Separation of Substances

Separation of Substances - Complete Guide for CBSE Class 6 Science

Introduction to Separation of Substances

Seperation of Substances: In our daily lives, we rarely encounter pure substances. Most materials around us are mixtures combinations of two or more substances. Understanding how to separate these mixtures is essential for obtaining pure substances needed for various purposes, from cooking to scientific research.

What is a Mixture?

A mixture contains molecules of two or more substances combined physically. The individual substances that make up a mixture are called components or constituents. Unlike compounds, mixtures can be separated using physical methods without changing the chemical nature of their components.

Real-World Examples of Mixtures

• Air: A mixture of oxygen, nitrogen, carbon dioxide, and water vapor
• Tap Water: Contains water plus dissolved minerals and salts
• Milk: A mixture of water, fats, proteins, and other nutrients
• Wood: Primarily composed of cellulose and water
• Jewellery Gold: Gold mixed with copper or silver for increased strength

Types of Mixtures

Mixtures are classified based on the visibility and distribution of their components:

1. Homogeneous Mixtures

In homogeneous mixtures, components are uniformly distributed and not visible to the naked eye. The mixture appears uniform throughout.

Examples:

• Sugar dissolved in water
• Salt solution
• Milk mixed with water
• Air (gaseous mixture)
• Alloys like brass and bronze

2. Heterogeneous Mixtures

In heterogeneous mixtures, components do not mix completely and remain clearly visible or distinguishable.

Examples:

• Sand and salt mixture
• Oil and water
• Rice and stones
• Chalk powder in water
• Fruit salad

Classification by State of Matter

Type of Mixture	Homogeneous Examples	Heterogeneous Examples
Solid Mixture	Jewellery (Gold + Copper/Silver)	Rice + Stone, Sand + Salt, Chalk + Sugar
Liquid Mixture	Milk + Water, Juice + Water	Oil + Water
Gaseous Mixture	Air	-
Solid-Liquid	Salt in water, Sugar in water	Sand + Water, Chalk + Water
Solid-Gas	-	Smoke (Soot + Air)
Liquid-Gas	Cold drinks (CO₂ in water)	Mist (Water droplets + Air)

Properties of Mixtures

Understanding these fundamental properties helps explain why separation is possible:

1. Variable Composition: Components can be present in any ratio
2. Retain Individual Properties: Each component maintains its original characteristics
3. Easy Separation: Components can be separated using simple physical methods
4. No Chemical Reaction: Mixing is a physical process, not chemical

Need for Separation of Substances

Separation techniques are essential for three primary reasons:

1. Removing Undesirable Constituents

Purpose: Eliminate harmful or unwanted materials

Examples:

• Removing stones from rice before cooking (prevents dental damage)
• Filtering impurities from water at purification plants
• Separating husk and dirt from food grains
• Removing tea leaves from prepared tea using a strainer

2. Obtaining Desirable Substances

Purpose: Extract useful components from mixtures

Examples:

• Extracting common salt from seawater
• Obtaining butter from milk or curd through churning
• Separating petroleum into petrol, kerosene, diesel, and tar from crude oil
• Extracting fruit juice from pulp

3. Obtaining Highly Pure Substances

Purpose: Achieve purity for specific applications

Examples:

• Producing distilled water for laboratories and medical use
• Purifying chemicals for research
• Preparing pharmaceutical-grade substances

Methods of Separation - Complete Guide

The choice of separation method depends on the physical properties of the mixture components, such as size, density, solubility, magnetic properties, and boiling point.

1. Hand-Picking

Principle: Manual selection based on visible differences

When to Use: When undesirable particles are large, visibly different in color, shape, or size, and present in small quantities

Examples:

• Removing stones from rice or pulses
• Separating rotten fruits from fresh ones
• Picking out different seeds mixed together
• Removing insects or foreign materials from grains

Advantages:

• Simple and requires no equipment
• Cost-free method

Limitations:

• Time-consuming for large quantities
• Not suitable for small particles

2. Threshing and Winnowing

Threshing

Principle: Separating grains from stalks through beating or trampling

Process:

• Beating harvested crop stalks on hard ground
• Using animals to trample over stalks
• Using mechanical threshers (modern method)

Application: First step after harvesting wheat, rice, or other grain crops

Winnowing

Principle: Separation based on density difference - wind blows away lighter particles while heavier ones fall down

Process:

1. Pour the mixture of grain and chaff from a height
2. Natural or artificial breeze carries away the lightweight chaff
3. Heavier grains fall almost vertically to the ground
4. Two separate heaps form - grains and chaff

Home Experiment: Take roasted groundnuts, peel them, and blow air over your palm. The outer covering (lighter) will blow away while nuts (heavier) remain.

