Are colloids homogeneous or heterogeneous?

[GotGoose]

Hi, I'm in chemistry and one of the questions is whether fog is homogeneous or heterogeneous. I've searched and found answers like, "it is homogeneous to the naked eye, but when you look deep down it is really heterogeneous" (paraphrase). I've also found descriptions of a colloid as being "between heterogeneous and homogeneous" (paraphrase)

I'd really like to understand this so please don't burst out an answer.

And yes, I did look through my text book.

 

[CycloWizard]

It depends on how you want to look at it. If you're concerned about very small length scales (say, the distance between two particles in solution) and the solution is concentrated, then it's heterogeneous. If you're looking at the length scale between two particles in a dilute solution, it may be treated as a homogeneous solution for some purposes. If you're concerned with the length scale of a flow (e.g. the diameter of a pipe), the solution is dilute, and the particle diameter is small compared to this scale, then you can treat it as homogeneous in some cases. If it's concentrated and/or the length scale of the particle approaches the flow length scale, you'll have to treat it as heterogeneous.

Of course, all of the above assumes a uniformly dispersed colloid. If you have concentration gradients, then it becomes much more difficult to say it's homogeneous. In the end, it all depends on your system and what you're trying to find out about your system.

 

[jagec]

Hmm, well, there's basically a continuum of how well-mixed something can be. A colloid is something that's in the "grey area" between something obviously homogeneous and obviously heterogeneous.

So, take a fruit salad. Obviously heterogeneous. Put it in a hyper-speed blender that will rip everything into its component molecules and mix them together. Homogeneous. Somewhere in the middle there is a state where there are small "clumps" of dissimilar materials, but they're so small and so well distributed that it's hard to call it heterogeneous. A fruit smoothie, maybe.

That's a conceptual answer, not a textbook answer, so come up with something better for the test

 

[MyK Von DyK]

Colloids...what ARE they?
What are Mixtures and Solutions?
Mixtures and Pure Substances
Colloids

Hope this helps, Best, MyK.

PS: I remember we did some experiments with colloids in our chemistry class back in my high-school times which is quite some time ago, so I don't remember much. One thing does linger in my mind though - light dispersion. Don't know if it's general characteristic of liquid colloids though, might be bollocks.

 

[CycloWizard]

Originally posted by: MyK Von DyK
PS: I remember we did some experiments with colloids in our chemistry class back in my high-school times which is quite some time ago, so I don't remember much. One thing does linger in my mind though - light dispersion. Don't know if it's general characteristic of liquid colloids though, might be bollocks.

Light scattering is generally caused by two things: particle size and discontinuties in refractive index. Any particle whose size is of the right magnitude to interfere with a light wavefront will scatter said light wavelength. A discontinuity in refractive index in colloids means that the refractive index of the particle is significantly different than its surrounding medium. Thus, a colloid may or may not scatter light depending on the kind of light (i.e. wavelength, depending on the particle size) and the refractive index of the particle relative to its surrounding medium.

 

[GotGoose]

Thanks for the help.

 

[Paperdoc]

CycloWizard's comments on the scale of things is a good view. From a chemist's perspective, though, an important factor is the composition of the "particles" dispersed in the "medium".

In a true solution, each solute "particle" is a single molecule, and the "medium" is the dominant component we usually call the solvent. So a simple solution has single solute melocules surrounded by many solvent individual melecules. This does NOT preclude some associations between individual molecules - sometimes a few solute molecules might stick together weakly, but they can be broken apart by small disruptions. Or very commonly, a solute molecule may be associated with several solvent molecules, but the attaching forces in these groupings are much smaller than those between atoms in a molecule, or between melecules in a crystal. For example, Al(OH)3 normally exists in a water solution with three water (H2O) molecules attached to it, too.

In a colloidal dispersion, the dispersed "particle" is MANY MANY molecules bonded tightly together - almost as tightly as in the original material from which they came. It's just that the original mass of material has been broken up into VERY tiny particles. Milk, fog, and whipped cream are good examples. Milk has both fat and protein particles suspended in water, along with some truly disolved solutes like trace metals. And note that milk, as it comes from the cow, will separate if undisturbed. The fat portion will float to the top as "cream", and eventually some of the proteins will sink to the bottom. But most milk we see has been homogenized. That is, the suspended particles have been modified to make them repel each other so strongly they refuse to collect in one area, and the colloidal dispersion is said to be stabilized. Fog is water droplets (each containing thousands (billions? more?) of water molecules, with the droplets suspended in air as the "medium". Whipped cream is air droplets as the colloidal material, themselves suspended in a colloid medium called cream (see milk above).

Regarding light dispersion, light will be scattered off the interface surface between any to media with different refractive indices. And since EVERYTHING has its own refractive index, there's always a difference IF the particles in contact are large compared to the light wavelength. For a true solution the solute individual molecule is MUCH smaller that the wavelength of visible light, so the light simply does not interact with it as a particle with a different refractive index. (There are other types of interaction - for example, abosrption of visible light by dye molecules - but that's another story.) But for a MANY-molecule colloidal particle, that particle is large compared to the light wavelength and the light "sees" an object very different from its surroundings; hence it is scattered at the surface. Thus, one old rule of thumb is that light will pass easily through a true solution, but will be scattered by a colloidal dispersion.