Why doesn't diffusion occur with everything? Why don't water and oil mix if you leave it to stand? Theoretically it should, right? If it comes to that, why doesn't everything mix up to become one giant homeogenous mixture?

[Inuyasha]

Oil and water don't mix because of many reasons but the main reason is because of density. Since the lighter will go onto the top and the heavier will sink that is the main reason.

[Solid Snake]

things settle as well as diffuse, due to weight just as Inuyasha said. Think about peanut butter thats left around to long.

[Cyranian]

Diffusion is the sudden, slow, and steady spreading of things like particles, heat, or momentum. This process is readily observed when a drop of colored water is added to clear water, or when smoke from a bbq or stove dispels itself into thin air. In terms of dissolved substances, this phenomenon moves the liquid from a higher concentration to a lower concentration. Like Snake said above, heavier things settle themselves on the bottom while the lighter sits on top. That's why they don't just mix into one big pile of stuff.

[biostudent84]

Oil and water don't mix because of many reasons but the main reason is because of density. Since the lighter will go onto the top and the heavier will sink that is the main reason.

Density is the reason that oil will be the topmost liquid in a water column. But the reason the two will not mix is that oil, a non-polar chemical, is insoluble in water, a very polar chemical.

The rate of diffusion is dependant mostly on the size of the molecules and the ambient heat. Smaller molecules diffuse more quickly. Warmer molecules will diffuse more rapidly as well.

[brownpanda]

Thanks all for the replies.

From what I gather from your replies, things have to be soluble in each other to diffuse?

Actually come to think of it, solubility is not as simple as it seems. From what I gather, solubility is the ability of something to dissolve. You can tell when something is soluble when it becomes part of a solution or a liquid. But how do you tell for gases?

How about the diffusion of solids?

P.S.: Come to think of it, solubilty and diffusion do seem to be very much related...When something is soluble in something else, it diffuses through it as well, and vice versa. I think (therefore I am, haha)?

[biostudent84]

Yes. Difusion is essentially the rate of dissolving, and solubility is the ability to dissolve.

I have never heard of two gasses being insoluble in each other...although different gasses with different higher atomic weights will settle slightly. The closer to the ground you are, the more oxygen there is. Molecular Oxygen weighs 32 AMU's, while Molecular Nitrogen only weights 28. While the difference is not much, there will be more oxygen at ground level.

Kyle

[brownpanda]

Ok, got it! Thanks!

[Premed]

oil and water don't mix not just because their densities are different but because the former is a non polar (hydrophobic) substance, this explains why oil aggregate together in water, and the later is a polar solvent.. Just remember like dissolves like...

[biostudent84]

Yes, please read my post above. This has already been explained.

[thank.darwin]

Hi, I'm a Secondary School student, which would be the American equivalent of a High School student, I think. I have a question about diffusion that's been bugging me for a long time:

Why doesn't diffusion occur with everything? Why don't water and oil mix if you leave it to stand? Theoretically it should, right? If it comes to that, why doesn't everything mix up to become one giant homeogenous mixture?

The reason why water will not mix with oil is because oil is a lipid and is hydrophobic(water hating)and water is held togetherwith polar covalent bonds(the Oxygen shares unevenly with the 2 hydrogens- you could call the oxygen an electron hog) So the oxygen has the electron more often and it becomes slightly negitive and the hydrogen has the electron less of the time so it becomes slightly positive. Water likes things with charges and oil doesn't have a charge so oil and water don't mix. - hoped that helped
- Thank.darwin

[Gerard]

I have never heard of two gasses being insoluble in each other...

I didn't until recently
The mixture Helium-Xenon forms two phases at 333.15 K (60 degrees celcius) and 0.3 GPa (2000 bar ! ). Both phases then contain He and Xe in different amounts.

see:

Jean-Louis Barrat and Willem L. Vos, Stability of van der Waals compounds and investigation of the intermolecular potential in helium-xenon mixtures http://www.tn.utwente.nl/cops/pdf/wvos/vdwhexe.pdf