Class 9 | Science | Chapter 5: Exploring Mixtures and their Separation

 Mixture :-

A mixture is a substance formed when two or more elements or compounds are physically combined in any proportion, without undergoing a chemical change.

Examples:

Air → mixture of nitrogen, oxygen, carbon dioxide, etc.

Salt solution → mixture of salt and water

Soil → mixture of minerals, sand, clay, organic matter, etc.

Classification of Mixtures:-

Mixtures are mainly classified into two types:

1. Homogeneous Mixture

A homogeneous mixture has a uniform composition throughout. Its components are completely mixed and cannot be distinguished separately.

Examples:

• Salt solution
• Sugar solution
• Air
• Brass (copper + zinc)

Key point: It has only one visible phase.

2. Heterogeneous Mixture

A heterogeneous mixture does not have a uniform composition throughout. Its components can often be distinguished separately.

Examples:

• Sand and water
• Oil and water
• Soil
• Iron filings and sulphur

Key point: It may have two or more visible phases.

Solutions:-

A solution is a homogeneous mixture of two or more substances.

For example, when salt dissolves in water, salt solution is formed.

1. Solute

The substance that gets dissolved in a solution is called the solute.

Example: In salt solution, salt is the solute.

2. Solvent

The substance that dissolves the solute is called the solvent.

Example: In salt solution, water is the solvent.

Remember:

Solute + Solvent → Solution

3. Concentration

The amount of solute present in a given amount of solution is called the concentration of the solution.

Mass percentage=(Mass of solute / Mass of solution)×100

where:

Mass of solution = Mass of solute + Mass of solvent

Example: If 10 g salt is dissolved in 90 g water:

Mass of solution = 10 + 90 = 100 g

Mass percentage of salt:

 (10/100) X 100 = 10%

4. Solubility

Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent at a particular temperature to form a saturated solution.

Example: If no more sugar dissolves in water at a particular temperature, the solution has reached its solubility limit.

5. Saturated Solution

A saturated solution is a solution that contains the maximum amount of solute that can dissolve in the solvent at a given temperature.

Example: If you keep adding sugar to water and eventually some sugar remains undissolved at the bottom, the solution is saturated.

Methods of Separation of Homogeneous Mixtures –

Homogeneous mixtures can be separated into their components using different physical methods, depending on the properties of the substances.

1. Crystallization

Crystallization is used to obtain a pure solid substance in the form of crystals from its solution.

Principle: Difference in solubility of the substance at different temperatures.

Example: Obtaining pure copper sulphate crystals from copper sulphate solution.

Basic process:

Solution → Heating/concentration → Cooling → Crystals

Important: Crystallization is generally preferred over simple evaporation when a pure crystalline solid is required.

2. Distillation

Distillation is used to separate a solvent from a solution by using the difference in boiling points.

Principle: Difference in boiling points.

Example: Obtaining pure water from salt solution.

Process:

1. Heat the solution.
2. The component with the lower boiling point changes into vapour.
3. Vapour is cooled in a condenser.
4. It changes back into liquid and is collected.

Salt solution → Heating → Water vapour → Cooling → Pure water

3. Fractional Distillation

Fractional distillation is used to separate two or more miscible liquids having close boiling points.

Principle: Difference in boiling points of liquids.

Example: Separation of different components of petroleum and separation of liquid air into nitrogen and oxygen.

A fractionating column is used to make the separation more effective.

Key point:

Distillation → usually liquids with a significant difference in boiling points

Fractional distillation → liquids with close boiling points

4. Paper Chromatography

Paper chromatography is used to separate and identify different components of a mixture based on their different rates of movement through a medium.

Principle: Different components have different solubilities in the solvent and different attractions towards the paper.

Example: Separating different colours present in black ink.

Basic process:

A spot of ink is placed on chromatography paper.

The paper is placed in a suitable solvent.

The solvent moves upward through the paper.

Different dyes move at different speeds.

Separate coloured spots appear.