Modern Application: Industrial grain cleaning facilities use mechanical winnowers

3. Sieving (Sifting)

Principle: Separation based on particle size difference using a mesh with specific pore sizes

When to Use: When mixture components have significantly different particle sizes

Types of Sieves:

• Fine sieves for flour and fine powders
• Medium sieves for rice, wheat grains
• Coarse sieves for separating stones from sand at construction sites

Common Applications:

Application	Purpose	Sieve Type
Wheat flour	Remove bran and lumps	Fine mesh
Rice grains	Remove dust and small impurities	Medium mesh
Construction sites	Separate stones from sand	Coarse mesh
Flour mills	Separate grains from stones	Slanting sieves

Note: While sieving wheat flour removes roughage, modern nutritionists consider this loss of dietary fiber unhealthy.

4. Sedimentation and Decantation

Sedimentation

Principle: Heavier insoluble particles settle at the bottom of a liquid due to gravity

Process:

1. Mix the insoluble solid with liquid
2. Allow the mixture to stand undisturbed
3. Heavier particles gradually settle as sediment
4. Clear liquid remains above - called supernatant liquid

Decantation

Principle: Carefully pouring out the clear liquid without disturbing the sediment

Process:

1. After sedimentation is complete
2. Gently tilt the container
3. Pour out the supernatant liquid slowly
4. Sediment remains in the original container

Everyday Examples:

Cleaning Rice or Pulses:

1. Soak grains in water
2. Dust and light impurities float or dissolve
3. Heavier grains settle at bottom
4. Pour away dirty water (decantation)
5. Clean grains remain

Washing Vegetables:

• Cut vegetables settle in water
• Dirt and impurities float
• Pour out dirty water
• Clean vegetables remain

Important Limitation: This method cannot separate dissolved solids from liquids (e.g., salt from salt water)

5. Filtration

Principle: Separation using a porous barrier (filter) that allows liquid to pass while retaining solid particles

Filter Types Based on Particle Size:

Filter Material	Particle Size	Example Application
Filter paper	Very fine	Separating mud from water
Muslin cloth	Fine	Filtering milk
Strainer	Medium	Separating tea leaves from tea
Sand layer	Medium	Water purification plants
Cotton	Fine	Home water filters
Ceramic pot	Very fine	Household water filters with UV

Process:

1. Pour mixture through the filter
2. Liquid passes through (called filtrate)
3. Solid particles remain on filter (called residue)

Applications:

At Home:

• Straining tea
• Using water purifiers with ceramic filters
• Filtering coffee

At Water Treatment Plants:

• Multi-stage filtration
• Ceramic porous pots filter solid impurities
• UV light kills germs and bacteria

Scientific Use:

• Laboratory filtration using filter paper and funnel
• Vacuum filtration for faster results

Important Note: Filtration cannot separate dissolved substances (like salt in water). Use evaporation or distillation for such mixtures.

6. Magnetic Separation

Principle: Using magnetic properties - separating magnetic materials from non-magnetic ones

When to Use: When one component is attracted to magnets (like iron, steel, nickel, cobalt)

Process:

1. Move a magnet over or through the mixture
2. Magnetic particles cling to the magnet
3. Non-magnetic particles remain behind
4. Remove magnet to release magnetic material

Examples:

Small Scale:

• Separating iron filings from sulphur powder
• Removing iron nails from sawdust
• Extracting iron pins from sand

Industrial Scale:

• Huge electromagnets on cranes lift scrap iron in junkyards
• Mining industry separates iron ore from rocks
• Recycling facilities sort metal waste

Advantages:

