Separation of Substances and Mixtures
Separation of Substances and Mixtures
Separation techniques give 1 to 2 questions and are almost always asked as a matching exercise: which method suits which mixture. Learn each method by the property it exploits, because that is what lets you answer an unfamiliar pairing.
The Methods and What They Exploit
| Method | Exploits | Used for |
|---|---|---|
| Handpicking | Visible difference | Stones from rice |
| Threshing | Force | Grain from stalks |
| Winnowing | Difference in weight | Husk from grain |
| Sieving | Difference in size | Flour from bran |
| Sedimentation and decantation | Density | Mud from water |
| Filtration | Particle size | Tea leaves from tea |
| Evaporation | Boiling point | Salt from sea water |
| Distillation | Boiling point | Pure water from salt water |
| Sublimation | Direct solid to gas | Camphor from salt |
| Magnetic separation | Magnetic property | Iron filings from sand |
| Chromatography | Different solubility | Dyes in ink |
The property column is the real content. Once you know that filtration separates by particle size and distillation by boiling point, an unseen mixture can be assigned to the right method without memorising every pair.
Evaporation Versus Distillation
Both use boiling point, but they keep different products.
- Evaporation keeps the dissolved solid and lets the liquid escape — used to obtain salt from sea water.
- Distillation keeps the liquid by condensing the vapour — used to obtain pure water from salt water.
- Choose by asking which component the question wants.
✗ Distillation is used to obtain salt from sea water | ✓ Evaporation gives the salt; distillation gives the pure water
Solutions, Solubility and Saturation
A solution is a homogeneous mixture of a solute dissolved in a solvent. Salt in water is a solution; the salt is the solute and the water the solvent, called the universal solvent because it dissolves so many substances.
- A saturated solution can dissolve no more solute at that temperature.
- Solubility of most solids increases with temperature.
- Solubility of gases decreases with temperature, which is why warm water holds less dissolved oxygen.
- A suspension has visible undissolved particles; a colloid lies between a solution and a suspension.
- Milk is a colloid, not a solution — a commonly asked classification.
The opposite behaviour of solids and gases with temperature is a genuine discriminator, and the aquatic-life consequence makes it a favourite in environment-linked questions.
Water Purification in Practice
For drinking water, sedimentation and filtration remove suspended matter, while boiling or chlorination kills microbes. A domestic candle filter works by particle size and does not remove dissolved salts, which is why filtered water is not necessarily pure.
That last distinction — clear water is not the same as pure water — is a public-health point a teacher is expected to convey, and it is asked in exactly that form.
वाष्पीकरण से ठोस मिलता है, आसवन से शुद्ध तरल।
TRE pointer: This topic is set as a matching question in almost every paper, so the property column of the table above is what to revise. The evaporation versus distillation pair is the most repeated single item, and the two products are offered as alternatives. The gas-solubility-falls-with-temperature fact is the best discriminator. With −1/3 negative marking, read whether the question asks for the solid or the liquid product.
60-Second Recap
- Each method exploits one property: size, weight, density, boiling point or magnetism.
- Winnowing uses weight; sieving uses size; filtration uses particle size.
- Evaporation keeps the solid; distillation keeps the liquid.
- Chromatography separates dyes by differing solubility.
- Solubility of solids rises with temperature; solubility of gases falls.
- Milk is a colloid; filtered water is not necessarily pure water.
Frequently Asked Questions
What is the difference between evaporation and distillation?
Both depend on boiling point, but evaporation lets the liquid escape and keeps the dissolved solid, as when salt is obtained from sea water. Distillation condenses and collects the vapour, keeping the pure liquid instead. Which one to use depends on which component you want.
Why does warm water hold less dissolved oxygen?
Because the solubility of gases decreases as temperature rises, unlike that of most solids. Warmer water therefore holds less dissolved oxygen, which is why thermal pollution of rivers harms fish. The opposite behaviour of solids and gases is asked as a comparison.
Is milk a solution?
No, milk is a colloid. Its particles are larger than those in a true solution but do not settle out as they would in a suspension, and they scatter light. Classification questions regularly place milk among options with salt water and muddy water.
Does filtered water become pure water?
Not necessarily. A candle filter removes suspended particles by size but leaves dissolved salts and microbes behind. Making water safe to drink also needs boiling or chlorination. Clear water and pure water are not the same thing, and questions test that distinction.