Surface Chemistry — NEET UG practice

1. Colloids are classified based on the physical state of the dispersed phase and the dispersion medium.

2. When both the dispersed phase and the dispersion medium are solids, the colloid is called a solid sol. $\text{Solid sol} = \text{solid dispersed in solid}$

3. Pumice stone is a porous volcanic rock in which tiny pockets of trapped gas are locked inside a solid rock matrix, so it fits the pattern of one solid phase dispersed within another solid.

4. This is different from a gel (liquid dispersed in solid, like jelly), a foam (gas dispersed in liquid, like whipped cream), and a sol (solid dispersed in liquid, like paint).

Hence, the answer is B. solid sol.

1. Colloidal sol particles are electrically charged because they selectively adsorb a particular type of ion from the dispersion medium onto their surface.

Given this, the sign of the charge on a sol depends on which substance was used to prepare it.

2. Positively charged sols are usually formed by basic substances such as basic dyes and metal hydroxides, since these preferentially adsorb positive ions. $\text{Example: Methylene blue sol, Fe(OH)}_3\text{ sol, Al(OH)}_3\text{ sol}$

3. Negatively charged sols are usually formed by acidic dyes, metal sols, and metal sulphide sols, since these preferentially adsorb negative ions. $\text{Example: Congo red sol, Silver sol, Sb}_2\text{S}_3\text{ sol}$

4. Comparing the given options with these two categories, only methylene blue sol belongs to the positively charged group.

Hence, the answer is A. Methylene blue sol.

1. A catalysed reaction is called homogeneous catalysis when the catalyst and the reactants are in the same phase, and heterogeneous catalysis when the catalyst is in a different phase from the reactants.

2. In option A, the $H^+$ catalyst and the sugar solution are both in the liquid (aqueous) phase. $C_{12}H_{22}O_{11(aq)} + H_2O \xrightarrow{H^+} C_6H_{12}O_{6(aq)} + C_6H_{12}O_{6(aq)}$ Since everything is in one phase, this is homogeneous catalysis.

3. In option B, gaseous NO catalyses the decomposition of gaseous ozone; both are gases. $2O_{3(g)} \xrightarrow{NO(g)} 3O_{2(g)}$ This is also homogeneous catalysis.

4. In option C, solid iron catalyses the reaction between gaseous nitrogen and gaseous hydrogen. $N_{2(g)} + 3H_{2(g)} \xrightarrow{Fe(s)} 2NH_{3(g)}$ Since the catalyst (solid) is in a different phase from the reactants (gas), this is heterogeneous catalysis.

5. In option D, gaseous oxides of nitrogen catalyse the oxidation of gaseous $SO_2$; both catalyst and reactants are gases, so this is homogeneous catalysis.

Hence, the answer is C.