Chapter 5 — Separation Of Substances

What is separation? Separation is the process of dividing a mixture into two or more parts (substances) so that each part has different composition or properties. In everyday life we often separate things to get the useful or pure part from a mixture.

Types of mixtures: Mixtures can be heterogeneous (different parts are visible, e.g., sand + water, rice + stones) or homogeneous (same throughout, e.g., sugar dissolved in water). The method used to separate depends on properties such as particle size, solubility, density and magnetic behavior.

Why do we need separation? There are several reasons:

• To obtain a pure substance (e.g., clean water from muddy water).
• To remove unwanted or harmful materials (e.g., stones from grains, dirt from clothes).
• To recover useful material for reuse or sale (e.g., recycling metal from waste).
• To make substances suitable for a specific use (e.g., salt from seawater by evaporation).
• For health and safety (e.g., removing microbes or harmful solids from drinking water).

Common separation techniques (short overview):

• Handpicking: removing large foreign particles by hand (rice and stones).
• Sieving: using mesh to separate different particle sizes (flour, sand).
• Winnowing: using wind to separate light particles from heavy ones (grain from chaff).
• Sedimentation and decantation: letting heavier particles settle, then pouring off the liquid.
• Filtration: passing a mixture through a filter to separate solids from liquids (tea leaves from tea).
• Evaporation: removing a solvent to obtain dissolved solids (salt from saltwater).
• Magnetic separation: using magnet to pull magnetic materials out of a mixture (iron filings from sand).

How to choose a method? Choose the method based on the physical property that differs between components: solubility (use evaporation, distillation), particle size (use sieving, filtration), density (use settling, centrifuging), magnetism (use magnets), or volatility (use distillation).

Important principle: Mass is conserved in separation processes — the total mass of all parts after separation equals the mass of the original mixture (ignoring losses). This helps check results and plan separations.

What is a mixture? A mixture is a combination of two or more substances in which each substance keeps its own chemical identity and properties. Mixtures can be separated into their components by physical methods (like filtration, evaporation, magnet, sieving).

Major types of mixtures

• Homogeneous mixtures (solutions): The components are evenly distributed and the mixture looks like a single substance. Examples: salt dissolved in water, sugar in water, air. Particles are very small and cannot be seen with the naked eye.
• Heterogeneous mixtures (mechanical mixtures): The components are not evenly distributed and different parts can be seen. Examples: sand in water, oil and water, salad. Particles are larger and may settle or can be picked out.

Sub-categories useful at Class 6 level

• Suspensions: Heterogeneous mixtures in which particles are large and settle on standing (e.g., muddy water). They can be separated by filtration or decantation.
• Colloids (basic idea): Mixtures with intermediate particle size that do not settle quickly and scatter light (e.g., milk, smoke). (Colloids may be introduced briefly at this level.)

Key characteristics of mixtures

• Components retain their original properties (e.g., sand in saltwater still feels gritty).
• Composition is variable — proportions can change (you can add more salt or water).
• Can be separated by physical methods (filtration, evaporation, magnetic separation, sieving, distillation for solutions).
• Particle size affects behaviour: smaller particles give homogeneous mixtures; larger ones give heterogeneous mixtures; intermediate give colloids.
• Appearance: homogeneous mixtures are uniform throughout; heterogeneous mixtures are non-uniform.

How to identify type quickly (simple tests)

• Look: Is the mixture uniform? If yes, likely homogeneous.
• Filter: If particles remain on filter, it is a suspension (heterogeneous).
• Settle: If particles settle on standing, it is a suspension.
• Light test (Tyndall effect): If a beam of light is scattered (visible path), the mixture may be a colloid; if not, it is likely a true solution.

Practical importance: Understanding mixtures helps in everyday tasks (making salt water, separating sand from grains, purifying water) and is the basis for later chemistry topics like compounds and chemical reactions.

What is a solution? A solution is a homogeneous mixture of two or more substances. It looks the same throughout and the different parts cannot be seen separately.

Solute and solvent:

• Solute: The substance that gets dissolved (usually present in smaller amount). Example: salt or sugar.
• Solvent: The substance that dissolves the solute (usually present in larger amount). Example: water. Water is called the ‘universal solvent’ because many substances dissolve in it.

