1. Why Do We Separate Mixtures?
Many materials around us are mixtures. We separate them when we want to obtain a useful component, remove an unwanted component, purify a substance, recover a solvent, or make a material suitable for use.
Useful substances are obtained from complex natural materials.
Components of blood can be separated for diagnosis and treatment.
Unwanted suspended particles can be removed through suitable processes.
2. How Can We Classify Mixtures?
Homogeneous mixture
A homogeneous mixture has a uniform composition throughout. A well-stirred sugar solution is equally sweet throughout. Examples include sugar solution, vinegar and many gas mixtures such as air.
Heterogeneous mixture
A heterogeneous mixture does not have a uniform composition throughout. Its different components may be visibly distinguishable or may form different phases. Examples include sand in water, oil and water, and muddy water.
| Feature | Homogeneous | Heterogeneous |
|---|---|---|
| Composition | Uniform throughout | Non-uniform |
| Components | Not separately visible in an ordinary solution | May be visible or form separate phases |
| Examples | Salt solution, vinegar, brass | Muddy water, oil + water, smoke |
3. Solutions
A solution is a homogeneous mixture in which one substance is dissolved in another.
The substance that gets dissolved.
Example: sugar in sugar solution.
The substance that dissolves the solute.
Example: water in sugar solution.
In a solution, the particles of the solute are extremely small and are uniformly distributed. A true solution does not show visible settling and does not scatter light in the way a colloid or suspension does.
Indian scientific contribution: Dilip Mahalanabis
Dilip Mahalanabis, an Indian paediatrician, made major contributions to oral rehydration therapy. His work on ORS helped make treatment for severe dehydration much more accessible and effective.
4. Concentration of a Solution
The concentration of a solution describes the amount of solute present in a given amount of solution. In this chapter, three percentage methods are important.
A. Mass by Mass Percentage (% m/m or % w/w)
Example: 10 g salt + 90 g water gives 100 g solution.
B. Mass by Volume Percentage (% m/v or % w/v)
It tells us how many grams of solute are present in 100 mL of solution.
Example: 5 g glucose made up to 100 mL of solution = 5% m/v.
C. Volume by Volume Percentage (% v/v)
This is useful when two liquids are mixed. For example, 1 mL pesticide made up to 100 mL solution gives 1% v/v.
| Method | Formula | Useful when |
|---|---|---|
| % m/m | mass solute / mass solution × 100 | Masses are measured |
| % m/v | mass solute / volume solution × 100 | Mass of solute and volume of solution are convenient |
| % v/v | volume solute / volume solution × 100 | Two liquids are mixed |
5. Solubility of Substances
Solubility is the maximum amount of a solute that dissolves in a fixed quantity of solvent at a given temperature.
A solution that cannot dissolve any more solute at that temperature is called a saturated solution.
| Substance | Effect of increasing temperature |
|---|---|
| Most solid solutes in liquids | Solubility generally increases |
| Gases dissolved in liquids | Solubility generally decreases |
Solubility curve
A graph of solubility against temperature is called a solubility curve. The horizontal axis normally represents temperature and the vertical axis represents solubility, often in grams per 100 g of water.
6. Crystallisation
Crystallisation is the formation of crystals from a saturated solution. It can be used to obtain a purer solid and to separate solids when their solubilities differ.
Basic process
Slow cooling generally gives larger and better-formed crystals than rapid cooling.
Examples
- Purification of solids in the laboratory.
- Formation of sugar crystals (mishri).
- Obtaining salt crystals from concentrated brine/seawater.
- Natural crystal formation, such as quartz and ice crystals.
Crystallisation vs evaporation
| Crystallisation | Evaporation |
|---|---|
| Designed to form crystals of a solid. | Solvent is allowed to escape, leaving the dissolved solid. |
| Controlled cooling/solubility is often important. | May be faster but can give less well-formed crystals. |
| Useful for purification. | Useful when recovery of the solvent is not required. |
7. Distillation
Distillation separates components by using a difference in boiling points. A liquid with the lower boiling point vaporises more readily; the vapour is then cooled in a condenser and collected as liquid.
