Separation of Substances

Why this chapter matters for UPSC: Prelims general science regularly asks which method separates which mixture, and the answer always turns on one property of the components: size, weight, solubility, boiling point or density. This chapter teaches that logic with kitchen and farm examples; NCERT Class IX then extends it to centrifugation, chromatography, distillation and the city water works. In NCERT's rationalised 2023-24 edition this is Chapter 3, "Separation of Substances", with the text essentially unchanged.

🧠 First Principles — Read This First

NCERT starts with things we already do: grain is separated from stalks at harvest, milk or curd is churned to take out butter, tea leaves are strained from tea, and chillies are picked out of poha. The question is why we separate at all, and how we choose a method.

Why: before we use a substance we often need to remove harmful or non-useful materials mixed with it (stones in rice), and sometimes we separate two useful components so that each can be used on its own (butter from curd).

How: the components of a mixture may differ in size, weight or the way they behave in water, and they may be solids, liquids or gases. Each method uses one such difference:

• Size: handpicking (large, visible bits), sieving and filtration (small particles pass, large ones stay).
• Weight: winnowing (wind carries the light husk away), sedimentation (heavy particles settle in water).
• Solubility: a dissolved solid passes through any filter, so it is recovered by evaporation.

Often one method is not enough, and several are used in turn.

PART 1 — Quick Reference

Why We Separate (NCERT Table 5.1)

NCERT Class VI Methods

PART 2 — Concepts & Narrative

Sedimentation, Decantation and Filtration

When rice or pulses are washed, the lighter dust goes into the water and the heavier grain sinks. Sedimentation is the settling of the heavier component after water is added; decantation is pouring off the water (with the dust) from above it. NCERT applies the same principle to two liquids that do not mix, such as oil and water: left to stand, they form two layers, and the top layer can be decanted.

Decantation leaves a few tea leaves in tea; a strainer catches them all. This is filtration. Muddy water left to stand for half an hour drops most of its soil (sedimentation); the water poured off (decantation) is still brown, so it is filtered, first through cloth and then through filter paper, which has very fine pores. Fruit and vegetable juices are filtered to remove seeds and pulp, and paneer is made by adding a few drops of lemon juice to boiling milk and filtering the solid paneer through a fine cloth.

NCERT ends the chapter with a useful trap: a mixture of milk and water cannot be separated by filtration, because nothing in it is large enough to be caught by a filter.

Evaporation, Condensation and Using More Than One Method

Evaporation is the conversion of water into its vapour; it takes place continuously wherever water is present. Heating salt water until the water boils away leaves the salt behind. Sea water contains many salts: when it stands in shallow pits, the sun heats it, the water evaporates over a few days, and the solid salts remain. Common salt is then obtained from this mixture of salts by further purification.

Condensation is the conversion of water vapour back into liquid. NCERT shows it by holding a metal plate cooled with ice over the spout of a boiling kettle: drops of water form on the plate and fall. The same thing happens under a plate used to cover a vessel of freshly boiled milk.

A mixture of sand and salt needs several methods. Add water: the salt dissolves and the sand settles. Remove the sand by decantation or filtration. Boil the salty water in a kettle: the steam condenses on the cold plate as pure water, and salt is left in the kettle. Sand, salt and water are thus separated by decantation, filtration, evaporation and condensation.

Can Water Dissolve Any Amount of a Substance?

Solutions, Suspensions and Colloids (NCERT Class IX)

NCERT Class IX explains why some mixtures pass straight through a filter. The answer is particle size.

More Separation Techniques (NCERT Class IX)

Drinking Water: Separation at City Scale

Petroleum Refining: Separation as an Industry

PART 3 — UPSC Integration

Separation is the step that turns a mixture into something usable, and the same handful of principles runs from the kitchen to national infrastructure. Size and weight clean grain on the threshing floor; settling, loading, filtering and chlorinating supply city water; evaporation in coastal salt pans and crystallisation give us common salt; fractional distillation turns crude oil into LPG, petrol, kerosene, diesel and bitumen, and liquid air into oxygen and nitrogen; centrifuges separate blood for diagnosis. In Prelims the skill tested is matching a mixture to its method by naming the property that differs. In Mains, the drinking-water treatment chain and petroleum refining are the two places where this chapter's vocabulary is used directly.

