Surface chemistry deals with the phenomenon that occurs at the surfaces or interfaces. The interface or surface is represented by separating the bulk phases by a hyphen or a slash. For example, the interface between a solid and a gas may be represented by solid-gas or solid/gas. Due to complete miscibility, there is no interface between the gases. The bulk phases that we come across in surface chemistry may be pure compounds or solutions. The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallization occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.
The essential difference between a solution and a colloid is of their
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Particle size
A true solution and a colloid differ in the size of the dispersed particles. In a solution the particles are ions or small molecules (below about 1 nm); in a colloid the dispersed phase consists of particles between about 1 nm and 1000 nm, which may be single macromolecules or aggregates of many atoms, ions or molecules.
Why the other options are incorrect
Option (1) Composition — Incorrect. The same substance can form either a solution or a colloid (e.g. sulphur, gold); composition is not what separates them.
Option (2) Density — Incorrect. Density depends on the substances and amounts used and does not mark the difference between the two systems.
Option (3) Boiling point — Incorrect. The boiling point of the mixture is not the basis of the classification; colloids are classified by particle size.
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):
Colloids – “… The essential difference between a solution and a colloid is that of particle size. While in a solution, the constituent particles are ions or small molecules, in a colloid, the dispersed phase may consist of particles of a single macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules. Colloidal particles are larger than simple molecules but small enough to remain suspended. Their range of diameters is between 1 and 1000 nm (10–9 to 10–6 m).”