Soluble and Insoluble:

• Soluble — A substance is called soluble if it dissolves in a solvent. Example: sugar is soluble in water.
• Insoluble — A substance is insoluble if it does not dissolve (or dissolves negligibly) in the solvent. Example: sand is insoluble in water.

Types of solutions (basic): solid in liquid (salt in water), gas in liquid (carbon dioxide in a soft drink), liquid in liquid (alcohol in water), solid in solid (alloy).

Saturated, unsaturated and supersaturated (simple idea):

• Unsaturated: More solute can still dissolve at that temperature.
• Saturated: No more solute can dissolve at that temperature (solution contains maximum dissolved solute).
• Supersaturated: The solution contains more dissolved solute than normally possible at that temperature (unstable, common in carefully prepared solutions).

Factors affecting solubility and rate of dissolving:

• Temperature: For most solids solubility increases with temperature; for gases solubility usually decreases with temperature.
• Stirring (agitation): Speeds up the rate of dissolving but does not change final solubility.
• Surface area: Finely powdered solute dissolves faster than large pieces.
• Nature of solute and solvent: "Like dissolves like" — polar solvents (water) dissolve polar solutes; non-polar solvents (oil) dissolve non-polar solutes.

Simple experiment to observe solubility: Add a spoon of sugar to a glass of water and stir — sugar disappears (dissolves) forming a solution. Add sand to another glass and stir — sand settles down showing it is insoluble.

Important notes for students: Solutions are uniform mixtures. Dissolved particles of true solutions are too small to be seen and do not settle on standing. Some mixtures that look similar (like milk) are colloids, not true solutions.

Physical properties are characteristics of matter that can be observed or measured without changing the chemical identity of the substance. In separation of substances (Class 6), we use such properties to separate mixtures into their components. Common physical properties used for separation include particle size, solubility, density (or relative density), magnetic property, boiling/volatility, and state of matter.

• Particle size — Larger particles can be removed from smaller ones by sieving or handpicking. Example: Separating stones from grains using a sieve or hand.
• Solubility — If one component dissolves in a solvent and the other does not, filtration followed by evaporation or crystallization can separate them. Example: Salt dissolves in water, sand does not; filter and then evaporate the filtrate to get salt crystals.
• Density / Relative density — Denser substances sink while lighter ones float. Decantation or flotation methods use this. Example: Oil floats on water and can be decanted or separated using a funnel.
• Magnetic property — Magnetic materials (like iron) can be separated from non-magnetic ones using a magnet. Example: Removing iron filings from a mixture of sand and iron.
• Boiling point / Volatility — Components with different boiling points can be separated by distillation: the lower boiling component vaporizes first and is condensed separately. Example: Separating alcohol from a water-alcohol mixture (simple distillation concept).
• State of matter — Solids, liquids and gases can be separated using evaporation, condensation or by exploiting immiscibility. Example: Collecting water by condensing steam (simple condensation) or separating air into gases (advanced idea).
• Adsorption / affinity (simple paper chromatography) — Some substances travel faster on paper with a solvent and separate as bands. Example: Separating inks or colors using paper chromatography (school experiments).

Each method uses one or more physical properties. In practice, a combination of methods is often used to get pure components. For students, focus on matching the right property to the right method (e.g., insoluble solid + liquid = filtration; different particle sizes = sieving; magnetic + non-magnetic = magnet).

Definition: Handpicking is a simple physical method of separation in which unwanted substances (impurities) are removed from a mixture by hand (or with the help of tweezers) based on visible differences such as size, shape, colour or texture.

Principle: It works when the components of a heterogeneous mixture are large enough and easily distinguishable by the naked eye.

When to use: Use handpicking for small-scale separations where particles are large, well-separated and easily identifiable—for example, removing stones from pulses or picking damaged fruits from a tray.

Procedure (simple steps):

• Spread the mixture on a flat, clean surface (tray or sheet) so components are visible.
• Identify the impurity or the desired component by colour, size or shape.
• Pick out the unwanted/desired pieces using fingers or tweezers and place them in separate containers.
• Repeat until the required purity is reached.

