Chapter 5 — Separation of Substances

What is a mixture?
A mixture is formed when two or more substances are combined without any chemical change. Each component keeps its own properties and can be separated by physical methods. Mixtures are common in everyday life — examples include air, soil, milk, salt dissolved in water and a bowl of fruit. Recognising mixtures helps us decide how to separate them.

Types of mixtures
Mixtures are classified into homogeneous and heterogeneous. A homogeneous mixture has the same composition throughout and individual parts are not visible to the naked eye. For example, salt dissolved in water or sugar solution appears uniform. A heterogeneous mixture shows visibly different parts; for example, sand mixed with pebbles or salad where you can pick out different ingredients.

Properties to check
When you see a mixture, ask: Can you see different parts? Does any part settle on standing? Is a part dissolved? Is any part magnetic? Are the particles of different sizes? These observations tell you which separation method to use — sieving for different sizes, filtration for insoluble solids, evaporation for dissolved solids, magnetic separation for magnetic pieces, and decantation for substances that form layers.

Why classification matters
Classification saves time and energy. For example, filtering will not remove dissolved salt from water; you must evaporate or distil in that case. Understanding whether a sample is homogeneous or heterogeneous also helps in laboratory work and in industries where purity is required. Learning to observe carefully and classify mixtures introduces students to planning simple experiments and prepares them to use appropriate tools and follow safety rules.

Hand picking
Hand picking is the most basic separation method. When the components are large and visible, they can be separated by hand. Examples include removing stones from rice or picking out spoiled fruits from a basket. This method is simple, needs no tool, and is useful for small quantities. It relies on visual difference and manual dexterity and works best when items are not mixed too finely.

Winnowing
Winnowing separates lighter parts from heavier parts using wind or a blast of air. It is traditionally used in farming to remove chaff from grains. The mixture is tossed into the air or placed on a winnowing tray; lighter chaff blows away while heavier grain falls back. Modern mills use air currents and aspirators to achieve the same effect on a larger scale. Winnowing exploits differences in weight and shape.

Grain cleaning in practice
Grain cleaning often uses a combination: first hand picking to remove large stones or sticks, then sieving to remove bigger debris, and winnowing or aspiration to remove lighter husk. In mills, mechanical versions of these methods work continuously. For home use, simple tools like a dry cloth, flat tray and a fan can help.

Advantages and limitations
Hand picking and winnowing are inexpensive and require little training. However, hand picking is slow for large quantities, and winnowing requires a wind or air current and works only if weight differences are clear. These methods do not remove dissolved or very fine impurities, so further processing like sieving or washing may be needed. Always work on a clean surface and wear gloves if items are sharp or dirty.

What is sieving?
Sieving separates particles based on size using a sieve or mesh with holes of a chosen size. When a mixture is passed through, smaller particles pass through the holes and larger particles remain on top. This method is used in kitchens to sift flour, in construction to grade sand and in laboratories to separate coarse from fine powders.

How to sieve
Place a sieve over a clean container. Pour the mixture onto the sieve and shake, tap or rub gently so that fine particles pass through. For more precise separation, use a stack of sieves with decreasing mesh sizes so that materials are sorted into several size categories. Cleaning the sieve between uses prevents clogging and cross-contamination.

When sieving works best
Sieving is suitable when components differ clearly in particle size and are dry and free-flowing. Wet or sticky materials will clog the mesh. Sieving does not separate dissolved substances or very similar-sized particles. Particle shape also affects passing through the mesh; flat particles may lie across holes and not pass even if narrow.

Applications and care
Sieving is used in cooking (sifting flour), agriculture (grading grains), and construction (separating fine sand from gravel). In practice, choose the correct mesh size for the material and clean the sieve after use. Sieving can be combined with other methods: for example, after removing large pebbles by hand picking, sieving can sort remaining sand by size. Record observations about the particle sizes and the amount retained to evaluate efficiency.

Filtration explained
Filtration separates insoluble solids from liquids by passing the mixture through a porous material such as filter paper, cloth, or sand. The solid particles that are larger than the pores of the filter are trapped as residue, while the liquid that passes through is called the filtrate. Filtration is one of the most commonly used methods in labs and households.

Set-up and procedure
Typical classroom filtration uses folded filter paper placed in a funnel over a clean container. Pour the mixture slowly into the funnel. The liquid passes through and collects below; the solid remains on the paper. For faster separation or larger volumes, a funnel with a porous plate or a cloth filter may be used. For very fine particles a vacuum filtration speeds up the process but it is a more advanced technique used under supervision.

What filtration can and cannot do
Filtration works when the solid is not dissolved. For example, mud in water or tea leaves in brewed tea can be removed by filtration. Filtration cannot separate a dissolved substance such as salt from water — the dissolved ions pass through the filter with the solvent. For dissolved substances, evaporation or distillation is needed. Filtration also may not remove very fine suspended particles; repeated filtration or finer filter material is required in such cases.

