Question

Which of the following substances gives a positively charged sol?
(A) Gold
(B) Arsenious sulphide
(C) Starch
(D) Ferric hydroxide

Answer 1

Hint: A colloid in which very small solid particles are constituted in a liquid medium are known as sols. They are stable and show the Tyndall effect. Some examples of sols are paint, mud, blood, antacids, etc.

Complete answer:
Now, when there is an adsorption of ions on the surface of the sols, charged sols are formed. These charged sols can be positive or negative depending upon the ions adsorbed.
There are various methods through which a sol can acquire charge.
1. Dissociation of surface molecules.
2. Frictional Electrification
3. Selective adsorption of ions
- Gold sols are prepared from the reduction method. Gold trichloride is reacted with tin (II) chloride and the chloride ions from gold chloride are added to tin chloride to form tin (IV) chloride and sols of gold. Now when in the dispersed phase, the gold sols adsorb negatively charged $AuO_{2}^{-}$ to form negatively charged sols.
- Arsenious Sulphide $A{{s}_{2}}{{S}_{3}}$ dissociates into $A{{s}^{3+}}$ ions and ${{S}^{2-}}$ ions. Since metals have a positive charge, they attract negative ions to form a negative sol.
- Starch sols are made by making a starch paste and boiling it in distilled water with constant stirring. They form negatively charged sols.
- Ferric Hydroxide sols are formed by reacting ferric chloride with NaOH in a solution. $Fe{{(OH)}_{3}}$ dissociates into $F{{e}^{3+}}$ ions and $O{{H}^{-}}$ ions. But since ferric hydroxide molecules selectively adsorb positive $F{{e}^{3+}}$ ions, they form positively charged sols.

So, option (D) ferric hydroxide gives a positively charged sol.

Note:
This question can be confusing as one can assume that since metals are electron donors, all of them will attract negatively charged ions to form negatively charged sols. But this inference is wrong as one does not take into account selective adsorption of ions.