Characteristics:

• Physical and reversible method (no chemical change).
• Requires no equipment or power—only manual effort.
• Best for small quantities and obvious impurities.

Advantages:

• Simple, cheap and easy to perform.
• No special apparatus or energy is needed.
• Does not alter the substances being separated.

Limitations:

• Not suitable for very fine or mixed particles that are hard to distinguish.
• Time-consuming and impractical for large quantities.
• Effectiveness depends on eyesight and manual dexterity.

Precautions / Tips: Work under good light, spread mixture thinly, use tweezers for small pieces, and change posture often to avoid fatigue if separating for long periods.

CBSE-style note: Handpicking is often the first step in cleaning seeds and grains at home or in small-scale processing; it is a basic separation technique taught under Separation of Substances.

What is winnowing?
Winnowing is a simple method used to separate lighter particles from heavier ones in a mixture by using air (natural wind or a stream of air). It is commonly used in agriculture to separate grain (heavier) from chaff or husk (lighter).

Principle:
When a mixture of particles is exposed to a moving air stream, lighter particles experience a larger effect of air resistance relative to their weight and are carried away, while heavier particles have greater downward force due to gravity and fall closer to the source. Separation occurs because different particles have different terminal (settling) velocities in air.

How winnowing is done (typical steps):

• Threshing: loosen the grain from the stalks (done earlier).
• Throwing: the mixture of grain and chaff is thrown into the air from a basket or winnowing fan so air can blow the light chaff away.
• Collection: heavier grains fall into a pile nearer the thrower; lighter chaff is carried away by wind and removed.

Types / tools: simple hand-held winnowing basket, winnowing fork, mechanical winnower, motorized fans.

Factors affecting winnowing:

• Wind speed (air velocity): too low — chaff won’t blow away; too high — grains may also be carried off.
• Particle size, shape and density: lighter, flatter or less-dense particles are easier to blow away.
• Height and angle of throw: control distance grains fall vs. distance chaff is carried.
• Airflow steady vs. gusty: steady flow gives controlled separation.

Advantages and limitations:

• Advantages: simple, inexpensive, quick, requires no chemicals.
• Limitations: depends on wind conditions and operator skill; not perfect for particles with very similar densities or sizes.

CBSE learning note: Understand the basic idea — using air to separate lighter from heavier particles — and be able to give examples and factors affecting the process. Detailed fluid dynamics is not required at Class 6 level, but the concept of heavier objects falling sooner than lighter ones in air is important.

What is Sieving?
Sieving is a method of separation that uses a sieve (a mesh or perforated sheet) to separate particles of different sizes. Particles smaller than the openings pass through (called the passed or fine fraction) while larger particles are retained (called the retained or coarse fraction).

How it works

• Place the mixture on a sieve.
• Shake or move the sieve; smaller particles fall through the holes.
• Collect the material that passes and the material that remains separately.

When to use sieving
Use sieving when the components of a mixture differ mainly in particle size (for example sand and pebbles, or flour and lumps).

Advantages: simple, quick, no chemicals, inexpensive.
Limitations: not suitable for particles of similar size or for separating dissolved substances; sticky or wet materials may clog the mesh.

Sedimentation is the process by which heavier particles in a suspension settle down at the bottom of a container under the action of gravity. The settled particles form a layer called the sediment and the clear liquid above is called the supernatant.

Decantation is the process of carefully pouring off the clear liquid (supernatant) after sedimentation, leaving the settled solid behind. Decantation is used to separate a liquid from insoluble solid particles that have settled.

Typical steps:

• Prepare the suspension and pour it into a container (beaker, jar).
• Allow the mixture to stand undisturbed until particles settle (sedimentation).
• Slightly tilt the container and slowly pour off the clear liquid into another container (decantation).
• If needed, repeat the process or use filtration/centrifugation to remove finer particles.

Key points to remember:

• Sedimentation works when particles are large/heavy enough to settle under gravity (not for true solutions or stable colloids).
• Decantation must be done slowly to avoid disturbing the sediment and mixing it back into the liquid.
• For very fine particles or emulsions, sedimentation is slow or ineffective; centrifugation or filtration may be used instead.

