Separation of Substances

• Why we need to separate mixtures.
• Methods: hand-picking, threshing, winnowing, sieving, sedimentation, decantation, filtration, evaporation, distillation, magnetic separation.
• Which method suits which type of mixture.

Why separate mixtures?

• Most substances found in nature are mixtures (impure).
• Separation removes unwanted components or recovers useful ones.
• Examples: separating grain from husk, purifying drinking water, extracting salt from seawater.

Methods of separation

1. Hand-picking

• Remove large, visible impurities by hand.
• Use: picking stones from rice or dal.
• Limitation: only works when impurities are large and few.

2. Threshing

• Beating harvested stalks against a hard surface to separate grain from stalk.
• Also done by trampling or using machines.
• Use: separating wheat/rice grains from stalks after harvest.

3. Winnowing

• Throwing mixture into air; wind blows away lighter husk; heavier grain falls down.
• Use: separating grain from husk/chaff.
• Principle: difference in weight/density + wind.

4. Sieving

• Passing mixture through a sieve (mesh); smaller particles pass through, larger ones remain.
• Use: separating fine flour from bran; separating sand from gravel.
• Principle: difference in particle size.

5. Sedimentation and Decantation

• Sedimentation: heavier, insoluble particles (sediment) settle to bottom of liquid when left undisturbed.
• Decantation: carefully pouring off the clear liquid above the sediment without disturbing it.
• Use: separating muddy water (mud settles → pour off clear water).
• Principle: difference in density; insoluble solid sinks.

6. Filtration

• Passing mixture through filter paper (or similar); solid particles are trapped (residue); liquid passes through (filtrate).
• Use: separating sand from water; purifying drinking water.
• Principle: difference in particle size — filter paper pores too small for solid particles.
• Residue = what stays on filter; Filtrate = liquid that passes through.

7. Evaporation

• Heating a solution so the liquid (solvent) evaporates, leaving behind the dissolved solid (solute).
• Use: obtaining salt from seawater; recovering sugar from syrup.
• Principle: liquid has lower boiling point than dissolved solid.
• Limitation: solid must not decompose on heating.

8. Condensation / Distillation

• Condensation: cooling a gas/vapour so it becomes liquid again.
• Distillation: heating a liquid to form vapour → cooling vapour → collecting the condensed liquid.
• Use: purifying water; separating liquids with different boiling points; making perfumes, alcohol.
• Simple distillation: used when one component is volatile and the other is not (e.g., water from salt solution — water evaporates, salt stays).

9. Magnetic Separation

• Using a magnet to separate magnetic materials (iron, nickel, cobalt) from non-magnetic ones.
• Use: separating iron filings from sulfur powder; removing iron impurities from food grains.
• Principle: magnetic materials are attracted to magnets; non-magnetic are not.

Choosing the right method

Saturated solution

• A solution = solute dissolved in solvent.
• Saturated solution: no more solute can dissolve at that temperature; any extra settles as undissolved solid.
• Heating increases solubility of most solids → more can dissolve.
• Cooling decreases solubility → solid crystallises out (crystallisation — used to purify salts).

• What is the difference between sedimentation and decantation?
• Name the method used to separate: (a) grain from husk (b) sand from water (c) iron from sulfur.
• What is the principle behind sieving?
• What is filtration? Name the residue and filtrate in filtering muddy water.
• How is salt obtained from seawater? Which method is used?

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