Understanding mixtures, solutions, and separation techniques
A pure substance is made up of only one type of particle. It has a fixed composition and fixed properties. Examples: Pure gold, pure water, common salt (NaCl), sugar.
Imagine a box containing only red balls - all identical. That's a pure substance! Now imagine a box with red, blue, and green balls mixed together - that's a mixture! In real life, pure substances are rare. Most things around us are mixtures.
A mixture is formed by mixing two or more substances in any proportion. It does not have fixed properties. Examples: Air, salt solution, soil, milk.
Pure substances are of two types:
An element is the simplest form of matter that cannot be broken down into simpler substances by chemical methods. Examples: Gold (Au), Iron (Fe), Oxygen (O₂), Hydrogen (H₂).
A compound is formed by the chemical combination of two or more elements in a fixed ratio. It can be broken down into elements by chemical methods. Examples: Water (H₂O), Carbon dioxide (CO₂), Common salt (NaCl), Sugar (C₁₂H₂₂O₁₁).
Water is a compound made of hydrogen and oxygen in a fixed ratio (2:1). No matter where you get water from - river, ocean, or rain - it always has the same composition: 2 hydrogen atoms + 1 oxygen atom = H₂O.
Fun fact: Pure water is colorless, odorless, and tasteless. If your water has taste or smell, it contains some impurities!
Mixtures are of two types based on their composition:
A homogeneous mixture has uniform composition throughout. You cannot see the different components with naked eyes. Also called solutions.
Think of sugar dissolved in water - it looks completely uniform. You can't see where sugar is and where water is. Or think of air - you can't see oxygen, nitrogen, or other gases separately. That's a homogeneous mixture!
• Salt solution (salt dissolved in water)
• Sugar solution
• Air (mixture of gases)
• Vinegar (acetic acid in water)
• Brass (copper + zinc)
• Steel (iron + carbon)
A heterogeneous mixture has non-uniform composition. You can see the different components clearly or distinguish them easily.
Think of your lunch plate with rice, dal, and vegetables - you can clearly see and separate each item. Or think of a fruit salad - you can pick out each piece of fruit. That's a heterogeneous mixture!
• Sand and water
• Oil and water
• Soil (sand, clay, organic matter)
• Fruit salad
• Concrete (cement, sand, gravel)
• Muddy water
A solution is a homogeneous mixture of two or more substances. It consists of:
• Solute: The substance that is dissolved
(present in smaller amount)
• Solvent: The substance that dissolves the
solute (present in larger amount)
When you make lemonade, you add lemon juice and sugar to water.
Here:
• Solutes: Lemon juice and sugar (they get
dissolved)
• Solvent: Water (it does the dissolving)
• Solution: Lemonade (the final uniform mixture)
Solutions can be solid, liquid, or gas depending on the state of solvent:
| Type | Solute | Solvent | Example |
|---|---|---|---|
| Solid Solution | Solid | Solid | Brass (zinc in copper), Steel |
| Liquid Solution | Solid/Liquid/Gas | Liquid | Salt in water, Alcohol in water, CO₂ in water |
| Gaseous Solution | Gas | Gas | Air (oxygen in nitrogen) |
The amount of solute present in a given amount of solution is called concentration.
Dilute Solution: Contains small amount of solute
in a large amount of solvent
Concentrated Solution: Contains large amount of
solute in a small amount of solvent
Imagine making tea. If you add 1 teaspoon of sugar, it's a dilute solution (less sweet). If you add 5 teaspoons of sugar, it's a concentrated solution (very sweet)!
Mass by Mass Percentage:
Concentration = (Mass of solute / Mass of solution) × 100
Mass by Volume Percentage:
Concentration = (Mass of solute / Volume of solution) × 100
Volume by Volume Percentage:
Concentration = (Volume of solute / Volume of solution) × 100
Question: 20g of salt is dissolved in 100g of
water. Find the concentration of solution.
Solution:
Mass of solute (salt) = 20g
Mass of solvent (water) = 100g
Mass of solution = 20 + 100 = 120g
Concentration = (20/120) × 100 = 16.67%
Answer: The concentration is 16.67%
A solution in which no more solute can be dissolved at a given temperature is called a saturated solution.
A solution in which more solute can be dissolved at a given temperature is called an unsaturated solution.
Take a glass of water at room temperature. Start adding salt and stirring. Initially, the salt dissolves (unsaturated). Keep adding more salt. After some time, the salt stops dissolving and settles at the bottom (saturated). No matter how much you stir, it won't dissolve more!
