question_answer 31)

  Explain the terms: (i) Electrophoresis          (ii) Coagulation (iii) Dialysis                        (iv) Tyndall effect.

Answer:

(i) Electrophoresis. The particles of the colloidal solutions possess electrical charge, positive or negative while the dispersion medium has no charge. The presence of the charge on the sol particles and its nature can be determined with the help of a phenomenon known as electrophoresis in which colloidal particles move towards oppositely charged electrodes under the influence electrical field. The phenomenon of movement colloidal particles under the influence of applied electric field is called electrophoresis. For example, the solutions of gold, arsenioussulphide (As2S3) are negatively charged and the particles move towards the positive electrode while those of Fe(OH)3,AI(OH)3 have positive charge and the particles move towards negative electrode. (ii) Coagulation. The phenomenon of the precipitation of a colloidal solution by the addition of the excess of an electrolyte is called coagulation or flocculation. Mechanism of coagulation. The particles of the dispersed phase i.e., colloids bear some charge. When an electrolyte is added to the sol, the colloidal particles take up ions carrying opposite charge from the electrolyte. As a result, their charge gets neutralised and this causes the uncharged particles to come closer and to get coagulated or precipitated. For example, if BaCl2 solution is added to As2S3 sol, the Ba2+ ions are attracted by the negatively charged sol particles and their charge gets neutralised. This leads to coagulation. (iii) Dialysis. It is based on the principle that impurities can pass through the parchment membrane while colloidal particle cannot. The apparatus used is known as dialyser. The impurities pass out through the membrane giving pure colloidal solution. This process is slow and time consuming. Thus, to fasten the process, an electric field is applied and the process is known as Electrodialysis. (iv) When a beam of light is passed through a colloidal solution, the path becomes illuminated by a bluish light when seen with a microscope at right angle to the path of light. This effect is called Tyndall effect and the illuminated path is called Tyndall cone. Cause. The colloidal particle first absorb light energy and become self luminous. Then a part of light energy is scattered from their surface. Tyndall effect is due to the scattering of light by the colloidal particles and the colloidal particles are seen to be moving as points of light moving against dark background.