At this level, simple distillation is particularly useful for a liquid from dissolved solids or for miscible liquids with a sufficiently large difference in boiling points. The chapter uses about 25 °C as a useful distinction; mixtures with smaller differences may require fractional distillation.
Example: acetone + water
Acetone boils at about 56 °C while water boils at about 100 °C, so the large difference in boiling points allows separation by distillation.
Traditional Indian distillation: Deg-Bhapka
The Deg-Bhapka method is a traditional distillation method associated with Kannauj in Uttar Pradesh, known for natural perfumes such as Mitti ka Ittar, which captures the earthy fragrance associated with the first rain.
Fractional distillation
Fractional distillation is used for mixtures whose components have relatively close boiling points. A major industrial application is separating crude petroleum into useful fractions such as petroleum gas, petrol, kerosene and diesel.
8. Paper Chromatography
Paper chromatography separates components because they move through a medium at different rates. The solvent travels up the paper and carries the components with it.
Basic procedure
- Draw a pencil line near the lower end of a strip of chromatography/filter paper.
- Place a small sample spot on the line.
- Put the lower end of the paper into a suitable solvent.
- Keep the solvent level below the sample spot.
- Allow the solvent to rise through the paper.
- Different components travel different distances and form separate spots.
Applications
- Separating pigments in coloured inks.
- Studying pigments in leaves and flowers.
- Checking the components of food colours.
- Analytical work in laboratories.
9. Separating Components of Heterogeneous Mixtures
Heterogeneous mixtures can contain components that differ in density, particle size, solubility or other physical properties. The method is chosen according to the relevant difference.
| Difference used | Possible technique | Example |
|---|---|---|
| Density of immiscible liquids | Separating funnel | Oil + water |
| Sublimation property | Sublimation | Camphor + sand |
| Particle size | Filtration | Sand + water |
| Density / rapid spinning | Centrifugation | Blood components |
| Ability to form clumps | Coagulation | Fine particles in muddy water |
10. Sublimation
Sublimation is the direct change of a solid into vapour without passing through the liquid state. When the vapour cools and changes directly back to solid, the process is called deposition.
Example: camphor + sand
On heating, camphor sublimes and deposits on a cooler surface, while sand remains behind. Naphthalene is another familiar sublimable substance.
Alloys
An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal. Examples include brass, bronze and stainless steel. Once formed as an alloy, its components are not generally separated by simple physical separation methods at this level.
11. Suspensions
A suspension is a heterogeneous mixture in which relatively large, insoluble particles remain dispersed for some time but settle when left undisturbed.
- Particles are large enough to be visible to the naked eye.
- Particles can settle under gravity.
- Many suspensions can be separated by filtration.
- Examples: sand in water, sawdust in water, tea leaves in water.
Muddy water
When muddy water is left undisturbed, heavier particles settle. Filtration can remove larger particles, but very fine particles may remain and keep the water cloudy. Centrifugation or coagulation can help in such situations.
12. Centrifugation
Centrifugation uses rapid spinning to separate components according to differences in density. In a spinning tube, denser particles move outward and settle, while the lighter liquid remains above them.
Applications
- Separation of blood components such as red blood cells and plasma.
- Laboratory sample preparation.
- Several industrial separation processes.
The Paperfuge
A paperfuge is a simple hand-powered device inspired by spinning toys. It can spin blood samples rapidly without an electric centrifuge and has been developed as a low-cost approach for some diagnostic applications in resource-limited settings.
13. Coagulation
Coagulation involves adding a substance called a coagulant that makes fine suspended particles clump together. The larger clumps can then settle and be removed by decantation or filtration.
Example: alum and muddy water
Alum (fitkari) can act as a coagulant in water purification. Another everyday example is the formation of paneer/cheese from milk when an acid such as lemon juice or vinegar causes milk proteins to coagulate.
