A true solution is a homogeneous solution in which the solute particles have diameters between 0.1 nm to 1 nm i.e., the solute particles are of molecular dimensions. Such dispersed particles dissolve in solution to form a homogenous system. These do not settle down when the solution is left standing. The particles are invisible even under powerful microscopes and cannot be separated through filter paper, parchment paper or animal membranes. For example, sodium chloride in water is a true solution.
Colloidal solution or colloidal state or colloidal dispersion, represent an intermediate kind of a mixture between true solution and suspension. The size of a colloidal particle lies roughly between 1-100 nm. Colloids are also a two-phase heterogeneous system consisting of the dispersed phase and dispersion medium. However, colloidal particles present in small amount as the dispersed phase component behave like a solute in a solution when suspended in a solvent phase or dispersing medium, because of their small size.