Question

Which of the following substances gives a positively charged sol?

• A. Gold
• B. Arsenious sulfide
• C. Starch
• D. Ferric hydroxide

Answer 1

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 $\left(\mathrm{Fe}{\left(\mathrm{OH}\right)}_3\right)$ is prepared by adding ferric chloride $\left({\mathrm{FeCl}}_3\right)$ 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 ${\mathrm{Fe}}^{3+}$ 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:

$$\begin{gathered} 2{\mathrm{AuCl}}_3+3{\mathrm{SnCl}}_2\to 3{\mathrm{SnCl}}_4+2\mathrm{Au} \\ \left(\mathrm{Sol}\right) \end{gathered}$$

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

B) Arsenious sulfide

• Arsenious sulfide (${\mathrm{As}}_2{\mathrm{S}}_3$) is a lyophobic colloid.
• It is produced in boiling distilled water by hydrolysis of arsenic oxide (${\mathrm{As}}_2{\mathrm{O}}_3$). Additional ${\mathrm{H}}_2\mathrm{S}$ 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.