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Explain the various chemical methods by which colloids can be prepared. |
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Answer» SOLUTION :Condensation Methods : Various chemical methods for the formation of colloidal particles. (i) Oxidation : Sols of some non-metals are prepared by this method. (a) When hydroiodic acid is treated with iodic acid, `I_2` sol is obtained. `HIO_3 + 5HI to 3H_2O + I_2` (Sol) (ii) Reduction : Many organic reagents like phenyl hydrazine, formaldehyde, etc are used for the formation of sols. For example: Gold sol is prepared by reduction of auricchloride using formaldehyde. `2AuCl_3 + 3HCHO + 3H_2O to 2Au("sol") + 6HCl + 3HCOOH` (iii) Hydrolysis : Sols of hydroxides of metals like chromium and aluminium can be produced by this method. For Example, ` FeCl_2 + 3H_2O to Fe(OH)_3 + 3HCl` (iv) DOUBLE decomposition : For the preparation of water insoluble sols this method can be used. When hydrogen sulphide gas is passed through a solution of arsenic oxide, a yellow coloured arsenic sulphide is obtained as a colloidal solution. `As_2 O_3 + 3H_2S to As_2S_3 + 3H_2O ` Decomposition : When few drops of an acid is added to a dilute solution of sodium thio sulphate, the insoluble free sulphur produced by decomposition of sodium thiosulphate accumulates into small, clusters which impart various colours blue, yellow and even red to the system depending on their growth WITHIN the size of colloidal dimensions. `S_2O_3^(2-) + 2H^(+) to S + H_2O + SO_2` By exchange of solvent : (i) Colloidal solution of few substances like phosphorous or sulphur is obtained by preparing the solutions in ALCOHOL and pouring them into water. (ii) As they are insoluble in water, they FORM colloidal solution. Pin alcohol + water`to`Psol. |
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