Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Why Separate Substances?
- To remove impurities — stones from rice, husk from grain.
- To get a pure substance — salt from seawater.
- To separate useful components — cream from milk.
Methods of Separation
1. Handpicking
- Used when impurities are large and few.
- Done by hand — picking out stones from pulses, spoiled fruits from good ones.
2. Threshing
- Used to separate grain seeds from stalks after harvesting.
- The crop is beaten against a hard surface or a machine is used.
3. Winnowing
- Used to separate lighter materials from heavier ones using wind or blowing.
- Example: Separating husk (light) from grain (heavy).
- How: Mixture is dropped from a height. Wind blows away the lighter husk; heavier grain falls in a pile.
4. Sieving
- Used to separate components of different sizes.
- A sieve (mesh with holes) allows smaller particles to pass through; larger ones are retained.
- Example: Separating bran from flour, sand from gravel.
5. Sedimentation
- Heavier solid particles in a liquid settle to the bottom when left undisturbed.
- The settled solid is called sediment.
- Example: Mud particles settle when muddy water is left still.
6. Decantation
- The clear liquid is carefully poured from the top after sedimentation, without disturbing the sediment.
- Example: Pouring clear water off settled mud.
7. Loading (Alum treatment)
- Fine suspended particles (like clay) are too light to settle on their own.
- Alum (phitkari) is added to cause fine particles to clump and settle faster.
- Used in water treatment plants.
8. Filtration
- Used to separate an insoluble solid from a liquid.
- The mixture is poured through a filter paper placed in a funnel.
- Solid stays on filter paper (called residue); liquid passes through (called filtrate).
- Example: Separating sand from water.
- Cannot be used for dissolved substances (e.g., salt dissolved in water).
9. Evaporation
- Used to separate a dissolved solid from a liquid.
- Liquid is heated — it evaporates as vapour. Solid is left behind.
- Example: Getting salt from saltwater, sugar from sugarcane juice.
10. Condensation
- Water vapour cools and turns back into liquid water.
- Used to collect pure water (distillation process).
Comparison Table
| Method | Used For | Example |
|---|---|---|
| Handpicking | Large impurities | Stones from rice |
| Winnowing | Light from heavy | Husk from grain |
| Sieving | Different sized particles | Bran from flour |
| Sedimentation | Settling of heavy particles | Mud from water |
| Decantation | Pouring off clear liquid | Water from mud |
| Filtration | Insoluble solid from liquid | Sand from water |
| Evaporation | Dissolved solid from liquid | Salt from seawater |
Saturated Solution
- A solution in which no more solid can be dissolved at a given temperature.
- Example: When you keep adding sugar to water and it stops dissolving — the solution is saturated.
Key Vocabulary
| Term | Meaning |
|---|---|
| Residue | Solid left behind on filter paper |
| Filtrate | Liquid that passes through filter |
| Sediment | Solid that settles at the bottom |
| Winnowing | Separation using wind/blowing |
| Threshing | Separating grain from stalks |
| Evaporation | Liquid changing to vapour on heating |
| Condensation | Vapour changing back to liquid on cooling |
Exam Tips
- Know the right method for the right mixture — a common 2-mark application question (e.g., "Which method would you use to separate salt from water?").
- Filtration works only for insoluble solids — salt dissolved in water cannot be filtered.
- Learn sedimentation + decantation as a pair — they usually go together.
- Evaporation is for dissolved solids; filtration is for undissolved solids — this distinction is tested.
Frequently Asked Questions
We separate substances to remove harmful or unwanted components (stones from rice), to get a pure substance (salt from seawater), or to separate two useful substances that are mixed together.