Ans. A substance shows different physical properties in different medium. It may exist as colloid or crystalloids under certain conditions. Eg. NaCl in water behaves like crystalloid while in benzene behaves like colloid. Similarly dilute soap solution behaves like crystalloid whereas concentrated soap solution like colloids. It is the size of particles which matters i.e. the state in which the substances exists. If the size of the particles lies in the range 1 to 1000 no., it is in colloidal state.