• Fast and efficient
• Works even with fine particles
• No waste of materials

7. Evaporation

Principle: Converting liquid into vapor by heating, leaving behind dissolved solid

When to Use: To separate dissolved solids from liquids

Process:

1. Heat the solution in an open container
2. Liquid converts to vapor (evaporates)
3. Dissolved solid remains as residue

Common Applications:

Salt Production:

• Seawater collected in large shallow ponds
• Sun's heat evaporates water
• Salt crystals remain
• Commercial salt production method

Laboratory Use:

• Recovering salt from salt solution
• Concentrating solutions
• Crystallization processes

At Home:

• Drying wet clothes (water evaporation)
• Reducing gravy while cooking

Factors Affecting Evaporation Rate:

• Temperature: Higher temperature = faster evaporation
• Surface Area: Larger surface = faster evaporation
• Wind Speed: More wind = faster evaporation
• Humidity: Lower humidity = faster evaporation

Limitation: Cannot separate liquids from liquids or obtain pure liquid back

8. Distillation and Condensation

Distillation

Principle: Separating liquids based on different boiling points

Process:

1. Heat the mixture in a distillation flask
2. Liquid with lower boiling point evaporates first
3. Vapor travels to the condenser
4. Cooling converts vapor back to liquid
5. Pure liquid collected in receiving flask

Condensation

Definition: The process of vapor cooling and converting back to liquid state

Equipment Used:

• Distillation Flask: Holds the mixture to be heated
• Liebig's Condenser: Cools vapor with circulating cold water
• Thermometer: Monitors temperature
• Receiving Flask: Collects distilled liquid

Applications:

Laboratory:

• Producing distilled water (pure water without dissolved minerals)
• Separating alcohol from fermented mixture
• Purifying chemicals

Industry:

• Petroleum refining (separating petrol, diesel, kerosene)
• Alcohol production
• Essential oil extraction
• Desalination plants (converting seawater to freshwater)

Process for Distilled Water:

1. Impure water heated in flask
2. Water evaporates at 100°C
3. Impurities (higher boiling points) remain in flask
4. Water vapor enters condenser
5. Cold water circulates outside condenser tube
6. Vapor cools and condenses
7. Pure water droplets collected

Why Distilled Water is Pure:

• All dissolved solids removed
• No minerals or salts
• Only H₂O molecules present
• Used in laboratories, batteries, medical equipment

9. Separating Funnel

Principle: Separating immiscible liquids (liquids that don't mix) based on density

Immiscible Liquids: Liquids that form separate layers when mixed

• Oil and water
• Kerosene and water
• Petrol and water

Equipment:

• Special funnel with a stopcock at the bottom
• Allows controlled drainage of lower layer

Process:

1. Pour mixture of immiscible liquids into funnel
2. Allow to stand undisturbed
3. Liquids separate into layers (denser liquid at bottom)
4. Open stopcock slowly
5. Drain out lower layer completely
6. Close stopcock
7. Upper layer remains in funnel

Alternative Method - Decantation:

• For simple separation without funnel
• Carefully tilt container and pour upper layer
• Requires steady hand and practice

Applications:

• Separating oil from water in laboratories
• Extracting organic compounds
• Chemical analysis
• Oil-water separation in industries

Quick Reference: Separation Methods Comparison

Method	Mixture Type	Based On	Example
Hand-Picking	Solid-Solid	Size, Color, Shape	Stones from rice
Threshing	Solid-Solid	Physical force	Grain from stalks
Winnowing	Solid-Solid	Density (weight)	Grain from chaff
Sieving	Solid-Solid	Particle size	Flour from bran
Sedimentation & Decantation	Solid-Liquid (insoluble)	Density	Mud from water
Filtration	Solid-Liquid (insoluble)	Particle size	Tea leaves from tea
Magnetic Separation	Solid-Solid	Magnetic property	Iron from sulphur
Evaporation	Solid-Liquid (dissolved)	Boiling point	Salt from water
Distillation	Liquid-Liquid (miscible)	Different boiling points	Pure water from impure
Separating Funnel	Liquid-Liquid (immiscible)	Density	Oil from water

How to Choose the Best Separation Method

Follow this decision-making process:

Step 1: Identify the Components

• Are they solids, liquids, or gases?
• Are they mixed or dissolved?