A suspension is a heterogeneous mixture in which solid particles do not dissolve but remain suspended in the liquid. The particles are large enough to be seen with naked eyes.
• Chalk powder in water
• Muddy water
• Sand in water
• Paints
• Dust in air
A colloid is a mixture in which particles are bigger than solution but smaller than suspension. The particles are not visible to naked eyes but can scatter light.
It consists of:
• Dispersed Phase: The substance that is
distributed (like solute)
• Dispersion Medium: The substance in which it is
distributed (like solvent)
• Milk (fat droplets in water)
• Blood (cells in plasma)
• Fog (water droplets in air)
• Smoke (solid particles in air)
• Butter (water droplets in fat)
• Jelly (solid in liquid)
The scattering of light by colloidal particles is called Tyndall effect. This is why you can see the path of a beam of light in a dusty room or in fog. The dust or fog particles scatter the light, making the beam visible!
Have you noticed sunlight coming through trees in a forest? You can see the beams of light because dust and tiny water droplets in air scatter the light. This is Tyndall effect! The same thing happens when you see car headlights in fog.
| Property | Solution | Colloid | Suspension |
|---|---|---|---|
| Particle Size | Less than 1 nm | 1-100 nm | More than 100 nm |
| Visibility | Not visible | Not visible | Visible |
| Settling | Do not settle | Do not settle | Settle down |
| Filtration | Cannot separate | Cannot separate | Can separate |
| Tyndall Effect | Do not show | Show | Show |
| Nature | Homogeneous | Heterogeneous | Heterogeneous |
| Example | Salt solution | Milk | Muddy water |
Different methods are used to separate mixtures based on the properties of their components:
Used to separate a solid dissolved in a liquid. The solution is heated so that the liquid evaporates, leaving behind the solid.
Use: Obtaining salt from sea water, recovering sugar from sugar solution.
Used to separate suspended particles by spinning the mixture at high speed. The heavier particles settle at the bottom.
Use: Separating cream from milk, blood components, muddy water.
Remember the spin dryer in washing machines? When it spins very fast, water is thrown out and clothes become dry. Centrifugation works on the same principle!
Used to separate two immiscible liquids (liquids that do not mix). The mixture is kept in a separating funnel. The denser liquid settles at the bottom and can be separated.
Use: Separating oil and water, separating kerosene and water.
Used to separate a sublimable solid from a non-sublimable solid. When heated, the sublimable solid directly changes to gas, which can be collected separately.
Use: Separating camphor from salt, ammonium chloride from sand.
Used to separate colors or pigments from a mixture. Different components move at different speeds on a medium (like paper), thus getting separated.
Use: Separating colors in dyes or ink, separating pigments in plants.
Take a black marker and make a dot on a filter paper. Dip the paper in water (keeping the dot above water level). You'll see the black ink separating into different colors as water rises! This is chromatography.
Used to separate a liquid from a solution or to separate two miscible liquids with different boiling points. The mixture is heated, the liquid with lower boiling point evaporates first, and then it is condensed back to liquid.
Use: Obtaining pure water from salt solution, separating acetone and water.
When two liquids have boiling points close to each other, fractional distillation is used. It uses a fractionating column for better separation.
Use: Separating crude oil into petrol, diesel, kerosene, etc. Separating different gases from liquid air.
A change in which no new substance is formed. Only physical properties like shape, size, or state change. The change is usually reversible.
Examples: Melting of ice, dissolving sugar in water, breaking glass, folding paper.
A change in which a new substance with different properties is formed. The change is usually irreversible.
Examples: Burning of paper, rusting of iron, cooking food, digestion of food.
When you boil an egg, the liquid inside becomes solid. You cannot reverse it back to liquid form. New substances are formed with different properties. This is a chemical change.
But when you boil water and it becomes steam, you can cool the steam to get water back. No new substance is formed. This is a physical change.
Q1. Why is air a mixture and not a compound?
Answer: Air has variable composition (oxygen,
nitrogen, CO₂, etc. in varying amounts). Compounds have fixed
composition. Air can be separated by physical means (fractional
distillation). Hence, it's a mixture.
Q2. Why do we see the path of light in a cinema
hall?
Answer: Dust particles in air act as colloid and
scatter light, making the light beam visible. This is Tyndall
effect.
Q3. Why can't we separate salt from its solution
by filtration?
Answer: Salt dissolves completely in water
forming a solution. The particle size is very small (less than 1
nm). These particles pass through the filter paper. We need
evaporation to separate salt.