1. Chapter at a Glance
Your grandmother picks small stones out of rice before cooking. A farmer tosses grain in the wind to blow away the husk. Tea leaves are strained out of tea. Butter is churned out of curd. Each of these is a way of separating one substance from a mixture. This chapter describes the common methods of separation used at home, on farms and in industries, and explains why each method works.
- What a mixture is and why we separate its components.
- Handpicking, threshing, winnowing and sieving for solid–solid mixtures.
- Evaporation for getting dissolved solids (like salt) from water.
- Sedimentation, decantation and filtration for insoluble solids in liquids.
- Churning and magnetic separation.
- Choosing the right method for a mixture with many components.
2. Mixtures and Why We Separate Them
A mixture is formed when two or more substances are mixed together. We separate the parts of a mixture to remove unwanted or harmful substances (stones from rice), to get useful substances (salt from seawater) or to obtain pure substances. The method chosen depends on the properties of the components — size, weight, colour, solubility or magnetism.
3. Separating Solids from Solids
| Method | How it works | Based on | Example |
|---|---|---|---|
| Handpicking | Unwanted pieces are picked out by hand | Different size, colour or shape; small quantity | Stones and husk from rice, dal or wheat |
| Threshing | Harvested stalks are beaten (on a log, by bullocks or a thresher machine) to loosen grain | Grains attached to stalks | Wheat and paddy after harvest |
| Winnowing | Mixture dropped from a height (with a soop) in wind; light husk blows away, heavy grain falls near | Different weights | Husk from grain; skin from roasted peanuts |
| Sieving | Fine particles pass through holes; bigger ones stay on the sieve | Different sizes | Bran from flour; pebbles from sand at construction sites |
4. Getting Dissolved Solids: Evaporation
Salt dissolved in water cannot be picked, sieved or filtered. But if the solution is kept in sunlight or heated, the water evaporates and the salt is left behind. Along sea coasts, seawater is collected in shallow pits; sunlight and wind evaporate the water and common salt is obtained. In the laboratory, a salt solution heated in a china dish leaves salt crystals behind.
5. Separating Insoluble Solids from Liquids
- Sedimentation — when muddy water is left undisturbed, the heavier insoluble particles settle at the bottom. The settled layer is the sediment.
- Decantation — the clearer liquid above is poured off gently by tilting the vessel, leaving the sediment behind. (Used for washing rice and dal.)
- Filtration — the liquid is passed through a filter (strainer, cloth or filter paper). Solids stay on the filter as residue; the clear liquid that passes through is the filtrate.
A home water filter can be made from layers of gravel, sand and cloth inside a cut plastic bottle. Our masks and even the hair in our nose act as filters, stopping dust from entering our lungs.
Filtration removes insoluble impurities only. Salt or sugar dissolved in water passes through filter paper. To get the dissolved salt back, use evaporation.
6. Churning and Magnetic Separation
- Churning — curd is churned with a mathni (churner). The fat (butter) separates and floats to the top because it is lighter, leaving buttermilk below.
- Magnetic separation — a magnet pulls out iron pieces from a mixture (iron nails from sawdust; scrap iron from garbage at recycling plants).
7. Separating Mixtures with Many Components
- Use a magnet to remove iron nails.
- Filter the remaining mixture — sand stays as residue.
- Evaporate the filtrate (salt water) — salt is left behind.
Handpick the potatoes. Add water to the remaining mixture: sawdust floats and can be removed (or filtered), and salt dissolves. Evaporate the salt solution to get salt.
8. Master Table: Key Terms
| Term | Meaning |
|---|---|
| Mixture | Two or more substances mixed together |
| Handpicking | Removing pieces by hand (size, colour, shape) |
| Threshing | Beating stalks to separate grain |
| Winnowing | Separating light and heavy parts using wind |
| Sieving | Separating by particle size through a sieve |
| Sedimentation | Heavier insoluble particles settle at the bottom |
| Decantation | Pouring off the liquid above the sediment |
| Filtration | Separating insoluble solid using a filter; residue and filtrate |
| Evaporation | Removing liquid as vapour to get dissolved solid |
| Churning | Separating butter from curd |
| Magnetic separation | Removing iron with a magnet |
9. Master Summary Table
| Mixture | Method | Property used |
|---|---|---|
| Stones in rice | Handpicking | Size, colour |
| Husk and grain | Winnowing | Weight |
| Bran in flour | Sieving | Size |
| Salt in water | Evaporation | Water evaporates, salt does not |
| Mud in water | Sedimentation → decantation → filtration | Insolubility, weight |
| Butter in curd | Churning | Butter is lighter |
| Iron in sawdust | Magnetic separation | Magnetism |
10. Mind Map
- Handpicking, threshing
- Winnowing, sieving
- Sedimentation, decantation
- Filtration
- Evaporation
- Salt from seawater
- Churning
- Magnetic separation
11. Tips and Tricks
- Ask “What is different?” — size → sieve; weight → winnow; magnetism → magnet; solubility → dissolve + filter + evaporate.
