Class 6 Science Chapter 9 Notes
(New NCERT "Curiosity" | 2025-26)
📌 Chapter Overview
This chapter explores the many ways we separate different components from mixtures in our daily lives — at home, on farms, and in industries. Through the story of Malli and Valli's summer vacation at their grandmother's house in Haryana, students learn how traditional methods like winnowing and threshing are still used today, and how scientific principles are behind every separation technique.
💡 Opening Story: Malli and Valli visit their grandmother's house during summer vacation. They observe farmers separating grains from stalks and removing impurities from grain — and they realise that separation is everywhere around them! Their curiosity leads them to discover various methods of separation used in everyday life.
🔑 Key Points
- A mixture is formed when two or more substances are mixed together in any ratio
- We separate mixtures to remove harmful substances and to obtain useful ones
- Different methods are used based on the properties of the components (size, weight, magnetic nature, solubility)
- Methods of separation: Handpicking, Threshing, Winnowing, Sieving, Evaporation, Sedimentation, Decantation, Filtration, Churning, Magnetic Separation
- The choice of method depends on the nature of the mixture
🔎 9.1 Why Do We Need to Separate Mixtures?
- Mixtures in real life often contain unwanted impurities (stones in rice, husk in grain, mud in water)
- Separation helps us:
- Remove harmful or unwanted substances
- Obtain useful substances in a pure form
- Make food safe and fit to eat
- Purify water for drinking
💡 Key Insight: We choose a method of separation based on the properties of the components — such as their size, weight, magnetic nature, or solubility in water.
🔎 9.2 Methods of Separation
✅ Method 1: Handpicking
- Definition: The process of manually separating unwanted substances from a mixture by hand, based on differences in size, colour, or shape
- Used when: The quantity of the component to be removed is small and it is easily visible
- Principle: Differences in size, colour, and shape
Examples:
- Removing small stones from rice or dal
- Removing black peppercorns from a mixed dish
- Picking out spoiled vegetables from fresh ones
- Removing insects from grain
| ✅ When to Use | ❌ When NOT to Use |
|---|---|
| Small quantity of impurity | Very large quantity to remove |
| Impurity is clearly visible | Particles are too small to pick |
| Particles are different in size/colour | Both components look the same |
✅ Method 2: Threshing
- Definition: The process of separating grains from the stalks of crops by beating or striking them
- Used when: Harvested crops need to be separated from their stalks
- Principle: Grains are loosely attached to stalks — beating dislodges them
How it is done:
- Farmers beat the stalks on a hard surface
- Bullocks are made to tread over spread stalks
- Modern threshing machines (combine harvesters) do it automatically
Examples:
- Separating wheat grains from wheat stalks
- Separating rice grains from paddy stalks
- Used widely in agriculture across India
📌 Malli and Valli observed farmers using threshing in Haryana — an ancient method still used today!
✅ Method 3: Winnowing
- Definition: The process of separating lighter components from heavier ones using wind or blowing air
- Used when: One component is much lighter than the other
- Principle: Lighter particles are carried away by wind; heavier ones fall down
How it is done:
- Mixture is taken in a bamboo tray (chhaj/winnow)
- It is poured from a height in a breezy place
- Lighter husk/chaff flies away with the wind
- Heavier grains fall straight down
Examples:
- Separating husk (chaff) from wheat/rice grains
- Separating light peanut skins from heavier peanuts
- Separating sand from sawdust
📌 Activity: Blow gently on peanuts with skin — the lighter skin flies away, heavier peanut stays! This is winnowing!
✅ Method 4: Sieving
- Definition: The process of separating components of a solid-solid mixture based on particle size, using a sieve with holes
- Used when: Two solid components have different sizes
- Principle: Smaller particles pass through the holes; larger ones remain on the sieve
How it is done:
- Mixture is placed on a sieve
- Sieve is shaken
- Smaller particles fall through → collected below
- Larger particles remain on sieve → collected separately
Examples:
- Sifting flour to remove bran or lumps
- Removing stones from wheat flour
- Separating fine sand from coarse gravel
- In flour mills — removing husk and stones before grinding
Types of sieves used:
- Kitchen sieve (fine holes) — for flour
- Construction sieve (large holes) — for sand/gravel
- Filter cloth/muslin — for liquids
📌 Valli's experience: Valli helped her grandmother sieve flour to remove bran — a perfect example of sieving!
