Answer the question on basis of passage given below:
Lyophobic sols like those of metals (Au, Ag, etc) are unstable and are easily precipitated by addition of electrolytes. However, it is observed that the addition of certain lyophilic colloids like gums, soaps, gelatine, etc. to lyophobic colloids (like a metal sol) render lyophobic colloids difficult to coagulate by the addition of electrolytes. The process is known as 'protection' and the lyophilic colloids are termed as Protective colloids. It is believed that the protective action of the lyophilic colloids is due to the covering up of the particles of the lyophobic colloid by those of the lyophilic colloid. Evidently, smaller the gold number of a protective colloid, the greater is its protective action.
Which of the following methods cannot be used for purification of colloidal solution?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Peptization
A freshly prepared colloidal solution carries electrolytes and other soluble impurities. Purification removes them, and it is done by dialysis, electro-dialysis and ultrafiltration. All three keep the colloidal particles back and let the small ions and molecules pass through a membrane or an ultra-filter.
Peptization works the other way round: a precipitate is shaken with the dispersion medium and a small amount of electrolyte (the peptizing agent), and the precipitate is broken up into a colloidal sol. It is a method of preparing a sol, and it needs an added electrolyte, so it cannot purify one.
Option (1) Dialysis — A method of purifying a colloidal solution. The sol is kept in a parchment or cellophane bag in running water; ions and small molecules diffuse out and the colloidal particles stay inside.
Option (2) Electrodialysis — A method of purifying a colloidal solution. Dialysis speeded up by an electric field across the membrane, used when the impurity is an electrolyte.
Option (3) Peptization — Not a purification method; a method of preparing a sol. A precipitate is broken up into a sol by adding a small amount of electrolyte, so it adds an electrolyte instead of removing impurities.
Option (4) Ultrafiltration — A method of purifying a colloidal solution. Filter paper made finer with collodion holds back the colloidal particles while the solvent and dissolved impurities pass through.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Purification of Colloidal Solutions – “… The process used for reducing the amount of impurities to a requisite minimum is known as purification of colloidal solution. The purification of colloidal solution is carried out by the following methods: (i) Dialysis: It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane. … (iii) Ultrafiltration: Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by specially prepared filters, which are permeable to all substances except the colloidal particles.”
Peptization
