Chapter 3: Separation of Substances
पदार्थों का पृथक्करण (हस्त चयन, थ्रेशिंग, निष्पावन, चालन, अवसादन, निस्तारण, निस्संदन, वाष्पन)
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- Handpicking (हस्त चयन): Separating slightly larger impurities (stones, dirt, husk) from food grains by hand when quantities are modest.
- Threshing (थ्रेशिंग / गहाई): Beating harvested crop stalks against a hard surface, stone, or using mechanical threshers to separate grain seeds from stalks.
- Winnowing (निष्पावन): Separating heavier components (food grains) from lighter components (husk / chaff) using natural wind or an artificial blower. Heavy grains drop straight down; light husk is carried away by air currents.
- Sieving (चालन): Separating solid particles of different sizes through a mesh screen. Smaller particles pass through; larger impurities remain on the sieve (e.g. flour sifting, separating pebbles from construction sand).
- Sedimentation & Decantation: In an insoluble solid-liquid mixture (e.g. muddy water), allowing heavy particles to settle to the bottom is Sedimentation (अवसादन). Pouring off clear supernatant water without disturbing the sediment is Decantation (निस्तारण).
- Filtration (निस्संदन): Separating fine insoluble suspended solids from a liquid using a porous medium (filter paper, muslin cloth). The liquid passing through is filtrate; the solid left behind is residue. Used in separating tea leaves, fruit pulp, and cottage cheese (paneer).
- Evaporation & Condensation: Evaporation converts liquid to vapour to recover dissolved solids (salt from sea water). Condensation cools vapour back into liquid.
- Saturated Solution: A solution in which no more solute can dissolve at that specific temperature. On heating, solubility increases and more solute can be dissolved.
- Trap: Winnowing works on difference in MASS/DENSITY with wind, NOT particle size. Particle size separation is Sieving.
- Trap: Multi-step separation order for Sand + Salt + Water mixture: 1. Dissolution of salt in water -> 2. Filtration (sand collected on filter paper) -> 3. Evaporation of filtrate (pure salt recovered) -> 4. Condensation (pure water collected).
- Trap: Does paneer making utilize filtration? YES, curdled milk solids are separated from liquid whey using a piece of fine cloth or strainer (Filtration).
NCERT Core Facts: Physical Separation Methods in Laboratory & Daily Life
CTET Paper 2 regularly tests multi-step procedural separation and daily-life examples:
1. Comparative Breakdown of Separation Methods
| Technique | Operating Principle | Common Application | CBSE Exam Trap |
|---|---|---|---|
| Threshing (थ्रेशिंग) | Mechanical impact separates grain from stalks | Wheat and paddy harvest | Done by bullocks, beating, or machines |
| Winnowing (निष्पावन) | Difference in weight/density acted on by wind | Separating dry husk from wheat grains | Heavy grains fall vertically; light husk drifts away |
| Sieving (चालन) | Difference in particle size | Sand vs gravel at construction sites | Cannot separate substances of identical particle size |
| Sedimentation (अवसादन) | Gravitational settling of denser insoluble solids | Mud settling at bottom of bucket | Requires standing undisturbed |
| Decantation (निस्तारण) | Gentle pouring of supernatant liquid | Removing water after washing rice/pulses | Separates immiscible liquids (oil and water) as well |
| Filtration (निस्संदन) | Liquid passes through microscopic pores; solid caught | Making paneer (chhena); brewing tea | Better than decantation for fine suspended particles |
| Evaporation (वाष्पन) | Liquid boils/evaporates leaving non-volatile solute | Salt recovery from shallow sea-water lagoons | Water is lost unless collected by condensation |
2. High-Yield Sequence: Separating Multi-Component Mixtures
To separate a mixture of Iron filings, Sand, and Common Salt:
1. Magnetic Separation: Pass a magnet over mixture ightarrow Iron filings attracted and removed.
2. Dissolution: Add water to remaining sand and salt mixture ightarrow Salt dissolves, sand remains insoluble.
3. Filtration: Pass through filter paper ightarrow Sand is retained as residue on filter paper; salt water passes through as filtrate.
4. Evaporation: Heat filtrate in an evaporating dish ightarrow Water evaporates, leaving dry white salt crystals.
Test Your Mastery: Separation of Substances
Authentic CBSE exam questions tested directly from this NCERT chapter.
A teacher asks students to separate a mixture of Sand and Common Salt. What is the correct chronological sequence of operations required?
एक शिक्षक छात्रों से रेत और साधारण नमक के मिश्रण को अलग करने के लिए कहता है। आवश्यक प्रक्रियाओं का सही कालानुक्रमिक क्रम क्या है?
Which of the following mixtures CANNOT be separated by the method of winnowing?
निम्नलिखित में से किस मिश्रण को निष्पावन (winnowing) विधि द्वारा अलग नहीं किया जा सकता है?
Which method is used by farmers to separate husk from grain seeds in open fields?
Winnowing (निष्पावन), where the farmer stands at a raised height and lets the mixture fall gently while wind blows away the lighter husk.
How is common salt obtained from sea water on a commercial scale?
Sea water is trapped in shallow pits (salt pans). Sunlight and wind cause the water to evaporate, leaving behind heaps of solid crude salt.