Solutions · Suspensions · Colloids · Crystallization · Distillation · Chromatography
Have you ever wondered how sweet, white crystals of sugar are obtained from tall, green sugarcane plants? Or how doctors detect diseases like malaria using just a few drops of blood? Many such everyday activities are made possible by techniques based on the fascinating science of separating mixtures.
In this chapter, you will explore mixtures in greater depth, including their properties, behaviour and the various techniques used to separate them. From industrial processes like sugar production to life-saving medical tests, the separation of mixtures plays a crucial role in our daily lives.
A mixture of sugar and water has a uniform composition throughout -- it is equally sweet in the first and last sip. Such a mixture is called a homogeneous mixture or a solution.
On the other hand, a stirred mixture of sand and water is not uniform. The sand particles are visible and settle with time. Such a mixture is called a heterogeneous mixture.
Uniform composition throughout (sugar solution, vinegar, soda)
Non-uniform composition (sand in water, oil in water)
A homogeneous mixture that always remains uniform
Three groups prepare mixtures: A (salt + water), B (chalk powder + water), C (milk + water). Pass a laser pointer through each beaker. Observe from the side:
Light path not visible -- true solution with tiny particles.
Light path clearly visible -- suspension with large particles that scatter light.
Light path visible -- colloid scatters light though mixture appears uniform.
Click each substance to sort it into "Homogeneous" or "Heterogeneous":
| Property | Solution | Colloid | Suspension |
|---|---|---|---|
| Particle Size | — | — | — |
| Light Beam | — | — | — |
| Filtration | — | — | — |
| Settling | — | — | — |
Solutions are homogeneous mixtures. A solution is prepared when a solute (the substance that gets dissolved) is mixed with a solvent (the substance that dissolves the solute).
The substance that gets dissolved. Example: sugar in sugar solution.
The substance that dissolves the solute. Example: water in sugar solution.
The amount of solute dissolved in a given amount of solvent or solution is called the concentration of the solution. The right proportion is always essential when preparing a solution.
ORS (Oral Rehydration Solution): Specific amounts of salt and sugar must be added to a fixed amount of water. Changing these amounts will not give ORS!
Pesticide Spray: Farmers must mix the right amount of pesticide with water. Too little may not protect crops, too much can damage crops, soil and the environment.
There are three main ways to express concentration as a percentage:
Formula: (Mass of solute / Mass of solution) × 100
Used for homogeneous & heterogeneous mixtures, packaged foods, spice mixtures.
Formula: (Mass of solute / Volume of solution) × 100
Used in medicines and labs. E.g., 5% glucose solution = 5 g glucose in 100 mL solution.
Formula: (Volume of solute / Volume of solution) × 100
Used when mixing two liquids -- perfumes, cosmetics, vinegar.
Q: If 10 g of salt is dissolved in 90 g of water, calculate the mass by mass percentage.
A: Mass of solution = 10 + 90 = 100 g
% m/m = (10/100) × 100 = 10%
Q: If 5 g of glucose is dissolved in water to make 100 mL of solution, find % m/v.
A: % m/v = (5/100) × 100 = 5%
Q: If 1 mL of liquid pesticide is mixed with water to form 100 mL of spray, find % v/v.
A: % v/v = (1/100) × 100 = 1%
Enter values to calculate mass by mass percentage:
Calculate mass by mass % and mass by volume % concentrations. Enter your values:
The maximum amount of solute that dissolves in a fixed quantity of solvent (100 mL or 100 g) is called its solubility at a given temperature. A solution that cannot dissolve any more solute at that temperature is called a saturated solution.
Solubility of solid solutes generally increases with temperature.
Solubility of gases generally decreases with increase in temperature.
If you take a saturated solution at a higher temperature and cool it slowly, the excess solute separates out as a pure solid, often as crystals. A crystal is a solid made up of particles arranged in a regular geometric pattern.
The process of forming crystals from a saturated solution in a laboratory is called crystallization. It can be used for:
Large, cubic crystals formed naturally from evaporation of salt water.
Candy sugar formed by slow crystallization from sugar solution.
Ice crystals formed when water vapour freezes in air.
Shiny blue crystals grown in the lab from saturated solutions.
Steps: Take 1 g copper sulfate in 25 mL water. Add a drop of dilute sulfuric acid. Heat gently in a water bath, adding more copper sulfate until saturated. Filter the hot solution. Allow to cool slowly. Beautiful blue crystals form!
Slow cooling = larger, well-shaped crystals. Rapid cooling = smaller, less well-formed crystals.
In Sohra (Cherrapunji), this cave has fascinating natural crystal formations. Quartz is one of the beautiful crystals found in nature.
Seawater is evaporated in shallow pools. As water evaporates, the salt solution becomes saturated and salt crystals form!
A homogeneous mixture of two miscible liquids can be separated by heating until the liquid with the lower boiling point vaporises. The vapour is then cooled and collected as a pure liquid. This process is called distillation.