Factors affecting the rate of sedimentation:

• Particle size: larger particles settle faster.
• Density difference: greater difference between particle and liquid densities increases settling rate.
• Viscosity of the liquid: higher viscosity slows down settling.
• Temperature: higher temperature usually lowers viscosity and increases settling rate.
• Shape of particles and presence of flocculants (which make particles clump and settle faster).

Limitations: Sedimentation and decantation are slow for very fine particles, and they cannot separate dissolved substances (solutions).

When used: Common in households (settling muddy water), water treatment plants (clarifiers), laboratories and industries where solid particles are separated from liquids.

Definition: Filtration is a method used to separate an insoluble solid from a liquid by passing the mixture through a porous medium (filter) that allows the liquid to pass but retains the solid particles.

Principle: Filtration works because the filter has pores smaller than the solid particles. The liquid (filtrate) passes through the pores while the solid (residue) is trapped on the filter.

Apparatus commonly used:

• Funnel and filter paper (or cloth/mesh)
• Beaker or flask to collect filtrate
• Glass rod (to guide pouring)
• Stirring rod and spatula (for handling solids)

Simple procedure (using filter paper):

1. Fold a circular filter paper into a cone and place it inside a funnel.
2. Place the funnel in a beaker or flask; wet the filter paper slightly with the solvent if needed so it sticks to the funnel.
3. Pour the mixture slowly into the center of the filter paper, using a glass rod to guide the liquid and avoid splashing.
4. Allow the liquid to pass through. The solid remains on the paper as residue; the clear liquid collected below is the filtrate.
5. If required, wash the residue on the filter with a small amount of clean solvent to remove any trapped soluble material.

Observations and result: The residue is the insoluble solid retained on the filter paper. The filtrate is the liquid that passes through and is largely free of the solid particles.

Applications / Uses: Filtering tea or coffee, clarifying muddy water (sand filters), laboratory separation of precipitates, kitchen straining, air filters (for dust) and water treatment plants.

Limitations: Filtration cannot separate dissolved substances from a solvent (for that evaporation or crystallization is needed). Very fine colloidal particles may pass through ordinary filter paper; special methods or finer filters are needed.

Safety tips: Pour slowly to avoid spills, use appropriate filter strength for hot liquids, and handle glass apparatus carefully.

What is evaporation?
Evaporation is the process by which molecules at the surface of a liquid escape into the air as gas. It occurs at all temperatures (not only at the boiling point) because, at any temperature, some molecules have enough kinetic energy to leave the liquid surface.

How it happens (simple explanation):
In a liquid, molecules move with different speeds. A few faster molecules near the surface can overcome the attractive forces of neighbouring molecules and fly off into the air as vapor. As these higher-energy molecules leave, the remaining liquid becomes cooler — this is why evaporation causes cooling.

Difference from boiling:
Boiling is a rapid vaporisation that happens throughout the liquid at a specific temperature (the boiling point) and produces bubbles. Evaporation is a slow surface process that can happen at any temperature.

Factors affecting the rate of evaporation:

• Temperature: Higher temperature → faster evaporation (more molecules have enough energy to escape).
• Surface area: Larger surface area → faster evaporation (more molecules at the surface).
• Humidity of air: Lower humidity → faster evaporation (drier air accepts more vapor).
• Air movement (wind): Wind removes vapor near the surface → faster evaporation.
• Nature of liquid: Volatile liquids (like alcohol) evaporate faster than less volatile liquids (like water).
• Pressure: Lower air pressure can increase evaporation rate.

Evaporation and energy (latent heat):
Evaporation requires energy (latent heat of vaporisation). The required heat comes from the liquid itself or the surroundings, causing cooling of the remaining liquid.

Crystallization is a method used to separate and purify a solid substance from a solution by forming solid crystals. When a solution becomes saturated or supersaturated with a dissolved substance, the extra solute comes out of the solution in an ordered, repeating pattern called a crystal.

