Separation of Substances — Class 6 Physical Science
'Separating mixtures is a skill we use every day — from picking stones from rice to filtering tea.'
1. Introduction
Most substances in nature are mixtures — they contain two or more components mixed together. We often need to separate mixtures to get pure substances.
Why Do We Need Separation?
- Remove unwanted components (stones from rice)
- Obtain useful components (sugar from sugarcane)
- Purify substances (drinking water)
- Reuse materials (recycling)
Examples in Daily Life
- Picking stones from dal before cooking
- Straining tea with a strainer
- Separating cream from milk
- Drying clothes (removing water)
2. Handpicking
Removing unwanted substances by hand.
When to Use
- When the quantity is small
- When the unwanted pieces are large enough to see and pick
- When the components are different in size, shape, or colour
Examples
| Mixture | What is Removed |
|---|---|
| Rice + stones | Stones picked by hand |
| Dal + small stones | Stones picked out |
| Dried chillies + damaged ones | Damaged chillies removed |
'Handpicking is the simplest method — it works when the impurity is visible and few in number.'
3. Threshing
Separating grains from stalks (harvested plants).
How It Works
- After harvesting, plants are beaten on the ground or with sticks
- The grains fall off the stalks
- In AP: paddy (rice), wheat, and millets are threshed this way
Traditional vs Modern
- Traditional: Beating stalks by hand or using bullocks to trample
- Modern: Threshing machines (combined harvester)
4. Winnowing
Separating grains from chaff (light husk) using wind.
How It Works
- The grain-chaff mixture is dropped from a height
- Wind blows away the lighter chaff
- The heavier grains fall straight down in a pile
↑ Wind
\
grain (mixture dropped)
/ \
Chaff Grains
(blows (fall
away) down)
When to Use
- When one component is much lighter than the other
- Needs wind or a fan
Examples
- Separating wheat/rice from husk
- In AP villages: winnowing paddy after threshing
5. Sieving
Separating mixtures based on particle size.
How It Works
- A sieve has holes of a specific size
- Smaller particles pass through
- Larger particles are retained
Examples
| Mixture | What Passes Through | What Stays |
|---|---|---|
| Flour + bran | Fine flour | Coarse bran |
| Sand + gravel | Fine sand | Large gravel |
| Rice + water | Water (when draining) | Rice |
Sieving in AP Kitchens
- Fine sieves for flour (atta, ragi flour)
- Coarse sieves for rice washing
6. Sedimentation and Decantation
Used for separating insoluble solids from liquids.
Sedimentation
Heavier solid particles settle at the bottom when the mixture is left undisturbed.
Decantation
Gently pouring out the clear liquid (supernatant) from the top, leaving the sediment behind.
Step by Step (Muddy Water)
- Add water to muddy soil in a glass
- Let it stand for 10-15 minutes
- Heavy sand and soil settles at bottom (sedimentation)
- Gently pour the clear water into another glass (decantation)
- Some fine particles remain — these can be removed by filtration
Daily Life Examples
- Settling of mud in pond water
- Decanting oil from pickles (top layer)
- Separating curd water (whey) from curd
'This is the simplest way to clean muddy water — let it settle, then pour off the clear part.'
7. Filtration
Passing a mixture through a filter to separate solids from liquids.
How It Works
- Filter paper (or cloth) has tiny pores
- Liquid passes through (filtrate)
- Solid particles are trapped (residue)
Simple Filter
┌──────────┐
│ Mixture │
│═══╗ ╔═══│
│ ═ ║ ║ ═ │
│═══╝ ╚═══│
└─────┬─────┘
│
┌─────┴─────┐
│ Filtrate │
│ (pure │
│ liquid) │
└───────────┘
Examples
| Mixture | Filter Used | Solid Retained |
|---|---|---|
| Tea + leaves | Tea strainer | Tea leaves |
| Sand + water | Filter paper | Sand |
| Milk (to remove dirt) | Cloth | Impurities |
Filtration vs Decantation
| Method | Best For | Speed | Purity |
|---|---|---|---|
| Decantation | Large, heavy solids | Faster | Less pure |
| Filtration | Small particles | Slower | More pure |
8. Evaporation
Removing water (liquid) to obtain dissolved solids.