Acetone boils at 56 °C, water at 100 °C. The large difference allows acetone to vaporise first while water stays behind.
The setup includes: a distillation flask containing the mixture, a thermometer to monitor temperature, a water condenser to cool the vapour, and a collection flask for the distillate.
As the mixture is heated, the lower-boiling liquid vaporises first. Vapours pass through the condenser where circulating water cools them back into liquid form.
When two miscible liquids have boiling points that differ by less than 25 °C, simple distillation won't work well. Instead, we use fractional distillation. This is how crude petroleum is separated into useful products like petroleum gas, petrol, kerosene, diesel, and lubricating oil.
| Substance | Boiling Point |
|---|---|
| Acetone | 56 °C |
| Alcohol (Ethanol) | 78 °C |
| Chloroform | 61 °C |
| Benzene | 80 °C |
| Water | 100 °C |
Have you noticed what happens when a drop of water falls on writing done with a sketch pen on paper? The colour spreads! If the pen is black, different colours might separate out. This is the principle behind paper chromatography.
Paper chromatography uses differences in how components interact with the solvent and the paper to separate them. The liquid carries substances up the paper, separating them based on how fast they move.
Steps:
Different components move at different speeds depending on their interaction with the solvent and paper.
The word chromatography comes from Greek: chroma (colour) + graphein (to write). It literally means "writing with colour" as it was first used to separate coloured substances.
Click a mixture on the left, then click the correct technique on the right to match them:
Oil and water do not mix -- they are immiscible liquids that form separate layers. A separating funnel is used to separate them based on their different densities.
Pour 5 mL mustard oil + 20 mL water into a separating funnel. Let it stand -- two layers form. The yellow mustard oil floats on top (lower density) and water settles at the bottom (higher density).
Open the stopcock to drain the water layer first, then collect the oil separately.
Sublimation is the process where a solid changes directly into vapour without passing through the liquid state (below its melting point). The reverse process (vapour back to solid) is called deposition.
Heat the mixture -- camphor sublimes and deposits on the funnel wall while sand stays behind.
Solid CO₂ (dry ice) also undergoes sublimation. It turns directly from solid to gas!
Naphthalene (moth balls) is another sublimable substance that can be separated from non-sublimable solids.
Suspensions are heterogeneous mixtures where solid particles do not dissolve but remain suspended in the liquid. Particles are visible to the naked eye (larger than 1000 nm) and settle over time.
Centrifugation involves spinning a mixture at high speed. The centrifugal force pushes heavier particles outward where they settle at the bottom of the tube, while the lighter liquid stays on top.
A simple hand-powered centrifuge inspired by a spinning toy. It can separate blood components without electricity, helping detect malaria and anaemia in remote areas!
Coagulation involves adding a substance (coagulant) to make small suspended particles clump together into larger clumps that can then settle by gravity.
Added to muddy water, alum causes fine particles to clump together (coagulate). The clumps settle down (sedimentation) and can be removed by filtration.
Acid (lemon juice or vinegar) acts as a coagulant, causing milk proteins to coagulate and form cheese (paneer).
When metals are melted and mixed at high temperatures, they form alloys -- homogeneous mixtures that cannot be separated by physical methods.
| Alloy | Composition | Properties |
|---|---|---|
| Brass | ~80% Copper + ~20% Zinc | Strong, corrosion-resistant |
| Bronze | ~80% Copper + ~20% Tin | Hard, durable |
| Stainless Steel | Iron + Carbon + Chromium + Nickel + Molybdenum | Strong, rust-resistant |
Blood is neither a solution nor a true suspension -- it is a colloid. Colloids are mixtures with particle sizes between solutions and suspensions (1 -- 1000 nm).
Particle size < 1 nm. Homogeneous. Transparent. No settling. No light scattering.
Particle size 1-1000 nm. Appears uniform. No settling. Scatters light (Tyndall effect).
Particle size > 1000 nm. Heterogeneous. Visible particles. Settles over time. Scatters light.
The components of a colloid are the dispersed phase (solute-like particles) and the dispersion medium (in which they are suspended).
Colloids with both the dispersed phase and dispersion medium as liquids are called emulsions.
Milk, vanishing cream -- oil droplets dispersed in water.
Butter, body lotion, cold cream -- water droplets dispersed in oil.
| Property | Solution | Suspension | Colloid |
|---|---|---|---|
| Nature | Homogeneous | Heterogeneous | Appears homogeneous |
| Particle size | < 1 nm | > 1000 nm | 1 -- 1000 nm |
| Visibility | Not visible | Visible to naked eye | Not visible (but scatter light) |
| Filtration | Cannot separate | Can separate | Cannot separate |
| Settling | Does not settle | Settles on standing | Does not settle |
| Tyndall effect | Not shown | Shown | Shown |
Click each substance and sort it into the correct category:
The scattering of light by particles in a colloid or suspension is called the Tyndall effect, named after scientist John Tyndall who first explained it.