How it works (simple steps):

1. Prepare a concentrated (often hot) solution of the substance in a suitable solvent (for example, sugar in hot water).
2. If needed, filter the solution to remove insoluble impurities.
3. Allow the solution to cool slowly or let some solvent evaporate gently. As the solution becomes supersaturated, tiny solid particles (nuclei) form.
4. These nuclei grow into visible crystals as more solute comes out of the solution.
5. Collect the crystals by decanting the remaining solution or by filtration and then dry the crystals.

Key ideas:

• Saturation: A solution that cannot dissolve more solute at a given temperature is saturated.
• Supersaturation: If more solute is present than can remain dissolved, the solution is supersaturated and tends to form crystals.
• Nucleation: The first small particles that act as centers for crystal growth.
• Rate effects: Slow cooling or slow evaporation usually produces larger, well-formed crystals; fast cooling/evaporation gives many small crystals.

Why we use crystallization: It is a simple, effective way to obtain a pure solid from a solution (for example, to purify table salt or sugar). It is also used in making rock candy, growing laboratory crystals like copper sulfate or alum, and in many industrial processes.

What is Magnetic Separation?

Magnetic separation is a method used to separate magnetic materials from non‑magnetic materials by using a magnet. In this technique, magnetic substances (like iron, nickel and cobalt or their compounds) are attracted by a magnet and removed from the mixture, while non‑magnetic substances (like sand, glass, or plastic) are left behind.

How it works (simple principle)

• Bring a magnet close to a mixture containing both magnetic and non‑magnetic particles.
• The magnetic particles are attracted to the magnet and stick to it.
• Remove the magnet (often covered with paper or cloth to collect the particles) and then take off the attracted magnetic material.

Materials involved

• Magnetic materials: iron filings, iron nails, some ores (iron ore).
• Non‑magnetic materials: sand, salt, rice, plastic, glass.

Simple classroom procedure / demonstration

1. Mix iron filings with sand in a tray.
2. Move a bar magnet above the mixture — iron filings will jump to the magnet.
3. To collect the iron separately, wrap the magnet in paper or cling film before using it; after collection, remove the wrapping so the iron falls off into a container.

Uses and importance

• In laboratories and classrooms for demonstrations and small separations.
• In recycling plants to remove iron and steel from mixed waste.
• In mining to separate iron ore from other materials.
• In food industry and manufacturing to remove metal contaminants.

Advantages

• Quick and simple method.
• No chemicals required — environmentally friendly.
• Can be used on dry mixtures or in slurry form (industrial systems).

Limitations

• Works only when one component is magnetic.
• Very small or weakly magnetic particles may not be separated fully without strong magnets or special equipment.
• Does not separate non‑magnetic substances from each other (e.g., sand from salt).

Safety note

Use caution with strong magnets: keep them away from electronic devices and small children, and avoid pinching fingers between magnets.

Note for Class 6: Only the idea and simple steps are required at this level; advanced formulas are not needed for school experiments.

What it means: 'Choice of Method and Comparative Suitability' explains how to select the best way to separate the components of a mixture. The right method depends on the physical properties of the components (particle size, solubility, density, magnetic property, volatility, etc.).

Key factors to check before choosing a method

• Particle size: Large visible pieces can be picked out by hand or sieved; very fine particles may require filtration or settling.
• Solubility: If one component dissolves in a solvent and another does not, use filtration (for undissolved) or evaporation/distillation (to recover dissolved solute or solvent).
• Density difference: Components with different densities (e.g., sand and water, oil and water) can be separated by decantation, sedimentation or using a separating funnel.
• Magnetic property: If one component is magnetic (iron filings), use a magnet.
• Volatility (boiling point): A more volatile component can be separated by evaporation or distillation.

Common methods and when to use them

• Handpicking: For large, easily visible components (e.g., stones from rice).
• Sieving: For separating different particle sizes (e.g., flour from husk; sand from pebbles).
• Winnowing: Use wind or air to separate light and heavy particles (e.g., husk from grains).
• Sedimentation + Decantation: Let heavy particles settle; pour off the liquid (e.g., water from muddy water).
• Filtration: Separate insoluble solids from liquids (e.g., chalk from water, tea leaves from tea).
• Evaporation: Remove a solvent to recover a dissolved solid (e.g., obtaining salt from salt solution).
• Distillation: Separate two liquids with different boiling points or recover a solvent (e.g., alcohol-water separation in simple distillation experiments).
• Magnetic separation: Use a magnet to extract magnetic material (e.g., iron filings from sand).

