Notes On Colloids: Properties Of Colloidal Solutions - II - CBSE Class 12 Chemistry
Apart from exhibiting colour, the Tyndal effect, Brownian motion and colligative properties, colloidal solutions also exhibit electrical properties. Electrical properties of colloids are Electrical charge on colloidal particles, electrophoresis, electro-osmosis and coagulation of sols. Electrical property: The particles of a colloidal solution possess a definite electrical charge, either positive or negative, on them. Due to the presence of the same charge, they repel each other and do not combine to form larger particles. This keeps them dispersed in the medium, and hence, a colloidal solution is stable. Based on the nature of the charge on the colloidal particles, colloidal solutions are classified as positively charged or negatively charged sols. Hydrated metallic oxides, like Al2O3.xH2O, Fe2O3.xH2O and Cr2O3.xH2O, and basic dyes like methylene blue, form positively charged sols. Starch sol, metal sols like copper sol and gold sol, metal sulphide sols and acid dyes like Congo red, are examples of negatively charged sols. The electrical charge on colloidal particles may be due to several reasons. These include electron capture by the colloidal particles during electro-dispersion of metals, preferential adsorption of ions from the solution, and the formation of an electrical double layer. The combination of two layers of opposite charges around a colloidal particle is called the "Helmholtz electrical double layer." The first layer is firmly held and is called the "fixed layer," while the second or the outer layer is mobile and is termed as the diffused layer. The presence of opposite charges on the fixed and diffused layers of the double layer results in a difference in potential. This potential is called "electro kinetic potential" or "zeta potential". Electrophoresis: The movement of colloidal particles towards a particular electrode under the influence of an electrical field is called electrophoresis. The direction of movement of colloidal particles towards a particular electrode is governed by the nature of the charge on them. If the colloidal particles carry a positive charge, then they move towards the cathode when subjected to an electrical field, while negatively charged colloidal particles move towards the anode. An important application of electrophoresis is in sewage disposal. Electro-osmosis: Electro-osmosis is the movement of a dispersion medium under the influence of an electrical field when the movement of colloidal particles is prevented by a suitable membrane. Under the influence of an electrical field, the colloidal particles and the dispersion medium both have a tendency to move towards the oppositely charged electrodes, but the semi-permeable membrane does not allow the passage of the colloidal particles. If the charge is destroyed, they are free to come near each other to form larger molecules. These larger molecules aggregate or coagulate, and then settle down under the force of gravity. This phenomenon is called coagulation or precipitation of the sol. The coagulation of a lyophobic colloidal solution can be achieved in a number of ways. One method is electrophoresis. Another method used to bring about coagulation is by mixing two oppositely charged sols. EX: If equal proportions of a positively charged sol of hydrated ferric oxide and a negatively charged sol of arsenious sulphide are mixed, then the coagulation of both the sols takes place. This type of coagulation is called mutual coagulation. A sol may also be coagulated by simple boiling. EX: Arsenious sulphide sol undergoes coagulation on boiling. The coagulation of a sol can also be brought by persistent dialysis. The most important and useful method of coagulating sols is by adding an electrolyte. The coagulation behaviour of different electrolytes can be explained on the basis of the Hardy-Schulze rule. The rule states that the greater the valence of the flocculating ion added, the greater is its power to cause precipitation. The coagulating power of different cations to coagulate a negative sol follows the order Al+3> Ba+2 >Na+. The coagulating power of different anions to coagulate a positively charged sol decrease in the order [ Fe CN6]-4 >PO4-3 >SO4-2 > Cl - Lyophilic sols, are much more stable than lyophobic sols, and do not get coagulated easily under similar conditions. The two factors responsible for the stability of lyophilic sols are The existence of the same charge on all the colloidal particles The extensive solvation of the colloidal particles of a lyophilic sol. A lyophilic sol can be coagulated either adding an electrolyte or a suitable solvent. A higher concentration of an electrolyte is needed to coagulate a lyophilic sol than is needed to coagulate a lyophobic sol. Protection of colloids: Lyophobic sols, such as metal sols, are very susceptible to coagulation or precipitation. The process of protecting them from coagulation is referred to as protection of colloids. It has been observed that in the presence of certain lyophilic colloids, lyophobic sols acquire greater stability. They do not get coagulated easily when an electrolyte is added. A layer of lyophilic sol particles around the lyophobic sol particles prevents them from coagulating. EX: Adding gelatine, a lyophilic colloid, to gold sol.