- Order for multi-part mixtures: Magnet → Water → Filter → Evaporate.
- Residue = what remains on the filter; filtrate = what flows through.
- Trying to filter salt out of salt water — dissolved salt passes through.
- Mixing up threshing (beating stalks) and winnowing (using wind).
- Using sieving for mud in water — use sedimentation and filtration.
12. Important One-liners
- A mixture contains two or more substances.
- Handpicking suits small amounts of visible impurities.
- Threshing separates grain from stalks.
- Winnowing uses wind to separate light husk from heavy grain.
- Sieving separates particles of different sizes.
- Common salt is obtained from seawater by evaporation.
- The solid left on filter paper is the residue; the liquid passing through is the filtrate.
- Butter is separated from curd by churning.
- Magnets separate iron from scrap.
13. Exam-pattern Questions with Detailed Answers
Which method separates husk from grain using wind?
- Sieving
- Winnowing
- Threshing
- Filtration
Show answer
Winnowing uses wind to blow away light husk. Threshing loosens grain from stalks; sieving uses size; filtration separates solids from liquids.
Answer: (B) Winnowing
How is common salt obtained from seawater?
Show answer
Seawater is left in shallow pits; the Sun and wind evaporate the water, leaving salt.
Answer: Evaporation
Match: (a) bran from flour (b) iron pins from sand (c) butter from curd — with churning, sieving, magnet.
Show answer
(a) Sieving (b) Magnet (c) Churning.
Answer: a–sieving, b–magnet, c–churning
True or False (give reason): Salt can be separated from salt water by filtration.
Show answer
False. Salt is dissolved, and dissolved substances pass through filter paper. Evaporation is needed.
Answer: False — use evaporation
Explain sedimentation and decantation with an example.
Show answer
When rice is washed, dust and small particles settle at the bottom (sedimentation). The water above is poured off gently by tilting the vessel (decantation).
Answer: Settling of heavy particles; pouring off liquid above
Why do we wear masks in dusty or polluted air? How is the nose similar?
Show answer
A mask acts as a filter, trapping dust and germs while letting air pass. Hair and mucus in the nose also trap dust particles before air reaches the lungs.
Answer: Both act as filters
When is handpicking a suitable method? When is it not?
Show answer
Suitable when unwanted particles are few, large enough and look different (stones in dal). Not suitable for large quantities or very small particles (sand in flour), where it would take too long.
Answer: Few, visible impurities only
Describe how you would make a simple water filter at home.
Show answer
Cut a plastic bottle and invert the top part. Put layers of cotton or cloth at the neck, then fine sand, coarse sand and gravel. Pour muddy water from the top; clearer water collects below. (It removes insoluble dirt but does not kill germs; boil before drinking.)
Answer: Layers of cloth, sand and gravel in a bottle
Outline the steps to separate a mixture of iron nails, sand, black pepper, common salt and water.
Show answer
(1) Handpick black pepper (large, distinct) and remove iron nails with a magnet. (2) Filter: sand remains as residue. (3) Evaporate the filtrate to obtain salt.
Answer: Handpick pepper + magnet → filter → evaporate
Sludge from a drain is left in heaps beside it for some days before being carried away. Why?
Show answer
Water evaporates and drains out of the heap, so the sludge becomes lighter and drier. It is cheaper to transport, less messy and safer to handle.
Answer: Evaporation makes it lighter and drier
Why is winnowing done in the open on a windy day and not inside a closed room?
Show answer
Winnowing depends on wind to carry away the light husk. In a closed room there is little air movement, so husk and grain would fall together and would not separate.
Answer: Wind is needed to blow away husk
14. Quick Revision Summary
- A mixture has two or more substances; we separate them to remove impurities or get useful parts.
- Solid–solid: handpicking, threshing, winnowing (weight), sieving (size).
- Dissolved solid: evaporation (salt from seawater).
- Insoluble solid in liquid: sedimentation → decantation → filtration (residue and filtrate).
- Churning gives butter; magnets remove iron. Choose the method by the property that differs.