(a) Same substance can act both as colloids and crystalloids.
(b)Artificial rain is caused by spraying salt over clouds.
(c)When a beam of lights is passed through a colloidal sol, the path of the beam gets illuminated.
Ans . (a) 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 1mm, it exists as a true solution and behaves like a crystalloid.
(b)Artificial rain is caused by spraying salt over clouds. The colloidal water particles of the clouds get neutralised by oppositely charged ions of the salt and coagulate to bigger water drops which cause artificial rain.
(c)When a beam of light is passed through a colloidal solution, the path of beam gets illuminated with visible light. This is due to scattering of light by colloidal particels (Tyndal effect)
Q.2 Explain what is observed when: [ 3]
(i) An electrolyte, KCl, is added to hydrated ferric oxide sol (ii) An electric current is passed through a colloidal solution.
(iii) A beam of strong light is passed through a colloidal solution.
Ans . (i)When an electrolyte like KCl is added of Fe(OH)3 sol, the positively charged colloidal particles of Fe(OH)3 get coagulated by the oppositely charged Cl– ions provided by KCl.
(ii)On passing the electric current, colloidal particles move towards the oppositely charged electrode where they lose their charge and get coagulated.
(iii)When a beam of strong light is passed through a colloidal solution scattering of light by colloidal particles takes place and the path of light becomes visible. This phenomenon is called Tyndall effect.
Q.3 Explain the following observations: [ 3]
(i)A beam of light passing through a colloidal solution has a visible path
(ii)Passing an electric current through a colloidal solution removes colloidal particels from it.
(iii) Ferric hydroxide sol coagulates on addition of a solution of potassium sulphate.
Ans . (i)This is due to the scattering of light by colloidal solution.This effect is called Tyndal effect.
(ii)The charged colloidal particles move towards the oppositely charged electrodes on passing current Through the colloidal solution.
(iii) Ferric hydroxide sol is positively charged.When a solution ofK2SO4 is added to it, the sol gets coagulated by the negatively charged SO42– ions furnished by the added electrolyte.
Q.4 What are detergents? Give their scheme of classification. Why are the detergents preferred over soaps? [ 3]
Ans . A detergent is a surface-active agent used for cleaning dirty surfaces. It contains a nonpolar hydrocarbon chain(hydrophobic part) and polar group (hydrophilicpart) with in the molecules and thus shows adsorption and micellization.
On the basis of charge on polar part, detergents are classified as follows :
(i)Anionic detergents in which large part of the molecule acts a anion e.g. alkyl benzene-sulphonate.
(ii)Cationic detergents which are most lyacetates or chlorides of quaternary amines e.g.
Cetyltrimethyl ammoniumchloride.
(iii)Nonionic detergents like esters of high molecular mass formed by reaction between polyethylene glycol and stearic acid.
Detergents are preferred over soaps as they work even in hard water and acidic pH where soaps become in soluble. They have powerful cleansing action.
Q.5 Explain what is observed when [ 3]
(i) An electrolyte is added to ferric hydroxide sol.
(ii) An emulsion is subjected to centrifugation.
(iii) Direct current is passed through a colloidal sol.
Ans. (i)When an electrolyte like NaCl is added to ferric hydroxide sol, the Cl– ions of NaCl neutralize the
positive charge of ferric hydroxide sol particles and coagulation of the sol take place.
(ii)When an emulsion is subjected ti centrifugation, its two liquids separate into different layers i.e. Demulsification occurs.
(iii)When an electric current is passed through a colloidal sol, its colloidal particles move towards oppositely charged electrodes, where they lose their charge and get coagulated.
Q.6 Define adsorption and write two important differences between physical adsorption and chemisorption. [ 3]
Adsorption: It is the phenomenon of attracting and retaining the molecules of a substance on the surface of liquid resulting into a higher concentration on the surface.
Physical adsorption Chemisorption
(i) Forces of attraction between adsorbent (i) Forces between adsorbent and adsorbate are weak Vander Wall’s forces. adsorbate are strong chemical forces.
(ii) Heat of adsorption is low (ii) Heat of adsorption is high
Q. 7 Explain the following terms giving an example in each case. [3]
(i) Electrophoresis (ii)Coagulation (iii) Dialysis (i) Electrophoresis
It is the migration of electrically charged colloidal particles in one direction under the influence of an electric field. When colloidal particles move towards positive electrode, they are negatively charged and vice versa. Electrophoresis is used to measure the rate of migration of sol particles.
(ii) Coagulation
It is the precipitation of colloids by removal of the charge associated with the colloidal particles.
Usually an electrolyte is added to affect coagulation. The sol can also be coagulated by boiling or freezing.
(iii) Dialysis
The process of removing a dissolved substance from the colloidal solution by means of diffusion through a suitable membrane is called dialysis. The apparatus used for this purpose is called dialyser (parchment paper or animal membrane).
Q.8 Differentiate between soaps and detergents. [3]
Q.9 What are the two classes of emulsions? give one example of each class. State on activity to test the type of an emulsion. [ 3]
The two types of emulsions are :
(i)Oil-in-water type in which small droplets of an oil are dispersed in water.
for example milk, cod liver oil.
(ii)Water-in-oil type in which water droples are dispersed in an oil medium.
for example, butter.
Dye test : Some oil soluble dye is added to the emulsion. If the background becomes coloured, the emulsion is water-in-oil type and if the coloured droplets are seen, then the emulsion is oil-in-water type.
Q.10 Explain what is observed when [ 3×1=3]
(a)A beam of light is passed through a colloidal solution ofAs2S3. (b)An electrolyte (NaCl) is added to ferric hydroxide sol.
(c)An electric current is passed through a colloidal solution.
(a)When a beam of light is passed through a colloidal solution ofAs2S3, the path of light becomes visible due to the scattering of light by dispersed phase particles in colloidal range. This is called Tyndal effect.
(b) The colloidal particles get precipitated i.e. ferric hydroxide is precipitated.
(c)When electric current is passed through a colloidal solution, his colloidal particles move towards oppositely charged electrodes where they lose their charge and get coagulated. This is known as electrophoresis.
Q.11 State what you will observe when [ 3]
(i) An electrolyte is added to a colloidal solution.
(ii) An electric current is passed through a colloidal solution.
(iii) A beam of light is passed across pure water and then across a colloidal solution of sulphur when you are not in the path of light.
(i)The addition of electrolyte causes coagulation of the colloidal solution. For example, when a solution of K2SO4 is added to the ferric hydroxide (positively charged) sol, the sol gets coagulated by the negatively charged SO4–2 ions furnished by the added electrolyte.
(ii) The charged colloidal particles move towards the oppositely charged electrode on passing electric current through the colloidal solution.
(iii) The path of the beam of light is not visible through pure water but it becomes visible the colloidal solution of sulphur due to the scattering of light by colloidal particels (Tyndal effect) Q.12 Explain the following terms. [3]
(i)Peptization (ii)Dialysis
(iii)Hardy-Schulze rule
(i) Peptization or Deflocculation is the process responsible for the formation of converting precipitate into colloid by shaking with it an electrolyte. This is particularly important in colloid chemistry or for precipitation reactions in an aqueous solution.