A colloid is a heterogeneous mixture characterized by the dispersion of one substance, the dispersed phase, throughout another, the continuous phase. These mixtures possess particle sizes intermediate between those of true solutions and suspensions, typically ranging from 1 to 1000 nanometers. One key physical property of many colloidal systems is the Tyndall effect, which describes the scattering of light as it passes through the mixture, allowing the presence of suspended particles to be visually confirmed. Examples of colloids include milk, fog, and gelatin, and they are fundamentally important in physical chemistry for understanding stability, transport, and chemical interactions within various natural and man-made systems.