Surface Chemistry Class 12 Chemistry Important Questions

Please refer to Surface Chemistry Class 12 Chemistry Important Questions with solutions provided below. These questions and answers have been provided for Class 12 Chemistry based on the latest syllabus and examination guidelines issued by CBSE, NCERT, and KVS. Students should learn these problem solutions as it will help them to gain more marks in examinations. We have provided Important Questions for Class 12 Chemistry for all chapters in your book. These Board exam questions have been designed by expert teachers of Standard 12.

Class 12 Chemistry Important Questions Surface Chemistry 

Very Short Answer Questions

Question. What happens when gelatin is added to gold solution?
Answer. Gold solution which is lyophobic solution starts behaving like a lyophilic colloid when gelatin is added to it.

Question. Define peptization.l
Answer. Peptization is the process of conversion of a precipitate into colloidal state in the presence of some electrolyte.

Question. Which complex ion is formed when undecomposed AgBr is washed with hypo solution in photography ?
Answer. The developed film is immersed in sodium thiosulphate (hypo) solution which removes unchanged silver bromide as a complex ion.
This is known as fixing.
AgBr + 2Na₂S₂O₃ → Na₃[Ag(S₂O₃)₂] + NaBr
After fixing, the film is not sensitive to light.

Question. Explain the following terms giving a suitable example :
Emulsification
Answer. Emulsification : The process of making emulsion is known as Emulsification. To stabilise an emulsion, an emulsifying agent or emulsifier is added. Soaps and detergents are most frequently used emulsifiers.

Question. Define the following term :
Electrophoresis
Answer. The movement of colloidal particles under an applied electric potential is called electrophoresis. Positively charged colloidal particles move towards the cathode, while negatively charged particles move towards the anode.

Question. Write the main reason for the stability of colloidal sols.
Answer. The main reason for the stability of colloids is the electrostatic stabilisation i.e., equal and same type of charge on the colloidal particles which causes repulsion between them and prevents the coagulation of the sol.

Question. Write the dispersed phase and dispersion medium of butter.
Answer. Dispersed phase : Liquid
Dispersion medium : Solid

Question. In reference to surface chemistry, Define dialysis.
Answer. Dialysis : It is the process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane. A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing. The molecules and ions diffuse through membrane into the water and pure colloidal solution is left behind.

Question. What is the difference between lyophobic sol and lyophilic sol ?
Answer. Lyophilic sols : The colloidal solution in which particles of the dispersed phase have a strong affinity for the dispersion medium. These colloidal sols, even if precipitated, change back to the colloid form simply by adding dispersion medium. So, lyophilic sols are reversible in nature. e.g., glue, starch, rubber, etc. Lyophobic sols : The colloidal solution in which particles of the dispersed phase have no or very little affinity for dispersion medium. These are irreversible in nature i.e., once precipitated, they have little tendency to get back into the colloidal form on simply adding dispersion medium e.g., As₂S₃ solution. Lyophobic sols need stabilising agents for their preservation.

Question. Which aerosol depletes ozone layer?
Answer. CFC (Chlorofluorocarbon)

Question. What are physisorption and chemisorption?
Answer. Physisorption : The phenomenon in which adsorbate and adsorbent are held by van der Waals’ forces.
It is reversible in nature. e.g., setting a layer of dust particles on the furniture.
Chemisorption : The phenomenon in which adsorbate and adsorbent are held by chemical bonds.
It is irreversible in nature. e.g., painting on a furniture.

Question. Out of BaCl₂ and KCl, which one is more effective in causing coagulation of a negatively charged colloidal sol? Give reason.
Answer. BaCl₂ is more effective in causing coagulation of negatively charged colloidal sol. Because greater the valency of the coagulating ion, greater is its power to bring about coagulation.

Question. Name the two types of adsorption phenomenon.
Answer. The two types of adsorption phenomenon are chemisorption or chemical adsorption and physisorption or physical adsorption.

Question. Why does physisorption decrease with the increase of temperature ?
Answer. Since, adsorption is exothermic and according to Le-chatelier’s principle, low temperature is favourable for physical adsorption hence, physisorption decreases with increase in temperature.

Question. Give one example each of ‘oil in water’ and ‘water in oil’ emulsion.
Answer. Oil in water emulsion : Milk
Water in oil emulsion : Butter

Question. Based on the type of dispersed phase, what type of colloid is micelles.
Answer. Associated colloids

Question. To which colloidal system does milk belong ?
Answer. Emulsion

Question. Explain the following :
Artificial rain is caused by spraying salt over clouds.
Answer. Clouds are colloidal dispersion of water particles in air. These water particles carry some charge over them. On spraying oppositely charged colloidal dust or sand particles over a cloud from an aeroplane, the colloidal water particles present in the cloud will get neutralized and as a result they will come closer and will grow in size to form bigger water drops and ultimately will coagulate or precipitate causing artificial rain.

Question. Give reasons for the following observations :
A delta is formed at the meeting point of sea water and river water
Answer. The two types of emulsions are :
(i) Oil-in-water type in which small droplets of an oil are dispersed in water.
Example : Milk, cod liver oil.
(ii) Water-in-oil type in which water droplets are dispersed in an oil medium.
Example : Butter.

Short Answer Questions

Question. Explain the cleaning action of soap. Why do soaps not work in hard water?
Answer. The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part is in the oil droplet and hydrophilic part interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus, soap helps in emulsification and washing away of oils and fats. The negatively charged sheath around the globules prevents them from coming together and forming aggregates.