✅ Method 5: Evaporation
- Definition: The process of heating a liquid so it turns into vapour, leaving behind the dissolved solid
- Used when: A solid is dissolved in a liquid and we want to recover the solid
- Principle: Liquid evaporates, solid remains behind
How it is done:
- The solution (liquid + dissolved solid) is taken in an open container
- It is heated (or kept in sunlight)
- Liquid evaporates slowly
- Solid is left behind
Examples:
- Getting common salt from seawater — seawater is collected in shallow pits, sun evaporates the water, salt remains
- White patches on dark clothes after sweating — sweat evaporates, salts remain
- Getting sugar from sugarcane juice
📌 Activity 9.2: Salt solution spread on black paper → water evaporates → white salt crystals remain! You can even make salt art with it!
✅ Method 6: Sedimentation
- Definition: The process where heavier insoluble particles settle down at the bottom of a liquid when the mixture is left undisturbed
- Used when: Insoluble solids are mixed with a liquid
- Principle: Gravity pulls heavier particles to the bottom
Examples:
- Mud settling at the bottom of muddy water
- Sand settling in a glass of dirty water
- Soil particles settling in floodwater
💡 Important: Sedimentation = settling down of heavy particles. It is the first step before decantation!
✅ Method 7: Decantation
- Definition: The process of carefully pouring off the clear liquid from the top after sedimentation, leaving the solid residue at the bottom
- Used when: After sedimentation, to collect the clear liquid separately
- Principle: Clear liquid on top is poured into another container, residue stays behind
Examples:
- Pouring clear water from a glass of muddy water (after mud has settled)
- Separating oil from water — oil floats on top, water below → pour oil separately
- Separating rainwater from mud at home
📌 Sedimentation + Decantation are always used together as a two-step process!
✅ Method 8: Filtration
- Definition: The process of separating insoluble solid particles from a liquid by passing the mixture through a filter medium (filter paper, cloth, etc.)
- Used when: Solid particles are too small to be separated by decantation but are insoluble in liquid
- Principle: Filter medium has tiny pores — liquid passes through, solid particles are trapped
Parts of filtration:
- Filtrate = the clear liquid that passes through the filter
- Residue = the solid particles left on the filter
Filter materials used:
- Filter paper (finest pores)
- Muslin cloth / cheesecloth
- Cotton
- Charcoal and sand (in water purifiers)
- Coffee filter paper
Examples:
- Separating sand from water using filter paper
- Making tea — strainer traps tea leaves (residue), tea liquid is filtrate
- Water purifiers — filter water through layers of sand, charcoal, cloth
- Leela's story — filtering muddy pond water through cloth to make it usable
💡 Important Difference: Filtration ≠ Sieving
- Sieving = solid-solid separation
- Filtration = solid-liquid separation (insoluble solid)
- Salt CANNOT be separated by filtration because it dissolves in water and passes through filter paper!
✅ Method 9: Churning
- Definition: The process of rapidly stirring or agitating a liquid mixture to separate lighter components that float to the top
- Used when: Two components of a liquid have different densities
- Principle: Lighter component rises to the top due to difference in density
Examples:
- Churning curd → butter rises to the top (butter is lighter than buttermilk)
- Cream separator in dairies — spins milk rapidly → cream separates
- Making lassi — butter floats on top
📌 Centrifugation is a modern, faster form of churning — used in laboratories and dairies with machines that spin rapidly!
✅ Method 10: Magnetic Separation
- Definition: The process of separating magnetic materials from non-magnetic ones using a magnet
- Used when: One component is magnetic (like iron) and the other is not
- Principle: Magnet attracts magnetic substances; non-magnetic ones remain
How it is done:
- A magnet is moved over or through the mixture
- Magnetic particles stick to the magnet
- Non-magnetic particles remain behind
Examples:
- Separating iron filings from sand using a magnet
- Separating iron nails from sawdust
- In scrap industries — giant electromagnets separate iron scrap from heaps of waste
- In food processing — magnetic separators remove iron particles from food
💡 Important: Not just iron — nickel and cobalt are also magnetic! So magnetic separation works for all magnetic substances, not only iron.
