Class 9th Science Is Matter Around Us Pure Notes

Explanation

• Mixtures are formed when two or more substances are combined physically without any chemical reaction.
• Homogeneous Mixtures have the same composition throughout the mixture. The components cannot be distinguished by the naked eye.
• Heterogeneous Mixtures have a non-uniform composition. Different components can usually be seen separately.

Examples

Important Differences

• Homogeneous mixtures have a uniform composition, whereas heterogeneous mixtures have a non-uniform composition.
• The components of a homogeneous mixture are not visible, while those of a heterogeneous mixture are usually visible.
• Homogeneous mixtures consist of a single phase, whereas heterogeneous mixtures have two or more phases.

Exam Tip

Explanation

• A solution is a homogeneous mixture of two or more substances.
• The solute is the substance that gets dissolved.
• The solvent is the substance that dissolves the solute.
• After mixing, a clear and uniform solution is formed.

Examples of Solutions

Characteristics of a Solution

• Homogeneous in nature.
• Particles are extremely small and cannot be seen with the naked eye.
• Particles do not settle down on standing.
• Cannot be separated by ordinary filtration.
• Does not scatter a beam of light.

Exam Tip

Explanation

• A suspension is a heterogeneous mixture in which insoluble solid particles are dispersed in a liquid.
• The suspended particles are large enough to be seen with the naked eye.
• These particles settle down at the bottom when the mixture is left undisturbed.
• A suspension can be separated easily by filtration.

Examples of Suspension

Characteristics of Suspension

• Heterogeneous mixture.
• Particles are visible to the naked eye.
• Particles settle down on standing.
• Can be separated by filtration.
• Scatters a beam of light (shows the Tyndall effect).

Quick Comparison

Exam Tip

Explanation

• A colloid is a heterogeneous mixture in which very small particles are uniformly dispersed in another substance.
• The particles are larger than those in a true solution but smaller than those in a suspension.
• Colloidal particles remain suspended and do not settle down even after standing for a long time.
• They cannot be separated by ordinary filtration.

Examples of Colloids

Characteristics of a Colloid

• Particles are intermediate in size.
• Particles cannot be seen with the naked eye.
• Particles do not settle on standing.
• Cannot be separated by ordinary filtration.
• Shows the Tyndall Effect (scatters light).

Exam Tip

Quick Revision

• Solution → Smallest particles, homogeneous, no Tyndall effect.
• Colloid → Medium-sized particles, shows Tyndall effect, particles do not settle.
• Suspension → Largest particles, settle on standing and can be filtered.

Exam Tip

Explanation

Different mixtures require different methods of separation depending on the size of particles, solubility, boiling point and physical properties of the components.

Common Separation Methods

Memory Tip

Solid + Liquid (Insoluble) → Filtration

Solid Dissolved in Liquid → Evaporation / Crystallisation

Two Liquids → Distillation / Fractional Distillation

Colours / Pigments → Chromatography

Exam Tip

• Which method is used for which type of mixture.
• One example of every separation method.
• The basic principle behind each method.

Principle of Filtration

Filtration is a method used to separate an insoluble solid from a liquid using a filter medium such as filter paper. The liquid passes through the filter paper, while the insoluble solid remains behind.

Key Terms

Examples of Filtration

• Separating sand from water.
• Filtering tea leaves from tea using a tea strainer.
• Water purification in filtration plants.
• Air filters used in vehicles and air purifiers.

Advantages

• Simple and inexpensive method.
• Quick separation of insoluble solids.
• Widely used in laboratories and industries.

Limitations

• Cannot separate dissolved substances such as salt or sugar from water.
• Not suitable for separating very fine colloidal particles.

Exam Tip

• Residue → Remains on the filter paper.
• Filtrate → Passes through the filter paper.
• Filtration works only for insoluble solids.

Principle of Evaporation

Evaporation is the process of converting a liquid into vapour by heating. It is used to separate a soluble solid from its solution. During evaporation, the liquid changes into vapour while the dissolved solid remains behind.

Working of Evaporation

1. Take a solution containing a dissolved solid (e.g., salt solution).
2. Heat the solution in an evaporating dish.
3. The liquid (water) gradually evaporates.
4. The dissolved solid remains as a residue.

Examples

Advantages

• Simple and economical method.
• Useful for obtaining dissolved solids.
• Commonly used in salt production from sea water.

Limitations

• The liquid cannot be recovered after evaporation.
• Not suitable if both the solid and liquid are required.

Exam Tip

• Evaporation → Recover the dissolved solid.
• Water changes into vapour and escapes.
• Salt remains behind.

Principle of Crystallisation

Crystallisation is a method used to obtain a pure solid in the form of crystals from its solution. When a hot saturated solution is cooled slowly, pure crystals separate out from the solution.

Steps of Crystallisation

1. Prepare a hot saturated solution of the substance.
2. Filter the solution to remove impurities.
3. Allow the solution to cool slowly.
4. Pure crystals begin to form.
5. Separate and dry the crystals.

Examples

Advantages Over Evaporation

• Produces pure crystals.
• Removes many impurities.
• Prevents decomposition of heat-sensitive substances.
• Widely used in laboratories and industries.

Difference Between Evaporation and Crystallisation

Exam Tip

• Evaporation → Obtain solid.
• Crystallisation → Obtain pure crystals.
• Crystallisation is considered a better method than evaporation for purification.