14. Colloids
A colloid has particles intermediate in size between those of a true solution and a suspension. The particles are not normally visible to the naked eye and do not settle like suspension particles.
| Mixture | Approximate particle size | Typical behaviour |
|---|---|---|
| Solution | Less than 1 nm | Very small particles; no settling; no Tyndall effect |
| Colloid | About 1–1000 nm | Particles do not settle; scatter light |
| Suspension | More than 1000 nm | Particles visible and settle |
Examples of colloids
Milk, blood, tomato sauce and ice cream are examples discussed in the chapter.
Dispersed phase and dispersion medium
The dispersed particles form the dispersed phase; the medium in which they are distributed is the dispersion medium.
Emulsions
An emulsion is a colloid involving liquids. Oil-in-water and water-in-oil emulsions are important types. Milk is commonly described as an oil-in-water emulsion; butter and some creams are examples of water-in-oil systems.
15. Tyndall Effect
The Tyndall effect is the scattering of light by particles in a colloid or suspension, making the path of a beam visible.
Beam path normally not visible.
Particles scatter light; beam path visible.
Particles scatter light; beam path can be visible.
Examples
- Sunlight entering a dusty or smoky room.
- Light beams visible through fog.
- Floodlights in a sports stadium when particles are present in air.
- Light passing through milk or another colloidal system.
16. Solution vs Suspension vs Colloid
| Property | Solution | Suspension | Colloid |
|---|---|---|---|
| Nature | Homogeneous | Heterogeneous | Heterogeneous |
| Particle size | < 1 nm | > 1000 nm | 1–1000 nm |
| Particles visible to naked eye? | No | Yes / often visible | No |
| Settles on standing? | No | Yes | No |
| Can ordinary filtration separate particles? | No | Generally yes | Not by ordinary filtration |
| Tyndall effect? | No | Yes | Yes |
| Example | Salt solution | Sand in water | Milk |
17. Separation in Daily Life, Medicine & Environment
Tea leaves are separated from tea; grains can be cleaned; oil and water can form separate layers; salt can be obtained from solutions.
Crystallisation, chromatography, distillation and centrifugation are used to separate or purify materials.
Blood components can be separated by centrifugation; separation is important for laboratory analysis and blood banking.
Water and sewage treatment use processes such as sedimentation, coagulation and filtration. Waste segregation supports recycling and composting.
Waste separation
Separating dry waste such as paper, plastic, glass and metal from wet organic waste allows recycling and composting. Separation and recovery of useful materials can also support a more sustainable use of resources.
18. Quick Revision Sheet
Separation-method memory map
19. Practice Questions
- Differentiate between homogeneous and heterogeneous mixtures with two examples each.
- Define solute, solvent and solution.
- What is concentration? Why is correct concentration important in ORS or agricultural sprays?
- Calculate % m/m when 15 g salt is dissolved in 85 g water.
- Calculate % m/v when 8 g solute is present in 200 mL of solution.
- What is % v/v? Give one example where it is useful.
- Define solubility and saturated solution.
- What generally happens to the solubility of most solids in water when temperature increases?
- Explain crystallisation and state two uses.
- Differentiate crystallisation and evaporation.
- Explain distillation using the terms boiling, vapour and condensation.
- Why can acetone and water be separated by distillation?
- What is fractional distillation? Give one industrial application.
- Describe the basic steps of paper chromatography.
- Why should the solvent level remain below the sample spot in paper chromatography?
- How can oil and water be separated?
- Explain sublimation with camphor and sand.
- What is an alloy? Give two examples.
- Define suspension and give two examples.
- Why may filtration alone fail to make very muddy water clear?
- Explain centrifugation and give one medical application.
- What is a paperfuge?
- What is coagulation? How does alum help in muddy water?
- Define colloid. Give three examples.
- Differentiate solution, suspension and colloid on the basis of particle size and settling.
- What is the Tyndall effect?
- Why does milk show light scattering while a salt solution normally does not?
- Identify a suitable technique: pigments from flower petals.
- Identify a suitable technique: naphthalene and sand.
- Explain why separation techniques are important for environmental sustainability.
20. Interactive 30-Question Quiz
Select one answer for every question and click Check Score.