🔎 9.3 Ayurveda and Separation — Indian Knowledge Systems
- Ayurveda (ancient Indian medical system) focuses on holistic well-being using natural remedies from herbs and plants
- Many Ayurvedic processes use separation methods:
- Drying plant materials — removes excess moisture, preserves medicinal properties
- Filtering herbal extracts — removes impurities
- Evaporation — to concentrate herbal medicines
- This shows that separation methods have been used in India for thousands of years for both food and medicine! 🇮🇳
🔎 9.4 Choosing the Right Method
| Mixture Type | Best Method |
|---|---|
| Stones in rice | Handpicking |
| Grains from stalks | Threshing |
| Husk from grains | Winnowing |
| Flour from bran | Sieving |
| Salt from seawater | Evaporation |
| Mud in water (large particles) | Sedimentation + Decantation |
| Sand in water (fine particles) | Filtration |
| Butter from curd | Churning |
| Iron from sand | Magnetic Separation |
| Oil from water | Decantation (oil floats) |
📖 Important Definitions
| Term | Definition |
|---|---|
| Mixture | Two or more substances mixed together in any ratio |
| Separation | Process of dividing components of a mixture into pure substances |
| Handpicking | Manually removing impurities by hand based on size, colour, shape |
| Threshing | Separating grains from harvested stalks by beating |
| Winnowing | Separating lighter and heavier components using wind or air |
| Sieving | Separating solid particles of different sizes using a sieve |
| Evaporation | Heating liquid to convert it to vapour, leaving dissolved solid behind |
| Sedimentation | Heavier particles settling at bottom of liquid when left undisturbed |
| Decantation | Carefully pouring clear liquid after sedimentation |
| Filtration | Separating insoluble solid from liquid using a filter medium |
| Filtrate | Clear liquid that passes through the filter |
| Residue | Solid left behind on the filter |
| Churning | Rapid stirring to separate lighter components (butter from curd) |
| Magnetic Separation | Using magnet to separate magnetic from non-magnetic materials |
| Dissolution | Process of dissolving a solid, liquid, or gas in a liquid |
❓ NCERT Exercise Questions & Answers
Q1. True [T] or False [F]:
(i) Heating salt solution until all water evaporates is a useful method to get salt from the solution. Answer: True ✅ — Water evaporates, salt is left behind. This is evaporation.
(ii) Handpicking should be used only when the quantity of one component is less. Answer: True ✅ — Handpicking is effective only for small quantities that are visible and distinguishable.
(iii) A mixture of puffed rice and rice grains can be separated by threshing. Answer: False ❌ — Threshing separates grains from stalks, not from each other. Correct method: Handpicking or Winnowing (puffed rice is lighter).
(iv) A mixture of mustard oil and lemon water can be separated by decantation. Answer: True ✅ — Oil and water form separate layers (oil floats), which can be decanted.
(v) Sieving is used to separate a mixture of rice flour and water. Answer: False ❌ — Rice flour particles are too fine and dissolve/suspend in water. Correct method: Filtration.
Q2. Match the mixtures with their method of separation:
| Mixture | Method |
|---|---|
| Husk and wheat grain | Winnowing |
| Iron nails and sawdust | Magnetic Separation |
| Salt and water | Evaporation |
| Sand and water | Filtration / Sedimentation + Decantation |
| Stones and rice | Handpicking |
| Butter and curd | Churning |
Q3. In what situations would you use decantation instead of filtration to separate solids from liquids?
Answer: Decantation is preferred over filtration when:
- The solid particles are heavy and coarse and settle quickly (like mud or sand)
- The quantity of liquid is large and filtration would be too slow
- We only need to roughly purify the liquid (not completely clean it)
- Filtration is better when particles are very fine and won't settle easily. Both methods are often used together — decantation first, then filtration for finer purification.
