Big Ideas (Why we separate)
- We separate to (a) remove unwanted parts (stones from dal) and (b) obtain two useful components (butter + buttermilk).
- The property that differs between components (size, mass, magnetism, state, etc.) decides the method we choose.
Separation Toolbox at a Glance
| Method | What it separates | Property used | Daily-life example |
|---|---|---|---|
| Handpicking | Solid–solid (small quantity of unwanted pieces) | Visible difference in size/shape/colour | Pick small stones from rice/dal |
| Threshing | Grains from stalks | Grains loosen from stalks on beating | Beating wheat bundles to free grains |
| Winnowing | Lighter husk from heavier grains | Wind/air carries light component farther | Farmer uses soop (bamboo tray) in wind |
| Sieving | Solid–solid of different sizes | Mesh size allows smaller to pass | Sieving flour to remove bran/stones |
| Sedimentation & Decantation | Insoluble solid from liquid; immiscible liquids | Heavier solid settles; lighter layer poured off | Tea leaves settling, then gently pour tea; oil–water separation |
| Filtration | Insoluble solid from liquid (finer) | Pores allow liquid through | Tea strainer; cloth; filter paper cone + funnel |
| Evaporation | Dissolved solid from solution | Liquid vaporises, solid remains | Salt from seawater in salt pans; salt from salt solution on heating |
| Churning | Butter from curd (liquid–liquid/solid) | Density difference (lighter butter floats) | Mathni/whisk to get butter; buttermilk remains |
| Magnetic separation | Magnetic vs non-magnetic | Magnetism | Pick iron nails from sawdust with a magnet; magnets in recycling yards |
Limits: Filtration does not remove dissolved substances (e.g., salt in water). Evaporation yields the solid but usually loses the water (getting both back needs advanced methods beyond this chapter).
"Which Method Do I Use?" – A Quick Decision Map
- 1Is one part magnetic (iron, steel)? → Magnetic separation.
- 2Are pieces visibly different & few? → Handpicking.
- 3Solid sticks to stalk? → Threshing, then winnowing (husk).
- 4Different particle sizes (solid–solid)? → Sieving.
- 5Insoluble solid in liquid (coarse → fine)?
• Coarse → Sediment → Decant
• Fine → Filtration (cloth/filter paper) - 6Two immiscible liquids (oil–water)? → Decantation (let layers form, pour upper).
- 7Solid dissolved in liquid (salt in water)? → Evaporation → recover the solid.
Concept + Classroom Mini-Labs
A) Handpicking
Idea: Works when the unwanted fraction is small and visibly different.
Try: Close your eyes and pick stones from grains—notice how sight helps this method.
B) Threshing → Winnowing
Threshing: Beat dry stalks → grains separate.
Winnowing: In wind/air, light husk blows farther; heavy grains fall close.
Home demo: Rub roasted peanuts between palms → blow gently → skins fly off, nuts fall near you.
C) Sieving
Mesh size matters: if holes are too big, unwanted pieces pass through; if too small, useful flour is lost.
Try: Compare different sieves; note that all holes are equal within one sieve, but sieves can have different mesh sizes.
D) Sedimentation → Decantation
Settle heavy solids; tilt and pour clear liquid slowly.
Demo: Make tea, let leaves settle, pour into cup; or leave oil–water to form layers, then pour the top layer.
E) Filtration
Filters: multi-layer cloth, cotton, sand, filter paper (fine pores).
Lab setup: Fold filter paper cone, place in funnel, collect filtrate in flask; residue stays on paper.
Everyday link: Fishing nets act like large meshes—water passes, fish stay.
F) Evaporation (getting the solid back)
Salt pans: Shallow seawater ponds under sun & wind → water evaporates, salt remains; later purified.
Activity: Drop salt solution on dark paper → after drying, white salt patches appear; heat in china dish to speed up.
G) Churning (butter from curd)
Whisk/Mathni: Lighter butter floats; buttermilk remains.
Question: What kitchen appliance today replaces the mathni?
H) Magnetic Separation
Move a magnet through sawdust mixed with iron nails → nails stick to magnet.
Industry: Magnets on cranes pull out scrap iron from mixed waste for recycling.
Science + Society (Eco & Health Angles)
Water safety:
Simple cloth/charcoal/sand filters improve clarity, but do not remove dissolved salts; boiling makes water microbiologically safer.
Plastic in rivers/oceans:
Nets catch trash too—reduce plastic pollution at source.
Value of recycling:
Magnetic separation helps recover iron for reuse.
HOTS / Exam-Style Practice
- 1
You have a mix of iron nails, sawdust, sand, stones, black pepper, salt, and water. Plan the sequence of methods to separate everything. Justify each step.
- 2
Why is winnowing possible only in moving air? Predict what changes on a still day.
- 3
Explain why filtration removes mud from water but not salt. Which method would you use for salt water? Why?
- 4
Sieving failed to clean flour at home. List three likely causes and fixes.
- 5
Compare sedimentation–decantation with filtration for muddy water—when would you choose each?
Visual Prompts to Add to Your Notes
- Winnowing diagram: farmer + soop, arrows showing wind, light husk carried farther than grains.
- Sieving close-up: sieve mesh vs particle sizes (bran stays, flour passes).
- Filtration setup: folded filter paper cone in funnel → residue/filtrate labels.
- Evaporation: china dish on wire gauze; water boiling off; salt left behind.
- Churning: butter floating; buttermilk below.
- Magnetic separation: nails sticking to a magnet from sawdust.
Quick Recap
- Choose the method by property: size → handpick/sieve; heaviness in air → winnow; settling → sediment/decant; fine insolubles → filter; dissolved solid → evaporate; magnetism → magnet; butter vs buttermilk → churn.
- Methods often work in sequence (e.g., threshing → winnowing).
- Know the limits (filters don't remove dissolved salts; evaporation loses water).