Careful practice and observations
Fold filter paper correctly to avoid leakage. Pour the mixture along a glass rod to control flow and avoid splashing. Rinse the residue if you need to collect the solid in a purer form and then dry it. Always record the appearance of filtrate and residue, time taken, and any changes in colour or clarity to judge how effective the filtration was.

Evaporation defined
Evaporation is the process of removing a liquid from a solution by heating or by leaving it exposed so the liquid changes into vapour. When the solvent evaporates, the dissolved solid is left behind. This is a simple way to recover salt from salt water or to concentrate solutions such as sugar syrup.

Crystallisation explained
Crystallisation is a controlled form of evaporation used to obtain purer solid crystals. The solution is often warmed so that more solid dissolves, then filtered if needed to remove insoluble impurities, and finally allowed to cool undisturbed. As it cools slowly, the dissolved substance forms orderly crystals which can be separated by filtration. Crystallisation produces larger and cleaner crystals compared to rapid evaporation.

Steps, tips and observations
To perform evaporation, pour the solution in a shallow dish to increase surface area and heat gently until most liquid evaporates. For crystallisation, concentrate the solution by gentle heating, then let it cool slowly; cover the container with a clean cloth to prevent dust. After crystals form, filter and dry them. Note the time taken, the shape and colour of crystals and any residue left. Gentle heating avoids decomposition of substances that are heat sensitive.

Limitations and uses
Evaporation is simple but may leave impurities mixed with the solid; crystallisation gives purer product but is slower. Both methods are used in kitchens, laboratories and industries: salt production uses large evaporation pans, and laboratories use crystallisation to purify chemicals. Both require care with heat and cleanliness to avoid contamination and loss of material.

Sedimentation
Sedimentation is the process by which heavier particles in a suspension settle down at the bottom under the influence of gravity when the mixture is left undisturbed. For example, muddy water kept in a jar gradually becomes clearer as soil particles settle to form a sediment at the bottom. Sedimentation is a slow but useful first step in many separation processes.

Decantation
Decantation is the next step after sedimentation. Once the solid has settled, the clear liquid on top can be poured off carefully into another container, leaving the sediment behind. This method is simple and useful for coarse separations where complete removal of all particles is not necessary. Decantation is often used in water treatment and in kitchens.

Procedure and improvements
To decant, tilt the container slowly and pour the clear liquid into another vessel without disturbing the sediment. For greater care, use a glass rod or a pipette to draw off the liquid, minimizing disturbance. If finer particles remain suspended, repeat sedimentation and decantation or follow with filtration to remove the remaining solids. In industry, settling tanks and clarifiers are large-scale versions of sedimentation followed by decantation.

Limitations and observations
Decantation does not remove all fine particles; some remain in the decanted liquid. It is best used where quick and rough separation is acceptable. Note how long it takes for sediment to form and whether the liquid becomes sufficiently clear. Record the volume of clear liquid obtained and inspect for cloudiness; this helps decide if further treatment is needed.

Principle of magnetic separation
Magnetic separation uses the force of a magnet to remove magnetic materials from mixtures. Materials such as iron, nickel and cobalt are attracted by magnets while others like sand, plastic and wood are not. This method is fast and selective when magnetic particles are present and is widely used in both small-scale and industrial cleaning processes.

How to perform magnetic separation
Cover a strong magnet with a piece of paper or cloth and pass it over the mixture. Magnetic particles will stick to the cover and can be removed by taking off the cover. Alternatively, place the magnet directly into the mixture and let it attract metallic pieces. In industry, magnetic separators are fixed over conveyor belts or inside pipes to remove metal fragments continuously and protect machines from damage.

Applications and care
Magnetic separation removes iron filings from grain, sugar, and powdered materials. It protects milling machines by removing stray metal. Magnets will not separate non-magnetic impurities or dissolved ions. Safety rules include keeping magnets away from electronic devices, pacemakers and magnetic cards. Use gloves when handling sharp metallic pieces and clean the magnet cover between uses to avoid contamination.

Limitations and combination with other methods
Magnetic separation works only for magnetic materials. For mixtures where particle sizes or solubilities differ, combine magnetic separation with sieving or filtration to reach desired purity. Observe how much magnetic material is removed and whether further cleaning steps are required; record the mass if possible for comparison.

Immiscible liquids and layers
Immiscible liquids are those that do not mix to form a single uniform phase; they form separate layers when combined. A common example is oil and water: oil floats on water because oil is less dense. The denser liquid forms the lower layer and the lighter liquid forms the upper layer. This difference in density allows separation by gravity-based methods.

Simple decantation of liquids
For small-scale separations at home, allow the mixture to stand undisturbed so clear layers form. Then gently tilt the container and pour the top layer into another clean container. Pour slowly to avoid disturbing the lower layer. This method works well when layers are clearly visible and the volume is small. It is commonly used to remove cooking oil from water or to separate grease from liquids in simple household tasks.

