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Question

Which of the following substances gives a positively charged sol?


A

Gold

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B

Arsenious sulfide

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C

Starch

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D

Ferric hydroxide

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Solution

The correct option is D

Ferric hydroxide


Explanation of correct option:

D) Ferric hydroxide

  • Ferric hydroxide is a positively charged sol.
  • The colloid of ferric hydroxide Fe(OH)3 is prepared by adding ferric chloride FeCl3 to hot water where the ferric chloride gets hydrated by the excess amount of water and forms the sol of ferric hydroxide from the ferric chloride.
  • The formed Fe3+ ions are positively charged and get adsorbed on the surface of the sol giving the overall charge of the sol a positive charge.

Explanation of incorrect options:

A) Gold

  • Gold is not a positively charged sol. The gold sol produced by the reduction process is negatively charged.
  • The chemical reaction can be depicted as:

2AuCl3+3SnCl23SnCl4+2Au(Sol)

  • Negative charge is due to the adsorption of negatively charged ions on the dispersed phase particles.

B) Arsenious sulfide

  • Arsenious sulfide (As2S3) is a lyophobic colloid.
  • It is produced in boiling distilled water by hydrolysis of arsenic oxide (As2O3). Additional H2S gas is bubbled into the solution.
  • Hence arsenious sulphide sol carries a negative charge, it can be coagulated by positive ions.

C) Starch

Starch is not a positively charged sol. It is a negatively charged sol.

Hence, the correct option is D i.e. Ferric hydroxide.


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