In chemistry, a colloid is a heterogeneous dispersed system in which one substance (the dispersed phase) is distributed throughout another (the continuous phase) in the form of very small particles, typically ranging between 1 nanometer and 1 micrometer. At this scale, the particles are not visible under an optical microscope, yet they do not fully dissolve—they remain stably dispersed without easily settling out.

Colloids can involve various combinations of phases (solid, liquid, gas), resulting in systems such as aerosols (fog, smoke), emulsions (milk, mayonnaise), gels (gelatin), and sols (ink, paint). A classic example is milk, a colloidal mixture of fat droplets dispersed in water.

A key feature of colloids is the Tyndall effect—the scattering of light by colloidal particles, which makes a light beam visible as it passes through the mixture.

Colloids are thermodynamically unstable, but kinetically stable, meaning they don’t separate quickly over time. To preserve this stability, stabilizers or emulsifiers are often used, especially in food, cosmetics, and pharmaceutical products.

From a physical-chemical perspective, colloids are intermediate systems between true solutions and suspensions, exhibiting unique properties related to interfacial interactions and the high surface area of dispersed particles.