Examples of mixtures include air, soil, seawater, milk, tea, brass, sugar solution, smoke, muddy water and crude oil. In a mixture, the components do not chemically combine. They retain their individual properties and can usually be separated by physical methods.

Why are mixtures called impure substances?

In chemistry, the word impure does not always mean dirty or harmful. It means the substance contains more than one type of particle. A pure substance has only one type of particle and fixed composition, while a mixture has variable composition.

1. Homogeneous Mixture

A homogeneous mixture has uniform composition throughout. Its components cannot be seen separately with the naked eye.

Examples: salt solution, sugar solution, air, brass, vinegar and alcohol-water mixture.

2. Heterogeneous Mixture

A heterogeneous mixture has non-uniform composition. Its components may be visible or may form separate phases.

Examples: muddy water, oil and water, sand and iron filings, smoke, milk and soil.

Solute and Solvent

The solute is the substance that gets dissolved. The solvent is the substance in which the solute dissolves.

Properties of a True Solution

• It is homogeneous.
• The particles are extremely small, usually less than 1 nm.
• The particles cannot be seen with the naked eye.
• The solute particles do not settle down on standing.
• It cannot be separated by ordinary filtration.
• It does not show the Tyndall effect.
• The solution is stable.

1. Mass by Mass Percentage

2. Mass by Volume Percentage

ORS and Osmosis: Do cells lyse after drinking ORS?

ORS means Oral Rehydration Solution. It contains water, salts and glucose in a carefully balanced proportion. It helps the body absorb water and electrolytes during dehydration.

Saturated Solution

A saturated solution is a solution that cannot dissolve more solute at a given temperature.

Unsaturated Solution

An unsaturated solution can dissolve more solute at a given temperature.

Supersaturated Solution

A supersaturated solution contains more dissolved solute than a saturated solution at the same temperature. It is unstable and may form crystals when disturbed or cooled.

Solubility

Principle Behind Crystallization

The principle is based on the difference in solubility of a substance at different temperatures. Many solids dissolve more in hot water and less in cold water. When a hot saturated solution cools, excess solute separates out in the form of pure crystals.

How do we get pure salt by crystallization?

1. Prepare a hot saturated solution of impure salt in water.
2. Filter the hot solution to remove insoluble impurities.
3. Allow the clear solution to cool slowly.
4. Pure salt crystals separate out.
5. Dry the crystals using filter paper.

Natural Crystals

Crystals found in nature include quartz, diamond, ruby, sapphire, snowflakes, rock salt and sugar crystals.

Lab Process: Making Copper Sulphate Crystals

1. Take water in a beaker and add a few drops of dilute sulphuric acid.
2. Heat the water gently.
3. Add copper sulphate powder slowly while stirring.
4. Keep adding until no more copper sulphate dissolves.
5. Filter the hot saturated solution.
6. Allow the solution to cool undisturbed.
7. Blue crystals of copper sulphate form.

Simple Distillation

Distillation is used to separate a volatile liquid from non-volatile impurities or to separate two miscible liquids having a large difference in boiling points.

Example: Separating water from salt solution. Water evaporates, condenses and is collected as distilled water. Salt remains behind.

Fractional Distillation

Fractional distillation is used to separate two or more miscible liquids whose boiling points are close to each other.

Fractional Distillation of Crude Oil

Crude oil is a mixture of many hydrocarbons. It is heated and passed into a fractionating column. Components with lower boiling points rise higher and condense near the top. Components with higher boiling points condense lower in the column.

Paper Chromatography of Ink

1. Take a strip of filter paper.
2. Put a small ink spot near one end.
3. Dip the end of the paper in water, keeping the ink spot above water level.
4. Water rises through the paper due to capillary action.
5. Different dyes move at different speeds and separate into coloured spots.

Suspension

Examples: muddy water, chalk powder in water, flour in water.

• Particles are large and visible.
• Particles settle down on standing.
• It is unstable.
• It can be separated by filtration.
• It shows Tyndall effect.

Colloid

Examples: milk, fog, smoke, clouds, butter, jelly, foam and blood.

Dispersed Phase and Dispersion Medium

In a colloid, the substance present in smaller amount is called the dispersed phase. The substance in which it is distributed is called the dispersion medium.

Tyndall Effect

The Tyndall effect is the scattering of light by colloidal particles or fine suspended particles, due to which the path of light becomes visible.

Examples: sunbeam entering a dusty room, headlights visible in fog, projector beam in a cinema hall.

1. Separating Funnel

A separating funnel is used to separate two immiscible liquids that form separate layers due to difference in density.

Example: oil and water.

2. Sublimation

Sublimation is the direct change of a solid into vapour without changing into liquid.

Examples: ammonium chloride, camphor, iodine, naphthalene.

3. Deposition

Deposition is the direct change of vapour into solid without becoming liquid. It is the reverse of sublimation.

4. Centrifugation

Centrifugation is used to separate fine suspended particles from a liquid by spinning the mixture at high speed.

Examples: separating cream from milk, blood component separation, washing machine drying.

5. Coagulation for Water Purification

Coagulation is the process in which small suspended impurities are made to clump together so that they settle down easily.

In water treatment, alum is commonly used as a coagulant. It helps tiny suspended particles join together and settle as larger particles.

6. Filtration

Filtration separates insoluble solid particles from a liquid using filter paper or a porous material.

Example: separating sand from water.

7. Evaporation

Evaporation is used to separate a dissolved solid from a liquid by heating the solution.

Example: obtaining salt from seawater.

8. Magnetic Separation

This method separates magnetic substances from non-magnetic substances.

Example: separating iron filings from sand.

• A mixture contains more than one substance and has variable composition.
• Solutions are homogeneous mixtures.
• Particles of a solution are smaller than 1 nm and do not scatter light.
• Suspensions are heterogeneous and unstable; particles settle down.
• Colloids show Tyndall effect but particles do not settle easily.
• Crystallization is better than evaporation for obtaining pure crystals.
• Distillation separates substances based on boiling point difference.
• Separating funnel is used for immiscible liquids.
• Chromatography separates components based on different rates of movement.

A. MCQs

B. Fill in the Blanks

C. True or False

D. Assertion-Reason Questions

E. Very Short Answer Questions

F. Short Answer Questions

G. Long Answer Questions

H. Numericals

I. Case Study Based Question

MCQs

1. C2. C3. B4. B5. C

Fill in the Blanks

1. Solute and solvent2. Tyndall effect3. Separating funnel4. Crystallization

True or False

1. False2. True3. False4. True

Assertion-Reason

1. A2. A

Numericals

Case Study Answers

1. Why is a mixture called an impure substance?
2. Differentiate between true solution, colloid and suspension.
3. Why does a true solution not show Tyndall effect?
4. Why do suspension particles settle down?
5. Explain crystallization with the example of copper sulphate.
6. How can salt be obtained from seawater?
7. How can two immiscible liquids be separated?
8. What is the principle of chromatography?
9. Why is fractional distillation used for crude oil?
10. Calculate concentration using mass by mass and mass by volume formulas.