How It Works
- Heat the solution
- Water turns into vapour and escapes
- Solid remains in the container
Examples
| Mixture | Heat Applied | Residue Obtained |
|---|---|---|
| Salt + water | Boil water | Salt crystals |
| Sugar + water | Boil water | Sugar |
| Seawater | Sunlight (natural) | Salt |
Natural Evaporation — Salt Production
- AP context: Salt production along the coast (Kakinada, Machilipatnam)
- Seawater is trapped in shallow ponds
- Sun and wind evaporate water over days
- Salt crystals are left behind
- 'India produces salt by natural evaporation of seawater — a process used for centuries'
Distillation (Advanced)
Condensing the water vapour back to liquid (to get pure water). This is how we get pure drinking water from seawater.
9. Choosing the Right Method
| Mixture | Components | Method(s) |
|---|---|---|
| Rice + stones | Solid + solid (different size) | Handpicking |
| Grain + stalk | Solid + solid | Threshing |
| Grain + husk | Solid + solid (different weight) | Winnowing |
| Flour + bran | Solid + solid (different size) | Sieving |
| Mud + water | Solid + liquid (heavy solid) | Sedimentation + Decantation |
| Sand + water | Solid + liquid (fine solid) | Filtration |
| Salt + water | Solid dissolved in liquid | Evaporation |
10. Common Mistakes — Fix Them Now
| # | Mistake | Correction |
|---|---|---|
| 1 | Thinking sieving and filtration are same | Sieving separates by SIZE (solid-solid). Filtration separates solid from liquid. |
| 2 | Calling the clear water after decantation 'pure' | It may still have dissolved impurities. It's 'clear' but not necessarily 'pure'. |
| 3 | Forgetting that winnowING needs wind | Winnowing does not work on a windless day without a fan |
| 4 | Using evaporation when the solid could spoil by heating | Some substances decompose on heating — use other methods |
| 5 | Thinking sedimentation happens instantly | Sedimentation takes TIME — the finer the particle, the longer it takes |
11. Exam Focus
Marks Blueprint
| Question Type | Marks | Topic |
|---|---|---|
| MCQ | 1 | Identify separation method |
| Match | 1 | Mixture → Method |
| Short answer | 2 | Explain winnowing / filtration |
| Short answer | 2 | Difference between two methods |
| Long answer | 3 | Step-by-step separation of a mixture |
Quick Self-Test (5 Questions)
Q1: How would you separate a mixture of sand and salt?
<details><summary>Answer</summary>Add water (salt dissolves), filter (sand stays on filter), evaporate water to get salt.</details>Q2: What is the principle behind winnowing?
<details><summary>Answer</summary>Lighter particles (chaff) are blown away by wind; heavier grains fall down.</details>Q3: Differentiate between sedimentation and decantation.
<details><summary>Answer</summary>Sedimentation: particles settle at bottom. Decantation: pouring out the clear liquid from top.</details>Q4: How is common salt obtained from seawater?
<details><summary>Answer</summary>Natural evaporation in shallow ponds using sunlight.</details>Q5: Why can't filtration be used to separate sugar from water?
<details><summary>Answer</summary>Because sugar is DISSOLVED in water — the particles are too small to be trapped by a filter.</details>12. Chapter Summary
- Handpicking: remove visible impurities by hand
- Threshing: separate grains from stalks (beating)
- Winnowing: separate grain from chaff using wind
- Sieving: separate by particle size
- Sedimentation + Decantation: heavy solids settle; pour off liquid
- Filtration: pass through porous material to trap solids
- Evaporation: heat to remove liquid, obtain dissolved solid
- Choose the right method based on the type of mixture
'Separation techniques are practical science — from the kitchen to industry, we use these methods every day!'