Q4. How would you separate a mixture of potatoes, salt, and sawdust?
Answer:
- Step 1: Use Handpicking to remove the potato pieces (large, visible, easily picked by hand)
- Step 2: Add water to the remaining salt + sawdust mixture. Salt dissolves; sawdust floats
- Step 3: Use Filtration to separate the floating sawdust from the saltwater
- Step 4: Use Evaporation — heat the saltwater to evaporate water, leaving salt behind
- ✅ All three components are now separated!
Q5. Leela's Story — Intelligent Leela (purifying muddy pond water):
Answer: To purify muddy pond water for drinking:
- Step 1: Sedimentation — Allow water to stand so mud settles at the bottom
- Step 2: Decantation — Carefully pour the clear water into another container
- Step 3: Filtration — Filter through a clean cloth or sand to remove fine particles
- This makes the water cleaner, but it still needs boiling to make it safe for drinking!
Q6. Why can't we use filtration to separate salt from saltwater?
Answer: Because salt dissolves completely in water to form a solution. Dissolved particles are too small — they pass through the filter paper along with the water. Filtration only works for insoluble solids. To separate salt from saltwater, we use Evaporation.
✅ MCQ Questions
1. Which method separates grains from stalks? a) Winnowing b) Sieving c) Threshing ✅ d) Handpicking
2. Which method uses wind to separate lighter particles? a) Sieving b) Threshing c) Winnowing ✅ d) Filtration
3. The liquid that passes through filter paper is called: a) Residue b) Sediment c) Filtrate ✅ d) Mixture
4. Salt is obtained from seawater by: a) Filtration b) Sieving c) Decantation d) Evaporation ✅
5. Iron filings from sand can be separated by: a) Sieving b) Winnowing c) Magnetic Separation ✅ d) Decantation
6. Butter is separated from curd by: a) Filtration b) Sieving c) Churning ✅ d) Decantation
7. Handpicking is suitable when: a) Components are dissolved b) Quantity to separate is large c) Quantity is small and visible ✅ d) Components are magnetic
8. Sedimentation means: a) Pouring liquid slowly b) Passing through filter c) Heavier particles settling at bottom ✅ d) Using magnet to attract particles
9. Which cannot be separated by filtration? a) Sand and water b) Salt and water ✅ c) Chalk and water d) Mud and water
10. The solid that remains on filter paper is called: a) Filtrate b) Solution c) Sediment d) Residue ✅
🧠Quick Revision Table
| Method | Based On | Example |
|---|---|---|
| Handpicking | Size, colour, shape | Stones from rice |
| Threshing | Grains loosely attached to stalks | Wheat from stalks |
| Winnowing | Difference in weight/density | Husk from grain |
| Sieving | Difference in particle size | Flour from bran |
| Evaporation | Liquid evaporates, solid remains | Salt from seawater |
| Sedimentation | Gravity pulls heavy particles down | Mud settles in water |
| Decantation | Clear liquid poured off top | Clear water from muddy water |
| Filtration | Insoluble solid trapped in filter | Sand from water |
| Churning | Density difference in liquids | Butter from curd |
| Magnetic Separation | Magnetic vs non-magnetic | Iron from sand |
🌟 Ancient Indian Connection!
The methods of winnowing and threshing have been used in India since the time of the Indus Valley Civilisation (2600–1900 BCE)! Ancient Indians were experts at processing and storing grain. Ayurveda also used separation techniques like drying, filtering, and evaporating for making herbal medicines — showing that Indians understood the science of separation thousands of years before modern chemistry! 🇮🇳
📌 Exam Tips
- Most asked: "Why can't filtration separate salt from saltwater?" → Salt dissolves = passes through filter = use evaporation instead!
- Know the difference between sedimentation and decantation — they are two steps of the same process
- Remember: Residue = solid on filter, Filtrate = liquid below filter
- Sieving = solid-solid, Filtration = solid-liquid (insoluble) — don't mix these up!
- The Leela/muddy water type question is very common — always answer: sedimentation → decantation → filtration → boiling 🎯