Materials (Classes VI–VIII) • Topic 2 of 5

Separation of Substances

From NCERT Class 6 'Separation of Substances': we separate the components of a mixture to remove an impurity, to get a useful component, or to remove a harmful one. The method is chosen from the PROPERTY that differs between the components. HANDPICKING — when impurities are few, large and visibly different (stones from rice). WINNOWING — separates heavier and lighter components using wind or blowing air (separating husk/chaff from grain). SIEVING — separates components of different particle SIZE (bran from flour, pebbles from sand). SEDIMENTATION (heavier insoluble settles), DECANTATION (pour off the upper liquid), and FILTRATION — separate an INSOLUBLE solid from a liquid by passing it through a filter (mud from water; tea leaves from tea). EVAPORATION — recover a DISSOLVED (soluble) solid from a liquid by evaporating the liquid (salt from sea water). CONDENSATION turns the vapour back to liquid. MAGNETIC SEPARATION — separate a magnetic component from a non-magnetic one (iron filings from sand or sulphur). Two related facts CTET likes: a SATURATED solution is one that can dissolve no more solute at that temperature, and solubility generally increases when water is heated. The hardest single distinction: filtration removes an UNDISSOLVED (insoluble) solid, whereas evaporation is needed to get back a DISSOLVED (soluble) solid — you cannot filter salt out of salt water. PEDAGOGY/MISCONCEPTION: children often think filtration can separate salt from water; demonstrating that salt passes through the filter and only reappears on evaporation fixes this. HOW TESTED: a mixture is described and you pick the correct method, or you are asked which property the method exploits.

✅ Solved examples

1. Which method would you use to separate salt that has DISSOLVED in water?
Evaporation. Salt is soluble, so it passes through any filter; evaporating the water leaves the salt behind. Filtration would NOT work here.
2. To separate mud (an insoluble solid) suspended in water, the correct method is:
Filtration — passing the mixture through a filter holds back the insoluble mud and lets clear water pass. (Sedimentation followed by decantation also helps but filtration gives clear water.)
3. Husk has to be separated from heavier grains of wheat after threshing. The traditional method that uses blowing air is:
Winnowing — the wind carries the lighter husk away while the heavier grains fall straight down. It exploits the difference in WEIGHT, not size.
4. A mixture of iron filings and sulphur powder is best separated by:
Magnetic separation — a magnet attracts the iron filings (magnetic) and leaves the sulphur (non-magnetic) behind.

✏️ Practice — try these, take hints as needed

1. Flour is to be separated from coarse bran of a different particle size. The suitable method is:
Think of the kitchen chhalni.
Separates by size of particles.
Sieving
2. Why can filtration NOT be used to recover sugar dissolved in water?
Dissolved particles are too small / pass through the filter.
You would need to remove the water instead.
Because the dissolved (soluble) sugar passes through the filter paper; it must be recovered by evaporation
3. A solution that cannot dissolve any more of the solute at a given temperature is described as:
It is full / cannot take more.
Heating it usually lets it dissolve more.
A saturated solution
4. Stones are to be removed from a small quantity of rice. The simplest method is:
Impurities are few and visible.
Done by hand.
Handpicking

📝 Topic test — 8 questions

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