Step 2: Check Physical Properties

• Size difference? → Use sieving or hand-picking
• Density difference? → Use winnowing or sedimentation
• Magnetic vs non-magnetic? → Use magnetic separation
• Soluble vs insoluble? → Use filtration or evaporation
• Different boiling points? → Use distillation
• Miscible or immiscible liquids? → Use separating funnel

Step 3: Consider Practical Factors

• Quantity of mixture
• Availability of equipment
• Time constraints
• Desired purity level
• Cost considerations

Decision Tree Example:

Mixture: Sand and Water

1. Solid in liquid? → Yes
2. Dissolved or suspended? → Suspended (insoluble)
3. Particle size? → Medium to large
4. Choose: Sedimentation and decantation OR Filtration

Mixture: Salt in Water

1. Solid in liquid? → Yes
2. Dissolved or suspended? → Dissolved (soluble)
3. Want solid back? → Yes
4. Choose: Evaporation

Understanding Solubility

What is a Solution?

When a substance dissolves in a liquid, it forms a solution - a homogeneous mixture where:

• Solute: The substance that dissolves (e.g., sugar)
• Solvent: The liquid that dissolves the solute (e.g., water)
• Solution: The resulting uniform mixture

Molecular Level Process:

1. Solute breaks into individual molecules
2. Molecules spread between solvent molecules
3. Distribution becomes uniform
4. Molecules too small to see - substance "disappears"

Water: The Universal Solvent

Water can dissolve more substances than any other liquid, earning it the title "universal solvent."

Substances Soluble in Water:

• Sugar, salt, glucose
• Many acids and bases
• Some gases (oxygen, carbon dioxide)
• Alcohol, vinegar

Substances Insoluble in Water:

• Sand, wood, plastic
• Oil, kerosene
• Most metals
• Glass, rubber

Types of Solutions Based on Saturation

Solution Type	Definition	Can Dissolve More?	Temperature Effect
Unsaturated	Can dissolve more solute at current temperature	Yes	-
Saturated	Cannot dissolve more solute at current temperature	Only if heated	More dissolves when heated
Supersaturated	Contains more solute than normally possible	No, even when heated	Unstable, crystallizes easily

Experiment to Understand Saturation:

1. Take 100 ml water at room temperature
2. Add 1 spoon sugar - dissolves (unsaturated)
3. Keep adding sugar - continues dissolving
4. At some point, sugar settles (saturated)
5. Heat the solution - more sugar dissolves
6. Cool slowly - excess sugar crystallizes

Importance of Water as a Solvent

For Human Body

1. Digestion: Food breaks down into water-soluble substances for absorption
2. Transportation: Nutrients dissolved in blood reach all body parts
3. Excretion: Waste products dissolve in water for removal through urine
4. Chemical Reactions: Most body reactions occur in aqueous medium
5. Temperature Regulation: Water's properties help maintain body temperature

For Plants

1. Nutrient Absorption: Minerals from soil dissolve in water for root absorption
2. Transportation: Water carries nutrients from roots to leaves (xylem)
3. Transportation: Food from leaves travels to other parts (phloem)
4. Photosynthesis: Water is a raw material, also serves as medium

For Aquatic Life

Dissolved Oxygen:

• Fish and aquatic animals breathe dissolved oxygen through gills
• Oxygen enters water from atmosphere
• Aquatic plants also produce oxygen through photosynthesis

Dissolved Carbon Dioxide:

• Aquatic plants use dissolved CO₂ for photosynthesis
• CO₂ enters from atmosphere and animal respiration

Dissolved Minerals:

• Essential nutrients for aquatic plants
• Maintain water chemistry

Temperature Effect on Gas Solubility

Important Principle: Solubility of gases decreases with increasing temperature

Observation:

• Heat water gently before boiling
• Small bubbles appear (dissolved air escaping)
• Bubbles appear before water reaches 100°C
• Demonstrates air leaving as temperature rises

Practical Implications:

• Hot water contains less dissolved oxygen
• Thermal pollution harms aquatic life (warmer water = less oxygen)
• Aerated drinks go flat when warm (CO₂ escapes)
• Cold drinks kept cold to retain fizziness

Simple Experiments for Students

Experiment 1: Demonstrating Filtration

Materials Needed:

• Muddy water
• Filter paper or clean cloth
• Funnel
• Two beakers

Procedure:

1. Place filter paper in funnel
2. Set funnel over clean beaker
3. Pour muddy water slowly through filter
4. Observe clear water collecting below
5. Observe mud particles on filter paper

Result: Insoluble mud particles are retained by filter, while water passes through.

