Notes On Colloids: Classification - Physical State And Nature Of Interactions - CBSE Class 12 Chemistry
Colloids are one of the three major types of mixtures, the other two being solutions and suspensions. The three kinds of mixtures are distinguished by the size of the constituent particles. SOLUTIONS: Solutions are homogeneous systems in which the diameter of the solute is < 10-9m. These particles are not visible to the naked eye. EX: Common salt in water Suspension: A suspension is a heterogeneous system. The particle size of the molecules in a suspension is >10-6m. These particles can be seen with the naked eye. EX: Sand in water. Colloids: Colloids are mixtures whose particles are larger than the particles of a solution, but smaller than the particles of a suspension. The suspended particles in a colloid are small enough to settle down due to gravity. A colloid is a heterogeneous solution in which the particle size ranges from 10-9 to 10-6. These particles are not visible to the naked eye, but can be seen under a microscope. EX: Milk, blood, honey and starch solution are all colloids. For a colloidal solution, we use the terms dispersed phase and dispersion medium. The phase that is scattered or present in the form of colloidal particles is called the dispersed phase. The medium in which the colloidal particles are dispersed is called the dispersion medium. Ex: In a starch solution, starch represents the dispersed phase, while water represents the dispersion medium. Colloids can be classified on the basis of: The physical state of the dispersed phase and the dispersion medium The nature of interaction between the dispersed phase and dispersion medium ·The type of particles of the dispersed phase Classification based on the physical state of the dispersed phase and the dispersion medium: Depending upon the physical state of the dispersed phase and dispersion medium, eight types of colloidal systems are possible. Phase of colloid Dispersing phase Dispersion medium Colloid Type Example Gas Gas Liquid Liquid Liquid Solid Solid Solid Liquid Solid Gas Liquid Solid Gas Liquid Solid Gas Gas Liquid Liquid Liquid Solid Solid Solid Aerosal Aerosal Foam Emulsion Sol Solid foam Solid emulsion Solid sol Fog Smoke Whipped cream Milk Paint Marshmallow Butter Ruby glass Classification is based on the nature of the interaction between the dispersed phase and the dispersion medium: Depending upon the affinity of the dispersed phase for the dispersion medium, colloidal systems can be classified into two categories - lyophilic and lyophobic sols. Lyophilic sols: The word 'lyophilic' means liquid loving or solvent loving. When substances like starch, gum and gelatine are mixed with a suitable liquid, that is, a dispersion medium, they readily form colloidal solutions. Such colloidal solutions are called lyophilic colloids. Lyophobic colloids: The word lyophobic means liquid hating. Lyophobic colloids cannot be formed by spontaneous dispersion in the medium but can be prepared only by special methods. Arsenic sulphide, ferric hydroxide, gold and other metals form lyophobic colloids. These metals are sparingly soluble and thus their molecules do not pass readily into the colloidal state.

#### Summary

Colloids are one of the three major types of mixtures, the other two being solutions and suspensions. The three kinds of mixtures are distinguished by the size of the constituent particles. SOLUTIONS: Solutions are homogeneous systems in which the diameter of the solute is < 10-9m. These particles are not visible to the naked eye. EX: Common salt in water Suspension: A suspension is a heterogeneous system. The particle size of the molecules in a suspension is >10-6m. These particles can be seen with the naked eye. EX: Sand in water. Colloids: Colloids are mixtures whose particles are larger than the particles of a solution, but smaller than the particles of a suspension. The suspended particles in a colloid are small enough to settle down due to gravity. A colloid is a heterogeneous solution in which the particle size ranges from 10-9 to 10-6. These particles are not visible to the naked eye, but can be seen under a microscope. EX: Milk, blood, honey and starch solution are all colloids. For a colloidal solution, we use the terms dispersed phase and dispersion medium. The phase that is scattered or present in the form of colloidal particles is called the dispersed phase. The medium in which the colloidal particles are dispersed is called the dispersion medium. Ex: In a starch solution, starch represents the dispersed phase, while water represents the dispersion medium. Colloids can be classified on the basis of: The physical state of the dispersed phase and the dispersion medium The nature of interaction between the dispersed phase and dispersion medium ·The type of particles of the dispersed phase Classification based on the physical state of the dispersed phase and the dispersion medium: Depending upon the physical state of the dispersed phase and dispersion medium, eight types of colloidal systems are possible. Phase of colloid Dispersing phase Dispersion medium Colloid Type Example Gas Gas Liquid Liquid Liquid Solid Solid Solid Liquid Solid Gas Liquid Solid Gas Liquid Solid Gas Gas Liquid Liquid Liquid Solid Solid Solid Aerosal Aerosal Foam Emulsion Sol Solid foam Solid emulsion Solid sol Fog Smoke Whipped cream Milk Paint Marshmallow Butter Ruby glass Classification is based on the nature of the interaction between the dispersed phase and the dispersion medium: Depending upon the affinity of the dispersed phase for the dispersion medium, colloidal systems can be classified into two categories - lyophilic and lyophobic sols. Lyophilic sols: The word 'lyophilic' means liquid loving or solvent loving. When substances like starch, gum and gelatine are mixed with a suitable liquid, that is, a dispersion medium, they readily form colloidal solutions. Such colloidal solutions are called lyophilic colloids. Lyophobic colloids: The word lyophobic means liquid hating. Lyophobic colloids cannot be formed by spontaneous dispersion in the medium but can be prepared only by special methods. Arsenic sulphide, ferric hydroxide, gold and other metals form lyophobic colloids. These metals are sparingly soluble and thus their molecules do not pass readily into the colloidal state.

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