Surface Chemistry - Class 12 Medical Chemistry - Extra Questions

Give reasons for the following observations:
(i) Physisorption decreases with increase in temperature
(ii) Addition of alum purifies the water
(iii) Brownian movement provides stability to the colloidal solution

  1. physisorption is exothermic in nature. Therefore, according to Le-Chatelier's principle, it decreases with an increase in temperature. This means that physisorption occurs more readily at a lower temperature.
  2. When Alum $$((Al_2)(SO_4)_3)$$ is added into water it forms a gel-like precipitate such as $$Al(OH)_3$$.This floc attracts other fine particles and suspended material in raw water and settles down at bottom of a container.
  3. Brownian movement explains the force of gravity acting on colloidal particles. This helps in providing stability to colloidal sols by not allowing to settle down.


Due to which forces physisorption arises?

Due to weak physical forces, called van der Waals forces, physisorption arises. van der Waals forces are similar to the forces that cause the condensation of gas into liquid.


A colloidal solution of ferric oxide is prepared by two different methods as shown below. [HOTS]
i. What is the charge on colloidal particles in two test tubes $$(A)$$ and $$(B)$$?
ii. Give reasons for the origin of charge.
1776006_6ab412ef026f4f18a1f733d8ed65ddf4.png

i. Colloidal particles of test tube $$(A)$$ are positively charged whereas colloidal particles of test tube $$(B)$$ are negatively charged.

ii. In test tube $$(A), Fe^{3+}$$ is adsorbed on the precipitate $$Fe_{2}O_{3} . xH_{2}O$$ [or $$Fe_{2}O_{3} . xH_{2}O/ Fe^{3+}$$ is formed].
In test tube $$(B), OH^{-}$$ ion is adsorbed on the precipitate $$Fe_{2}O_{3} . xH_{2}O$$ [or $$Fe_{2}O_{3} . xH_{2}O/ OH^{-}$$ is formed].


A colloidal solution of $$AgI$$ is prepared by two different methods as shown in figure below.
i. What is the charge of $$AgI$$ colloidal particles in the two test tubes $$(A)$$ and $$(B)$$?
ii. Give reasons for the origin of charge.
1776002_6460aa825ebb4379b9117ea5669f0e47.png

i. Test tube $$(A)$$ has negative charge whereas test tube $$(B)$$ has positive charge on the colloidal particles.

ii. In test tube $$(A), I^{-}$$ is adsorbed on precipitate $$AgI$$ [or $$AgI/ I^{-}$$ is formed] and in test tube $$(B), Ag^{+}$$ is adsorbed on precipitate $$AgI$$ [or $$AgI/ Ag^{+}$$ is formed].


Class 12 Medical Chemistry Extra Questions