THE COLLOIDAL STATE:
While studying the diffusion of solutions through an animal membrane, ThomasGraham (1861) observed that certain substances such as sugar, urea, sodium chloride etc. in the dissolved state passed through the membrane, while the solutions of substances such as glue, gelatin, gum Arabic etc. did not. This observation led him to classify the soluble substances into two categories:
(i) Crystalloids (ii) Colloids
According to Graham, crystalloids were those substances which could be obtained in crystalline form and whose solutions were able to pass through an animal membrane. On the other hand, colloids were those substances which were amorphous in nature and whose solutions were unable to pass through the membrane. However, it was soon realized that the classification of dissolved substances made by Graham was not tenable because certain substances could act both as crystalloids and colloids.
Later on it was found that the diffusibility of crystalloids and non-diffusibility of colloids through an animal membrane was due to the difference in the size of their particles. Crystalloids formed smaller particles in solutions and therefore passed through the membrane. On the other hand, colloids formed larger particles (larger than the dimensions of the pores of the membrane) in solutions and were unable to pass through the membrane.
On the basis of the size of particles, the systems containing dispersed particles can be divided into following three categories.
1. True solutions: True solutions are homogeneous system and have the size of dispersed particles less than 1 nm, i.e. 10-9. The particles of solute present in a true solution are either single molecules or irons and are homogeneously distributed throughout the solutions. These particles are invisible and cannot be seen even with a microscope. Due to very small size of dispersed particles, true solutions pass through ordinary filter paper as well as through animal membranes. Sodium chloride, sugar, urea etc. form true solutions in water.