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Lyophobic sols

  1. Chemical methods: Colloidal solutions can be prepared by chemical reactions involving double decomposition, oxidation, reduction, and hydrolysis.
    • Double decomposition: A colloidal solution of arsenious sulphide is obtained by passing hydrogen sulphide into solution of arsenious oxide in distilled water.
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    • Oxidation: A colloidal solution of sulphur can be prepared by passing hydrogen sulphide into a solution of sulphur dioxide in water or through a solution of an oxidizing agent such as bromine water or nitric acid.
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    • Reduction: A colloidal solution of a metal such as silver, gold, or platinum can be prepared by the reduction of its salt solution with a suitable reducing agent such as stannous chloride, formaldehyde, and hydrazine.
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    • Hydrolysis: A red sol of ferric hydroxide is obtained by the hydrolysis of ferric chloride with boiling water.
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  2. Physical methods
    • Exchange of solvent: The addition of an alcoholic solution of sulphur to excess water causes the formation of colloidal sulphur, sulphur being more insoluble in water than in alcohol.
    • Excessive cooling: The colloidal solution of ice in an organic solvent such as chloroform or ether can be obtained by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution separately combine to form particles of colloidal size.
  3. Dispersion methods: In these methods, large particles of a substance are broken into particles of colloidal dimensions in the presence of dispersion medium. Since the sols formed in this manner are unstable, they are stabilized by adding suitable stabilizers. Some of the methods employed for carrying out the dispersion are described as follows.
    • Electrical disintegration or Bredig’s arc method: This process involves dispersion as well as condensation. Colloidal solutions of metals such as gold, silver, copper, and platinum can be prepared by this method. In this method, electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vaporizes some of the metal which then condenses to form particles of colloidal size. A slight trace of electrolyte stabilizes the sols formed.
    • Peptization: This is a process of converting a precipitate into colloidal solution by shaking it with dispersion medium in the presence of small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent. This method is generally applied to convert fresh precipitate into colloidal solutions because such precipitates are simply aggregates of colloidal particles held by weak forces.

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