Class 12 Science Practical Projects Viva Voce Guide
SURFACE
CHEMISTRY (Part - I)
Question 1. What is a true
solution? Give an example.
Answer: A homogenous mixture of two substances, in which particles of
both possess molecular dimensions (i.e., 10-10 to 10-9
m). For example, a solution of common salt in water.
Question 2. What is a
colloidal solution? Give an example.
Answer: A colloidal solution is a two-phase heterogeneous system in
which a substance is distributed in colloidal state (particles having diameter
between 1 to 100 nm) in a medium. The particles of the dispersed substance (of
colloidal size) are called dispersed phase; while the medium in which
they are dispersed, is called dispersion medium. For example, milk, butter, smoke, etc.
Question 3. What is the
size of colloidal particles?
Answer: Diameter of colloidal particles is between 1 - 100 nm.
Question 4. What are
lyophilisation and lyophobic sols?
Answer: A lyophilic sol is a colloidal solution in which there is great
is affinity between colloidal particles and the dispersion medium. For example,
starch sol, gum sol etc.
A lyophobic sol is a colloidal solution in which there is little
affinity between colloidal particles and the dispersion medium. For example,
sulphur sol, gold sol, etc. For example, starch sol is a lyophilic sol whereas
sulphur sol is a lyophobic sol.
Question 5. Give two
examples of positively charged sols.
Answer: Ferric hydroxide sol and aluminium hydroxide sol.
Question 6. Give example of
negatively charged sol.
Answer: Arsenious sulphide sol.
Question 7. Why cannot we
prepare a colloidal solution of a gas in a gas?
Answer: The mixture of any two gases is always a homogeneous mixture
whereas a colloidal solution is always heterogeneous.
Question 8. How can a
colloidal solution and a true solution of the same colour be distinguished from each other?
Answer: A colloidal solution and a true solution of the same colour can
be distinguished by passing a powerful beam of light through true and colloidal
solutions, each kept in glass vessel. Only colloidal solution exhibits Tyndall
Effect whereas true solution does not.
Question 9. Define the term
“Dialysis”. Tell about the use of dialysis.
Answer: The process of separating electrolytes from a colloid by means
of diffusion of the former through an animal or vegetable membrane is called Dialysis.
Dialysis is used for purifying colloidal solution. In the presence of
excess of electrolytes, the colloidal solutions get coagulated.
Question 10. Can we make
Dialysis fast? If yes, how?
Answer: By circulating hot water instead of cold water, in container or
by applying electric field across the dialyser.
Question 11. What is “Coagulation”?
Answer: Coagulation is the process of changing the colloidal state into
an insoluble precipitate, by including aggregation of colloidal particles.
Question 12. Name a few
methods of coagulating lyophilic colloids.
Answer: (i) Addition of electrolyte. (ii) Addition of a liquid in which
the dispersion medium is soluble.
Question 13. What is meant
by “Peptization”?
Answer: The method of breaking down a precipitate into colloidal form by
shaking with the dispersion medium in presence of an electrolyte is called
“Peptization”.
(To
be continued ...)
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