Question
Why is a colloid considered stable?
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RRB ALP 2025 CBT 2 Wiremen Question Paper (28-Jul-2026) (Shift 2)
The correct answer is
Its particles do not settle down when left undisturbed.
Colloid Stability Explained
A colloid is considered stable primarily because its dispersed particles are kept suspended within the dispersion medium and do not separate out easily.
Key Reason for Colloid Stability
- Non-Settling Particles: The main reason a colloid is stable is that its dispersed particles are small enough and possess enough kinetic energy (due to Brownian motion) or surface charge repulsion to remain suspended indefinitely. They do not settle down under the influence of gravity when left undisturbed. This is the defining characteristic of a stable colloidal system.
Analyzing Other Options
- Option 1 (Dissolve completely): This describes a true solution, where particles are molecularly dispersed and completely dissolved, not a colloid.
- Option 2 (Separated by filtration): Colloidal particles cannot be easily separated by ordinary filtration, though they can be separated by ultrafiltration. This is a property, not the primary reason for stability.
- Option 4 (Chemical reaction): Stability in colloids is generally due to physical factors (Brownian motion, electrostatic repulsion), not chemical reactions with the medium. Chemical reactions can often lead to destabilization.
Therefore, the inability of colloidal particles to settle down is the hallmark of their stability.
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