colloid
/ KOL-oyd /
Look at a glass of milk. It is not clear like sugar water, yet it never settles into layers like muddy water left overnight. Milk sits in between: tiny droplets of fat and clusters of protein, far too small to see, drift permanently through the watery liquid. That in-between state — particles too big to dissolve but too small to settle — is a colloid.
More precisely, a colloid is a mixture in which one substance is dispersed as fine particles, roughly one nanometre to one micrometre across, throughout another substance, without dissolving into single molecules. The scattered particles are the dispersed phase; the medium they float in is the continuous phase. Because the particles are so small, the random battering of molecules around them (Brownian motion) keeps them suspended against gravity, so a good colloid stays mixed for a long time.
Colloids are everywhere: milk, fog, smoke, jelly, paint, blood, whipped cream, and clay in river water. They matter because so much of food, biology, and industry lives in this realm, where a vast amount of surface is exposed and surface forces rule. The caveat is that a colloid is a halfway house, not a permanent one — given a nudge it can break, with the particles either dissolving fully or clumping and settling out.
Fog is a colloid: microscopic water droplets dispersed through the air. They are too small to fall as rain, so they hang in place — which is exactly why fog drifts rather than pours down.
Particles too big to dissolve, too small to settle: that's a colloid.
A handy three-way scale by particle size: a solution (molecules, under ~1 nm, clear, never settles), a colloid (~1 nm to 1 µm, often cloudy, scatters light, stays suspended), and a suspension (over ~1 µm, like sand in water, settles out). Colloids are the cloudy, light-scattering middle.