Scattering occurs in colloids and suspensions but NOT in true solutions. This is because solution particles are too small to scatter light.
Watch how a beam of light behaves differently in a solution, colloid, and suspension:
Particles < 1 nm. Light passes straight through -- no scattering. Beam is nearly invisible.
Particles 1-1000 nm. Light is scattered by particles -- beam becomes clearly visible. This is the Tyndall effect!
Particles > 1000 nm. Large particles block and scatter the beam heavily. Light cannot pass through cleanly.
Answer: Clouds are a colloid (specifically an aerosol -- liquid dispersed in gas). The tiny water droplets are small enough to stay suspended in air without settling, yet large enough to scatter light (which is why we can see them and why they look white). They show the Tyndall effect when sunlight passes through them!
Answer: Smoke and dust particles in the air form a heterogeneous mixture (colloid/suspension). These particles scatter sunlight (Tyndall effect), making the air look hazy and reducing visibility. The more particles in the air, the more light is scattered, creating the hazy appearance.
Mixtures are classified as homogeneous (uniform) or heterogeneous (non-uniform), and further as solutions, suspensions, or colloids.
Solutions are homogeneous mixtures. Concentration is expressed as % m/m, % m/v, or % v/v depending on the context.
A technique to get a pure solid from its saturated solution by cooling. Based on differences in solubility at different temperatures.
Separates two miscible liquids with boiling point difference of at least 25 °C. Fractional distillation for smaller differences.
Separates compounds based on differences in their rates of movement on paper using a solvent.
Separates two immiscible liquids based on their different densities.
Solid changes directly to vapour (without liquid state). Reverse process is deposition. Used for camphor, naphthalene.
Rapid spinning separates heavier solid particles from a solid-liquid mixture using centrifugal force.
A coagulant (like alum) makes small particles clump together and settle down. Used in water purification.
Dispersed particles in colloids and suspensions scatter light, making the light beam visible. Solutions do not show this.
A. Mass by Mass Percentage (% m/m): Tells how many grams of solute are in 100 g of solution. Formula: (Mass of solute / Mass of solution) × 100. Example: 10 g salt in 90 g water = (10/100) × 100 = 10% m/m. Used for packaged foods, spice mixtures.
B. Mass by Volume Percentage (% m/v): Tells how many grams of solute are in 100 mL of solution. Formula: (Mass of solute / Volume of solution) × 100. Example: 5 g glucose in 100 mL solution = 5% m/v. Used in medicines and labs.
C. Volume by Volume Percentage (% v/v): Tells how many mL of solute are in 100 mL of solution. Formula: (Volume of solute / Volume of solution) × 100. Example: 1 mL pesticide in 100 mL solution = 1% v/v. Used for perfumes, cosmetics, vinegar.
Solutions: Homogeneous, particle size < 1 nm, particles invisible, transparent, cannot be filtered, do not settle, do not show Tyndall effect. Example: salt solution.
Suspensions: Heterogeneous, particle size > 1000 nm, particles visible to naked eye, opaque, can be separated by filtration, settle on standing, show Tyndall effect. Example: sand in water, muddy water.
Colloids: Appear homogeneous, particle size 1--1000 nm, particles not visible to naked eye, translucent, cannot be separated by ordinary filtration, do not settle, show Tyndall effect. Example: milk, blood, fog, tomato sauce.
The key distinguishing feature is particle size, which determines visibility, settling behaviour, and light scattering properties.
Distillation separates a homogeneous mixture by heating it until the lower-boiling liquid vaporises. The vapour passes through a condenser (cooled by circulating water) and condenses back into liquid, which is collected separately.
Simple distillation is used when the boiling points of the two liquids differ by at least 25 °C. Example: acetone (56 °C) and water (100 °C) -- the 44 °C difference allows clean separation.
Fractional distillation is used when boiling points differ by less than 25 °C. This uses a fractionating column that provides multiple stages of evaporation and condensation for better separation. Example: crude petroleum is separated into petroleum gas, petrol, kerosene, diesel, lubricating oil, and bitumen using fractional distillation.
Distillation can also separate a liquid from dissolved solids and recover the solvent from a solution.
Step 1 -- Sublimation: Heat the mixture. Naphthalene sublimes (changes directly to vapour) and deposits on a cool surface, separating from sand and salt.
Step 2 -- Dissolving: Add water to the remaining sand and salt mixture. Salt dissolves in water (it is soluble) while sand does not.
Step 3 -- Filtration: Filter the mixture. Sand remains as residue on the filter paper. The filtrate contains salt dissolved in water.
Step 4 -- Evaporation/Crystallization: Heat the salt solution to evaporate water. Salt crystals are left behind. Alternatively, allow slow evaporation for better-formed crystals.
This demonstrates how different properties (sublimation point, solubility, particle size) are used to separate a complex mixture step by step.