Exam Strategy

Prelims traps:

• Winnowing uses wind (or blowing air) to separate heavier and lighter components; threshing beats grain free of the stalk; sieving sorts by size.
• Sedimentation is the settling; decantation is the pouring off. They are two steps, not one.
• Filtration cannot separate a dissolved solid (salt or sugar in water) or a colloid (milk): NCERT's own true-or-false says milk and water cannot be separated by filtration.
• Tyndall effect: shown by colloids and suspensions; not by true solutions.
• Distillation separates miscible liquids with different boiling points; fractional distillation when the difference is under 25 K (air, petroleum). Immiscible liquids (oil and water) need a separating funnel or decantation.
• Sublimation separates camphor, ammonium chloride, naphthalene or anthracene from a non-subliming solid such as salt.
• Chromatography separates solutes dissolved in the same solvent; the more soluble component travels faster.
• Loading = alum (phitkari) used to settle fine mud.
• Saturated = no more of the substance can dissolve; heating lets more dissolve.

Mains angle: Explain the stages of drinking-water treatment as a sequence of separation methods, or explain how refining separates crude oil into its fractions.

Practice Questions

Practice (UPSC-pattern, not past papers).

Prelims:

1. Lighter husk is separated from heavier grain seeds by wind or by blowing air. The method is called:
   (a) Sieving
   (b) Winnowing
   (c) Threshing
   (d) Decantation

2. Two miscible liquids whose boiling points differ by less than 25 K are best separated by:
   (a) Simple distillation
   (b) A separating funnel
   (c) Fractional distillation
   (d) Centrifugation

3. Consider the following statements about a colloid such as milk:
   

   1. Its particles scatter a beam of light passing through it.
      
   2. It can be separated into its components by filtration.
      
   3. Its particles do not settle when it is left undisturbed.
      Which of the statements given above are correct?
      (a) 1 and 2 only
      (b) 2 and 3 only
      (c) 1 and 3 only
      (d) 1, 2 and 3

4. Sublimation is the appropriate method to separate:
   (a) Sand from salt
   (b) Camphor from salt
   (c) Kerosene from water
   (d) Cream from milk

5. According to NCERT, which of the following is the correct order of stages in a water works?
   (a) Filtration, sedimentation, chlorination, loading
   (b) Sedimentation, loading, filtration, chlorination
   (c) Loading, chlorination, sedimentation, filtration
   (d) Chlorination, sedimentation, loading, filtration

6. Consider the following pairs:
   

   1. Threshing : separating grain from stalks
      
   2. Churning : separating butter from curd
      
   3. Loading : using alum to settle fine mud in water
      How many of the pairs given above are correctly matched?
      (a) Only one
      (b) Only two
      (c) All three
      (d) None

7. To make lemonade with as much sugar dissolved as possible, the sugar should be added:
   (a) Before adding ice, because more sugar dissolves in the warmer liquid
   (b) After adding ice, because cold water dissolves more sugar
   (c) At any time, because temperature does not affect how much dissolves
   (d) After adding ice, because ice adds more water

Mains (UPSC-pattern):

1. "Every stage of drinking-water treatment except disinfection is a separation method." Explain with reference to the stages of a water works. (150 words)
2. Why are the fractions of petroleum separated by fractional distillation rather than simple distillation? Name four fractions and their uses. (150 words)

📦 Revision Capsule

Sources

Sources

• NCERT, Science (Class VI), ch. 5 "Separation of Substances" (2020-21 print) and ch. 3 in the Rationalised 2023-24 edition: NCERT's book archives as captured by the Wayback Machine, 2020 copy and 2023-24 copy.
• NCERT, Science (Class IX), ch. 2 "Is Matter Around Us Pure?": 2021-22 print (separation techniques, water works, Fig. 2.13) and the Reprint 2025-26 book (solutions, suspensions, colloids).
• NCERT, Science (Class VIII), ch. 3 "Coal and Petroleum", Reprint 2024-25: book archive.