Learning: Filtration separates insoluble solids from liquids based on particle size.

Experiment 2: Demonstrating Evaporation

Materials Needed:

• Salt water solution
• Flat dish or plate
• Sunny spot or heat source

Procedure:

1. Prepare salt solution (mix 2 spoons salt in 100 ml water)
2. Pour solution into flat dish
3. Keep in sunny spot or apply gentle heat
4. Observe daily for 3-4 days

Result: Water gradually evaporates, leaving behind white salt crystals.

Learning: Evaporation separates dissolved solids from liquids.

Experiment 3: Sedimentation and Decantation

Materials Needed:

• Chalk powder or sand
• Water
• Glass or beaker

Procedure:

1. Mix chalk powder thoroughly in water
2. Allow mixture to stand undisturbed for 10 minutes
3. Observe chalk settling at bottom
4. Carefully pour clear water into another container
5. Observe chalk remaining in first container

Result: Heavier chalk particles settle (sedimentation), allowing clear water to be poured off (decantation).

Learning: Insoluble heavier particles can be separated by sedimentation and decantation.

Experiment 4: Magnetic Separation

Materials Needed:

• Iron filings
• Sulphur powder or sand
• Bar magnet
• Paper sheet

Procedure:

1. Mix iron filings with sulphur/sand
2. Spread mixture on paper
3. Move magnet over mixture (don't touch)
4. Observe iron filings attracted to magnet
5. Remove magnet over separate paper
6. Iron filings fall off

Result: Magnetic iron separates from non-magnetic sulphur/sand.

Learning: Magnetic properties can separate magnetic from non-magnetic materials.

Formulas and Concepts

Concept	Formula/Definition	Explanation
Mixture	Two or more substances physically combined	Components retain individual properties
Solution	Solute + Solvent = Solution	Homogeneous mixture where solute dissolves
Solubility	Maximum solute that dissolves in given solvent at specific temperature	Varies with temperature and pressure
Saturation	Point where no more solute dissolves	Depends on temperature
Concentration	Amount of solute per unit volume of solution	Can be expressed as percentage or ratio
Evaporation Rate	Affected by temperature, surface area, wind, humidity	Increases with temperature and surface area
Distillation	Separation based on different boiling points	Lower boiling point component evaporates first
Filtration	Liquid + Solid (insoluble) → Filtrate + Residue	Based on particle size difference

Common Mistakes to Avoid

1. Using filtration for dissolved substances: Filtration only works for insoluble particles
2. Expecting evaporation to give back pure liquid: Evaporation leaves solid behind, not liquid
3. Confusing sedimentation with dissolution: Sediment remains solid, dissolved substances disappear
4. Using fine sieve for large particles: Match sieve pore size to particle size
5. Disturbing sediment during decantation: Pour slowly and carefully
6. Expecting magnetic separation for all metals: Only works with magnetic materials (iron, nickel, cobalt)

Conclusion

Understanding separation of substances is fundamental to science and everyday life. From cleaning food grains to purifying water, from industrial processes to laboratory work, these methods are essential. The key is identifying the physical properties that differ between components and choosing the appropriate method.

Important Notes:

1. Mixtures contain physically combined components that retain their individual properties
2. Separation methods exploit property differences (size, density, solubility, magnetism, boiling point)
3. Choose the method based on mixture type and desired outcome
4. Multiple methods may be needed for complex mixtures
5. Water's role as universal solvent makes it crucial for life

Author's Note: This comprehensive Science notes for class 6 is designed for the students to understand the fundamental concepts of separation of substances. The content aligns with the NCERT curriculum and incorporates scientific accuracy with student-friendly explanations. For additional clarification or advanced concepts, consult your science teacher or NCERT textbook.