question_answer

  Give reasons for the following observations. (a) Peptizing agent is added to convert precipitate into colloidal solution. (b) Cottrell's smoke precipitator is fitted at the mouth of chimney used in factories. (c) Colloidal gold is used for intramuscular injection.                                                                                        [3 x 1]

Answer:

  (i) When peptising agent is added to freshly prepared precipitate, it changes into colloidal sol. It is called peptisation. During peptisation, the precipitate adsorbs one of the ions of the electrolyte on its surface. Thus, there is  formation of positive or negative charge on the precipitate which ultimately break up into smaller particles of the size of the colloidal sol. Agl is precipitated in the following reaction If Kl is added to Agl ppt, (common ion) is adsorbed on this surface and negatively-charged colloidal sol is formed.                       [1]  (ii) Smoke is a colloidal sol of solid particles such as carbon, arsenic, dust dispersed in air. They are charged particles. The smoke before it comes out from the chimney is led through the chamber containing plates having a charge opposite to that carried by smoke particles. (This chamber is called Cottrell precipitator). The particles, on coming in contact with these plates lose their charge and get precipitated. [1] (iii) Colloidal gold (in the form of medicine) are more effective because they have large surface area and therefore, easily assimilated. Thus, colloidal gold sol is used for intramuscular injection.                                       [1]