Question

Describe a conspicuous change observed when:
(i) A solution of NaCl is added to the solution of hydrated ferric oxide.
(ii) A beam of light is passed through a solution of NaCl and then through a solution.

Answer 1

Hint: When sodium chloride is added to the hydrated ferric oxide the negatively charged chloride ion forms whereas the hydrated ferric oxide is positively charged. The colloidal particles are big then particles of pure solution.

Complete answer:
(i) Colloidal particles carry a charge. The colloidal solution can be positively charged or negatively charged.
The hydrated ferric oxide is a positively charged colloidal solution.
When sodium chloride is added into the solution of hydrated ferric oxide, sodium chloride dissociates into sodium and chloride ions.
The dissociation of sodium chloride is shown as follows:
${\text{NaCl}}\, \to {\text{N}}{{\text{a}}^{\text{ + }}}\, + \,{\text{C}}{{\text{l}}^ - }$
The sodium ion has the same charge as the hydrated ferric oxide so, the presence of sodium ions has no effect on the hydrated ferric oxide colloidal solution but the chloride ions have a negative charge so, positively charged hydrated ferric oxide coagulate in presence of negatively charged chloride ions.
(ii) When light is passed through the colloidal solution, the particles of the colloidal solution, the light scatters due to the big size of the colloidal particles, and due to the scattering of light, the path of light eliminates.
So, an opaque colloidal solution appears when light is passed through the sodium chloride and then through the solution.
The effect is known as the Tyndall effect.

Note: All the particles of colloidal solution carry the same charge, so the charge of colloidal particles represents the charge of colloidal solution. For the observance of the Tyndall effect, the diameter of colloidal particles should not be too small than the wavelength of light.