Class 12 Viva Voce guide with questions and answers for CBSE Science - Surface Chemistry


Class 12 Science Practical Projects Viva Voce Guide
 SURFACE CHEMISTRY (Part - I) - 12th Viva Voce guide representative image
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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