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I want to understand what are the particles of a colloid in the microscopic scale. According to Wikipedia:

A colloid is a phase separated mixture in which one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance.

What I can't understand is how we can have a phase seperation when we have soluble particles? I looked for some pictures and I got more frustated. For example in the following picture Colloids: enter image description here

the particles seem to be like ordinary atoms/molecules just scaled up. So how can this particles exhibit bulk properties and classified as "solid" particles? Is the picture a simplification? I mean should colloid particles be represented as clusters forming a phase (e.g. solid) that is invisible to human eye?

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    $\begingroup$ Colloid particles could either be individual particles or clusters of particles where the cluster is still small enough to not precipitate. However the point here is that there should be no active "clustering". If there is active clustering then the clusters will get large enough to precipitate. $\endgroup$
    – MaxW
    Commented Mar 6, 2021 at 20:50
  • $\begingroup$ So the term colloid "particle" should refer to either a "solute" like particle (with just greater size) or to a small cluster of particles so that Brownian motion prevent its precipitation (phase seperation)? $\endgroup$
    – Anton
    Commented Mar 6, 2021 at 21:08
  • $\begingroup$ It can be one big protein molecule or thousand gold atoms or a million water molecules, whatever's kinda too big for "true" solution and too small for typical suspension. $\endgroup$
    – Mithoron
    Commented Mar 6, 2021 at 21:34
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    $\begingroup$ Op the cartoon just gives a qualitative comparison of size. Left the dots are ions or molecules, midlle the dots are small particles in a more everyday sense, eg small pieces of solids (they might differ from the same in bulk, at least they have a high surface / inner potential ratio) and right the dots are the same just bigger, or cluster of them. $\endgroup$
    – Alchimista
    Commented Mar 7, 2021 at 11:00
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    $\begingroup$ @Anton you can write a nice answer trying to merge all comments and answers into one. Also goldbook.iupac.org/terms/view/C01172 and colloidal fluids (emulsions). $\endgroup$
    – Alchimista
    Commented Mar 7, 2021 at 14:23

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Colloidal solutions may contain particles made of liquids or of solids. The particles are liquid in substances like mayonnaise or moisturizer, whose particles are droplets of oil (in water) for mayonnaise or droplets of water (in oil) for moisturizer. They can be solid in substances like acrylic paints, where the particles are made of semi-solid paint dispersed in water. The great advantage is that these paints can be diluted with water. Fresh spots are easily removed. Colloidal gold does exist. The colloidal particles can even be gas in whipped cream, froth and foams.

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  • $\begingroup$ Thank you for your answer. Because also the comments cleared up some of my confusion I will come up with an answer including your's and the comments above. $\endgroup$
    – Anton
    Commented Mar 7, 2021 at 18:22

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