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I've been teaching chemistry for 25 years, but now I'm confused. Is a colloidal dispersion a heterogeneous or homogeneous mixture? I've seen both answers on the web.

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    $\begingroup$ +1 for the "teaching chemistry for 25 years and am still learning it" - one of the best things about chemistry $\endgroup$ – Sean Doris Feb 26 '16 at 6:32
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Words like "homogeneous" and "heterogeneous" do not have a well-defined meaning. Different authors use them differently. The fact is, all matter is made of atoms, and atoms are not homogeneous. Thus, from a certain point of view, nothing is homogeneous. Matter is lumpy!

But suppose we don't care about things on the scale of atoms, i.e. angstroms. Then the question is "what scale do you care about?" Nanometers? Hundreds of nanometers? Chemists are making materials that have lumpiness on more more less all of these scales.

Back before folks invented centrifuges, filtration, and microscopes, bacterial cultures, with cells as small as 1 μm in diameter, probably seemed homogeneous. But van Leeuwenhoek peered through a microscope and dispelled that notion.

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A colloid is a homogeneous substance, consisting of large molecules or ultramicroscopic particles of one substance dispersed through a second substance. Colloids are classified as homogenous mixtures because the particles (generally of sizes $1-1000\:\mathrm{nm}$) do not settle out and cannot be separated out by ordinary filtering or centrifuging like those in a suspension, though the particle sizes are still greater than those in a true solution.

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    $\begingroup$ "particles do not settle out and cannot be separated out by ordinary filtering or centrifuging" that is factually incorrect. Many unstable systems that considered textbook examples for colloids, yet can be separated. $\endgroup$ – Greg Feb 26 '16 at 5:32

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