Colloids were wrongly classified as two different types of substances; colloids and crystalloids. Crystalloids were crystalline and readily pass through membranes while colloids were amorphous and were unable to pass through membranes. However, the classification of dissolved substances made by Graham was not tenable because certain substances could act both as crystalloids and colloids. Crystalloids or colloids simply represent a state of matter in which every substance can be brought by suitable methods. NaCl is a crystalloid in water but a colloid in benzene while soap is a colloid in water but a crystalloid in benzene. The permeability of crystalloids and non-permeability of colloids through a 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.
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