Q.Explain what is observed

Step 1 – (i) Light passed through a sol → Tyndall effect. Colloidal particles are large enough to scatter light. So the path of the beam through the sol is seen as an illuminated cone (the Tyndall cone) when viewed at right angles. This does not happen in a true solution.

Step 2 – (ii) Adding $NaCl$ to hydrated ferric oxide sol → coagulation. Hydrated ferric oxide, $Fe(OH)_3$ / $Fe_2O_3\!\cdot\! xH_2O$, is a positively charged sol. The added electrolyte supplies $Cl^{-}$ (opposite charge), which is adsorbed and neutralises the charge on the particles; the particles then aggregate and coagulate/precipitate (Hardy–Schulze law). …

Map each observation to its named phenomenon: light scattering → Tyndall; electrolyte on a charged sol → …

• Calling (i) reflection/refraction instead of scattering (Tyndall).
• Saying $NaCl$ simply dissolves in the sol rather than coagulating it. …