How to compare suitability: List the properties of the mixture first, then match methods that use those properties. For example, if a mixture has a dissolved solid, prefer evaporation; if one part is magnetic, prefer magnetic separation; if two liquids are immiscible and density differs, use decantation or a separating funnel.

Simple decision flow (summary): Visible large pieces → handpick/sieve. Soluble + insoluble → filtration (then evaporation to recover solute). Two immiscible liquids → decantation/separating funnel. Magnetic + non-magnetic → magnet. Different boiling points → distillation.

Practical tip: Often a combination of methods is needed (e.g., muddy water: sedimentation → decantation → filtration → evaporation to get solid).

Separation of substances is the process of removing one or more components from a mixture so that each component can be used or studied separately. In everyday life we use different separation methods depending on the physical properties of the components — particle size, solubility, magnetic properties, density, or boiling point. Common methods include filtration, sedimentation and decantation, evaporation and crystallization, distillation, magnetic separation, sieving, and paper chromatography.

How methods are chosen (simple rules):

• Use filtration or sieving when solids and liquids (or solids of different sizes) are mixed.
• Use sedimentation and decantation when heavier particles settle in a liquid over time.
• Use evaporation/crystallization to get a dissolved solid from its solution (e.g., salt from seawater).
• Use distillation when two liquids with different boiling points need separation (e.g., water from impure water).
• Use magnetic separation when one component is magnetic (e.g., iron filings from sand).
• Use paper chromatography to separate different coloured substances (e.g., inks or plant pigments).

Everyday benefits: cleaning drinking water, preparing food (tea/coffee), recovering salt or sugar, removing dust from air (filtration), and separating recyclable materials.

Simple safety and quality tips: always let solids settle before decanting, use a clean filter paper or sieve to avoid contamination, and heat solutions gently for evaporation to avoid splattering or burning.

This topic introduces the important words used when we study how different substances are mixed and how they can be separated. Knowing these terms helps you choose the right method to separate a mixture.

• Substance: Matter with definite properties (e.g., pure water, oxygen).
• Pure substance: Contains only one kind of particle and has fixed properties (e.g., distilled water, pure salt).
• Mixture: Combination of two or more substances where each keeps its own properties (e.g., salt + sand).
• Homogeneous mixture (Solution): A mixture in which particles are uniformly distributed and cannot be seen separately (e.g., sugar dissolved in water). Also called a solution.
• Heterogeneous mixture: A mixture in which different parts can be seen (e.g., oil + water, sand + water).
• Solute: The substance that gets dissolved in a solution (e.g., salt in saltwater).
• Solvent: The substance in which the solute dissolves (e.g., water is the common solvent).
• Solubility: The maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature (often given as grams per 100 g of solvent).
• Saturated solution: A solution that contains the maximum amount of dissolved solute at a given temperature.
• Unsaturated solution: A solution that can dissolve more solute at the same temperature.
• Suspension: A heterogeneous mixture in which fine particles are temporarily suspended in a fluid but settle on standing (e.g., muddy water).
• Filtration: A method to separate solids from liquids or gases using a porous medium (filter paper) that allows the fluid to pass but retains solid particles.
• Sedimentation and Decantation: Sedimentation is allowing heavier particles to settle under gravity; decantation is pouring off the clear liquid after settling.
• Evaporation: Removing a liquid by turning it into vapor, leaving dissolved solids behind (used to obtain salt from saltwater).
• Crystallization: Formation of solid crystals from a solution, often used to purify a dissolved substance (e.g., obtaining sugar crystals).
• Sieving (Sieving): Separating particles of different sizes using a sieve or mesh (e.g., removing pebbles from sand).
• Magnetic separation: Using a magnet to separate magnetic materials (like iron filings) from mixtures.
• Distillation (basic idea): Separating substances based on different boiling points (e.g., to get pure water from seawater).

These terms are linked to the physical properties of substances such as particle size, solubility, and magnetic behaviour. By identifying which property differs between components of a mixture, we can pick the appropriate separation method.