Q.What are the observations in following cases - [1+1+1=3]
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Start your 14-day free trial to unlock the full solution →(a) Tyndall effect - light scattering makes the beam visible. (b) The electrolyte's oppositely-charged ions neutralise the sol's charge and coagulate it. (c) Electrophoresis - charged particles migrate to and deposit at the oppositely charged electrode.
(a) When a beam of light is passed through a colloidal sol and viewed at right angles, the path of the beam becomes visible as a bright, illuminated cone within the sol. This is called the Tyndall effect. It occurs because colloidal particles (being of a size comparable to the wavelength of visible light) scatter light in all directions, unlike in a true solution where particles are too small to scatter visible light noticeably.
(b) Colloidal (lyophobic) sol particles carry a net electric charge (all particles of a given sol carrying the same sign of charge, which is what keeps them from aggregating due to mutual repulsion). When an electrolyte such as NaCl is added to a hydrated ferric oxide sol (which is positively charged), the ions of opposite charge to the sol particles (here, Cl- ions) are preferentially adsorbed on the particle surfaces. This neutralises the charge on the particles, removes the electrostatic repulsion that was stabilising the sol, and the particles aggregate and settle out - i.e. the sol is coagulated (precipitated/flocculated). This is governed by the Hardy-Schulze rule (coagulating power increases with the charge of the oppositely-charged ion).
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