#### Summary

Apart from exhibiting colour, the Tyndal effect, Brownian motion and colligative properties, colloidal solutions also exhibit electrical properties. Electrical properties of colloids are Electrical charge on colloidal particles, electrophoresis, electro-osmosis and coagulation of sols. Electrical property: The particles of a colloidal solution possess a definite electrical charge, either positive or negative, on them. Due to the presence of the same charge, they repel each other and do not combine to form larger particles. This keeps them dispersed in the medium, and hence, a colloidal solution is stable. Based on the nature of the charge on the colloidal particles, colloidal solutions are classified as positively charged or negatively charged sols. Hydrated metallic oxides, like Al2O3.xH2O, Fe2O3.xH2O and Cr2O3.xH2O, and basic dyes like methylene blue, form positively charged sols. Starch sol, metal sols like copper sol and gold sol, metal sulphide sols and acid dyes like Congo red, are examples of negatively charged sols. The electrical charge on colloidal particles may be due to several reasons. These include electron capture by the colloidal particles during electro-dispersion of metals, preferential adsorption of ions from the solution, and the formation of an electrical double layer. The combination of two layers of opposite charges around a colloidal particle is called the "Helmholtz electrical double layer." The first layer is firmly held and is called the "fixed layer," while the second or the outer layer is mobile and is termed as the diffused layer. The presence of opposite charges on the fixed and diffused layers of the double layer results in a difference in potential. This potential is called "electro kinetic potential" or "zeta potential". Electrophoresis: The movement of colloidal particles towards a particular electrode under the influence of an electrical field is called electrophoresis. The direction of movement of colloidal particles towards a particular electrode is governed by the nature of the charge on them. If the colloidal particles carry a positive charge, then they move towards the cathode when subjected to an electrical field, while negatively charged colloidal particles move towards the anode. An important application of electrophoresis is in sewage disposal. Electro-osmosis: Electro-osmosis is the movement of a dispersion medium under the influence of an electrical field when the movement of colloidal particles is prevented by a suitable membrane. Under the influence of an electrical field, the colloidal particles and the dispersion medium both have a tendency to move towards the oppositely charged electrodes, but the semi-permeable membrane does not allow the passage of the colloidal particles. If the charge is destroyed, they are free to come near each other to form larger molecules. These larger molecules aggregate or coagulate, and then settle down under the force of gravity. This phenomenon is called coagulation or precipitation of the sol. The coagulation of a lyophobic colloidal solution can be achieved in a number of ways. One method is electrophoresis. Another method used to bring about coagulation is by mixing two oppositely charged sols. EX: If equal proportions of a positively charged sol of hydrated ferric oxide and a negatively charged sol of arsenious sulphide are mixed, then the coagulation of both the sols takes place. This type of coagulation is called mutual coagulation. A sol may also be coagulated by simple boiling. EX: Arsenious sulphide sol undergoes coagulation on boiling. The coagulation of a sol can also be brought by persistent dialysis. The most important and useful method of coagulating sols is by adding an electrolyte. The coagulation behaviour of different electrolytes can be explained on the basis of the Hardy-Schulze rule. The rule states that the greater the valence of the flocculating ion added, the greater is its power to cause precipitation. The coagulating power of different cations to coagulate a negative sol follows the order Al+3> Ba+2 >Na+. The coagulating power of different anions to coagulate a positively charged sol decrease in the order [ Fe CN6]-4 >PO4-3 >SO4-2 > Cl - Lyophilic sols, are much more stable than lyophobic sols, and do not get coagulated easily under similar conditions. The two factors responsible for the stability of lyophilic sols are The existence of the same charge on all the colloidal particles The extensive solvation of the colloidal particles of a lyophilic sol. A lyophilic sol can be coagulated either adding an electrolyte or a suitable solvent. A higher concentration of an electrolyte is needed to coagulate a lyophilic sol than is needed to coagulate a lyophobic sol. Protection of colloids: Lyophobic sols, such as metal sols, are very susceptible to coagulation or precipitation. The process of protecting them from coagulation is referred to as protection of colloids. It has been observed that in the presence of certain lyophilic colloids, lyophobic sols acquire greater stability. They do not get coagulated easily when an electrolyte is added. A layer of lyophilic sol particles around the lyophobic sol particles prevents them from coagulating. EX: Adding gelatine, a lyophilic colloid, to gold sol.

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