Hard water contains calcium and magnesium ions. These ions form insoluble calcium and magnesium salts when sodium or potassium soaps are dissolved in hard water. These insoluble soaps separate as scum in water and are useless as cleansing agent.

Question. Give reasons for the following observations :
(i) Leather gets hardened after tanning.
(ii) Lyophilic sol is more stable than lyophobic sol.
Answer. (i) Animal hides are colloidal in nature. When a hide, which has positively charged particles is soaked in tannin, containing negatively charged colloidal particles, mutual coagulation takes place. This results in the hardening of leather.
(ii) Lyophilic sol is more stable than lyophobic sol because It is highly hydrated in the solution.

Question. (i) Out of MgCl₂ and AlCl₃, which one is more effective in causing coagulation of negatively charged sol and why ?
(ii) Out of sulphur sol and proteins, which one forms multimolecular colloids?
Answer. (i) According to Hardy-Schulze rule, for negatively charged sol greater the valency of positive ion added to it, greater is its coagulation power. In AlCl₃, Al has +3 charge which is more than Mg with +2 charge in MgCl₂. us, AlCl₃ is more effective in causing coagulation of negatively charged sol.
(ii) Proteins are macromolecules which cannot form multimolecular colloids while sulphur sol have smaller S8 molecules which can form multimolecular colloids.

Question. (i) Same substances can act both as colloids and crystalloids. Explain
(ii) What will be the charge on AgI colloidal particles when it is prepared by adding small amount of AgNO₃ solution to KI solution in water? What is responsible for the development of this charge ?
Answer. (i) The same substance can act as both colloid and crystalloid. It depends on the size of the particles.
When the size of the particles lies between 1 to 1000 nm, it behaves as a colloid. If particle size is less than 1 nm, it exists as a true solution and behave like a crystalloid.
(ii) When AgNO₃ solution is added to aqueous KI solution, a negatively charged sol of Agl is formed.
This is due to selective adsorption of l– ions from the dispersion medium.

Question. (i) Based on type of particles of dispersed phase, give one example each of associated colloid and multimolecular colloid
(ii) Write an important characteristic of lyophilic sols.
Answer. (i) Associated colloid : Soap
Multimolecular colloid : Sulphur sol
(ii) Lyophilic sols are reversible sols. These are quite stable and cannot be easily precipitated.

Question. Write the dispersed phase and dispersion medium of the following colloidal systems :
(i) Smoke
(ii) Milk
Answer. (i) Dispersed phase of smoke = Solid
Dispersion medium of smoke = Gas
(ii) Dispersed phase of milk = Liquid
Dispersion medium of milk = Water (liquid)

Question. Define the following terms giving an example of each :
(i) Emulsion
(ii) Hydrosol
Answer. (i) An emulsion : It is a colloidal system when both the dispersed phase and the dispersion medium are in the liquid state. e.g., milk.
(ii) A hydrosol : It is a colloidal solution of a solid in water as the dispersion medium. e.g., starch solution.

Long Answer Questions

Question. How do the size of particles of adsorbent, pressure of gas and temperature influence the extent of adsorption.
Answer. Size of adsorbent particles : Smaller the size of adsorbent particles, larger is the surface area and hence, higher is the adsorption.
Pressure : Increase in pressure forces gas molecules to come closer to the surface of adsorbent leading to increase in the amount of adsorption.
Temperature : Adsorption is an exothermic reaction hence is favoured at lower temperature, at higher temperature the K.E. of adsorbate is high and hence extent of adsorption is low.

Question. Explain what is observed when :
(i) A beam of light is passed through a colloidal solution.
(ii) NaCl solution is added to hydrated ferric oxide sol.
(iii) Electric current is passed through a colloidal solution.
Answer. (i) Scattering of light by the colloidal particles takes place and the path of light becomes visible (Tyndall effect).
(ii) The positively charged colloidal particles of
ferric hydroxide sol get coagulated by the oppositely charged Cl^– ions provided by NaCl.
(iii) On passing electric current through a sol, colloidal particles start moving towards oppositely charged electrodes where they lose their charge and get coagulated (electrophoresis).

Question. Giving appropriate examples, explain how the two types of processes of adsorption (physisorption and chemisorption) are influenced by the prevailing temperature, the surface area of adsorbent and the activation energy of the process?
Answer. Effect of temperature : Physisorption decreases with increase of temperature and chemisorption first increases then decreases with increase of temperature.
Surface area : Greater the surface area, greater is the physisorption and chemisorption.
Activation energy : In physisorption, no appreciable activation energy is needed. In chemisorption, sometimes high activation energy is needed.

Question. What is meant by coagulation of a colloidal solution? Describe briefly any three methods by which coagulation of lyophobic sols can be carried out.
Answer. The process of setting of colloidal particles is called coagulation of the sol. It is also known as precipitation. Following are the three methods by which coagulation of lyophobic sols can be carried out :
(i) Electrophoresis: In this process, the colloidal
particles move towards oppositely charged electrodes and get discharged resulting in coagulation.
(ii) Mixing of two oppositely charged sols: When equal proportions of oppositely charged sols are mixed, they neutralise each other resulting in coagulation.
(iii) Dialysis: By this method, electrolytes present in sol are removed completely and colloid becomes unstable resulting in coagulation.