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NCERT Solutions for Class 9th Science Chapter 5 .4.3 Suspensions — In-text Questions

Book page 855 Updated on2026-09-08

Q1.
How can we separate mud from water?
Answer

Start with the simplest method and add another only if the water is still not clear.

  1. Sedimentation and decantation. Let the muddy water stand. The heavier mud particles settle at the bottom; pour off the clearer water carefully without disturbing the sediment.
  2. Filtration. Pass it through filter paper or a clean cotton cloth. Particles larger than the pores are held back as residue.
  3. Coagulation, then filtration. Add powdered alum (fitkari). It makes the fine particles clump into larger flocs that settle by gravity and can then be removed by decantation or filtration.
  4. Centrifugation. Spin a sample at high speed; the centrifugal force drives even very fine particles to the bottom of the tube.
Why one method is often not enough: Muddy water contains particles over a very wide range of sizes. The big ones settle in minutes and are caught by filter paper. The finest clay particles are so light that molecular jostling keeps them suspended, and they are small enough to pass straight through the pores of the paper — which is why the water often stays cloudy even after filtering. Those particles must first be made bigger (coagulation) or be pushed down by a force stronger than gravity (centrifugation).
Q2.
If the muddy water is still not clear even after keeping for some time, how can it be cleaned?
Answer

By centrifugation and/or coagulation — the two techniques the chapter introduces for exactly this situation.

Coagulation. Add a pinch of powdered alum, a white crystalline substance used in water purification. Alum makes the fine suspended particles stick together in clumps. These larger, heavier clumps settle rapidly under gravity — the process called sedimentation — and the clear water above can then be decanted or filtered off.

Centrifugation. Take the cloudy water in a tube and spin it at high speed. The tubes swing out to the horizontal, and the centrifugal force acting outwards on each particle is many times greater than its weight, so even the finest particles are driven to the bottom of the tube while clear liquid remains on top.

Why these work when filtration does not: Filtration is a sieve, and it can only remove particles bigger than its pores. Coagulation attacks the problem from the other side — instead of making the sieve finer, it makes the particles bigger until they settle on their own. Centrifugation changes the force instead of the particle: it replaces gravity with a much stronger outward force, so particles that would take days to settle sink in minutes.
Did you know? A sewage treatment plant uses this very sequence — sedimentation, then coagulation, then filtration — before the treated water is released or reused for flushing and watering plants.
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