Using a separating funnel
In laboratory and industrial practice, a separating funnel gives a more precise separation. Pour the mixed liquids into the funnel and allow them to settle into distinct layers. Open the tap at the bottom to release the lower layer into a receiver, then close the tap before the upper layer flows out. The separating funnel can separate two immiscible liquids with minimal mixing if handled carefully.

Precautions and combined methods
Open the tap slowly to avoid sudden mixing; if emulsions form, allow more time for separation or add a small amount of salt in some cases to help layers separate. If tiny droplets remain suspended, follow with filtration or centrifugation. Record the volumes of each layer, appearance and any emulsion; these notes help adjust the method for better separation next time.

Idea of chromatography
Chromatography is a technique used to separate small amounts of substances based on their different rates of movement through a medium when carried by a solvent. It is especially useful for coloured mixtures like ink or plant pigments. The method helps to see components that are otherwise invisible to the eye in small amounts.

Paper chromatography procedure
Draw a faint pencil line near the bottom of a strip of filter paper and place a small spot of the ink or extract on it. Put the paper upright in a container with a small amount of solvent (water or alcohol) so that the spot is above the level of the solvent. As the solvent climbs the paper by capillary action, it carries the different components at different speeds. Components that dissolve more easily in the solvent travel farther; those that stick more to the paper travel less. When dry, separate coloured spots appear at different heights.

Why it separates and uses
Separation occurs because of differences in solubility and attraction to the paper. Chromatography is used for testing food colours, separating dyes, and in science to analyse mixtures. It is not a method for large-scale separation but is excellent for identification and small-scale analysis. Students should note the distance each component travels, which can be used qualitatively to compare substances.

Observations and care
Ensure the solvent does not touch the ink spot initially. Use pencil (not ink) to mark the origin because pencil does not dissolve. Keep the container covered to prevent rapid evaporation. Record the colours and their positions; if needed, measure the distances from the origin to compute retention factors later in higher classes.

Centrifugation — speeded sedimentation
Centrifugation uses rapid spinning to separate substances of different densities. When a sample is spun in a centrifuge, heavier particles move outward and collect at the bottom of the tube while lighter liquid stays above. This speeds up sedimentation and separates very fine particles that settle very slowly under gravity. Centrifuges are widely used in laboratories to separate blood cells from plasma and in dairies to separate cream from milk. In class, a demonstration model or supervised display helps students see how layers form quickly.

How to use a centrifuge safely
Balance tubes in the rotor by placing tubes opposite each other with equal mass. Close the lid before starting and never open while spinning. Use recommended speeds and times. After stopping, carefully remove tubes and observe separated layers. Centrifugation is powerful and must be done under teacher supervision with attention to safety rules.

Distillation — separating liquids by boiling point
Distillation separates a liquid from a solution by heating it to form vapour and then cooling the vapour to collect the purified liquid. In simple distillation, heat a salt solution so water vaporises and leaves salt behind; the vapour is cooled and collected as distilled water. Distillation is used to purify drinking water, collect essential oils and in many chemical processes. It requires glassware such as a boiling flask, condenser and receiver and must be performed with care to avoid overheating and loss of material.

When to choose which
Choose centrifugation when solids are suspended but not dissolved and settle very slowly; choose distillation when you need to separate a liquid from dissolved solids or separate two liquids with quite different boiling points. Both methods are more advanced than simple filtration or sieving and teach students how physical properties like density and boiling point can be used for separation.

Choosing the right method
To select a separation method, first identify the type of mixture and the properties of each component: solubility (dissolved or not), particle size (big or small), density (lighter or heavier), and magnetic nature. Make a list of observed properties and match them to methods: sieving for different sizes, filtration for insoluble solids, evaporation or distillation for dissolved solids, magnetic separation for magnetic parts, and decantation for layers. Often a combination of methods is needed to reach the desired purity.

Purity and practical applications
Purity means a substance has few or no impurities. Separation improves purity: water treatment uses sedimentation, filtration and sometimes distillation to provide safe drinking water; food mills use sieves and magnets to remove stones and metal; industries combine multiple steps to reach required purity. Test purity by observing physical appearance, and when available, simple tests like chromatography for mixtures of dyes. Knowing how to improve and test purity is important for health and safety.

Safety and classroom procedures
Always follow teacher instructions. Wear protective gear when needed, handle hot glassware with tongs, and keep magnets away from electronic devices and pacemakers. Balance centrifuge tubes and never open a centrifuge while it is spinning. Do not taste or smell chemicals unless the teacher allows safe samples. Clean equipment after use and label containers clearly. Dispose of residues as instructed and avoid pouring unknown substances down drains.

Record keeping and thoughtful practice
Keep a lab notebook with the method used, quantities, observations and results. Note time taken for sedimentation, appearance of filtrate, and weight of residue if measured. Good notes help repeat experiments and understand which methods worked best. Practice planning: for a given household mixture, write down the steps you would use to separate components and the reasons for choosing them.