Class 9 Science - Chapter 2: Is Matter Around Us Pure?

Is Matter Around Us Pure?

Introduction to Pure and Impure Substances

In our daily life, we encounter various types of matter. Some are pure substances while others are mixtures. Understanding the difference between pure and impure substances is essential in chemistry.

Pure Substance: A pure substance is made up of only one kind of particle (atoms or molecules). It has a fixed composition and definite properties. Examples: Gold, water, salt, sugar.

Impure Substance (Mixture): A mixture is made up of two or more pure substances mixed together in any proportion. Examples: Air, sea water, milk, soil.

Types of Mixtures

Mixtures can be classified into two main types:

1. Homogeneous Mixture

                    Properties:
                    Has uniform composition throughout
Components are not visible separately
Also called solutions
Cannot be separated by simple physical methods
Examples: Sugar solution, salt solution, air, brass, bronze

                

2. Heterogeneous Mixture

                    Properties:
                    Does not have uniform composition throughout
Components are visible separately
Can be separated by simple physical methods
Examples: Sand and water, oil and water, salad, soil

                

Solutions

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

Solute: The substance which is dissolved in a solvent is called solute. It is present in smaller quantity.

Solvent: The substance in which solute is dissolved is called solvent. It is present in larger quantity.

Types of Solutions

Solute	Solvent	Example
Solid	Liquid	Sugar in water, salt in water
Liquid	Liquid	Alcohol in water, vinegar
Gas	Liquid	Carbon dioxide in water (soda)
Solid	Solid	Alloys (brass, bronze)
Gas	Gas	Air (mixture of gases)

Concentration of Solution

The concentration of a solution tells us how much solute is dissolved in a given amount of solvent.

Concentration Formula:
Concentration of solution = (Mass of solute / Mass of solution) × 100

Types of Solutions Based on Concentration

Saturated Solution: A solution in which no more solute can be dissolved at a given temperature.
Unsaturated Solution: A solution in which more solute can be dissolved at a given temperature.
Supersaturated Solution: A solution which contains more solute than required for saturation at a given temperature.

Solubility

Solubility: The maximum amount of solute that can be dissolved in 100 grams of solvent at a specific temperature is called solubility.

Factors Affecting Solubility

Temperature: Solubility of solids in liquids increases with increase in temperature. However, solubility of gases in liquids decreases with increase in temperature.
Pressure: Solubility of gases in liquids increases with increase in pressure.
Nature of solute and solvent: "Like dissolves like" - polar solutes dissolve in polar solvents.

Suspensions

Suspension: A heterogeneous mixture in which solid particles remain suspended in a liquid and settle down after some time.

Properties:

Heterogeneous mixture
Particles are visible to naked eye
Particles settle down on keeping
Can be separated by filtration
Scatters light (Tyndall effect)
Examples: Chalk in water, muddy water, flour in water

Colloids

Colloid: A heterogeneous mixture in which particle size is between solution and suspension (1 nm to 100 nm).

Properties:

Heterogeneous but appears homogeneous
Particles are not visible to naked eye
Do not settle down
Cannot be separated by filtration
Shows Tyndall effect
Examples: Milk, fog, smoke, clouds, blood

Tyndall Effect

Tyndall Effect: The phenomenon of scattering of light by colloidal particles is called Tyndall effect. This effect helps us distinguish between solution and colloid.

Separation of Mixtures

Various methods are used to separate components of mixtures:

1. Evaporation

Used to separate non-volatile solid from a liquid. The liquid evaporates leaving behind the solid.

Example: Separation of salt from salt solution

2. Centrifugation

Used to separate suspended particles from a liquid by spinning at high speed.

Example: Separation of cream from milk, separation of blood cells from plasma

3. Separating Funnel

Used to separate immiscible liquids (liquids that do not mix).

Example: Separation of oil from water

4. Sublimation

Used to separate sublimable solid from non-sublimable solid.

Example: Separation of camphor from salt, iodine from sand

5. Chromatography

Used to separate components of a mixture that dissolve in the same solvent.

Example: Separation of pigments in ink, dyes, plant extracts

6. Distillation

Used to separate miscible liquids with different boiling points or to obtain pure liquid from solution.

Example: Separation of acetone from water, purification of water

7. Fractional Distillation

Used to separate miscible liquids having boiling points close to each other.

Example: Separation of petroleum into fractions, separation of gases from liquid air

8. Crystallization

Used to obtain pure solid from impure solid or solution.

Example: Purification of salt, copper sulphate

Physical and Chemical Changes

                    Physical Change: A change in which no new substance is formed. The composition remains same. Examples: Change of state, dissolving sugar in water.
                

                    Chemical Change: A change in which a new substance with different properties is formed. Examples: Burning of paper, rusting of iron.
                

Elements, Compounds and Mixtures

Element: A pure substance which cannot be broken down into simpler substances by chemical methods. Examples: Hydrogen, oxygen, gold, carbon.

Compound: A pure substance made up of two or more elements chemically combined in a fixed ratio. Examples: Water (H₂O), carbon dioxide (CO₂), salt (NaCl).

Differences

Mixture	Compound
Components mixed in any ratio	Elements combined in fixed ratio
Components retain properties	New properties different from elements
Can be separated by physical methods	Can be separated only by chemical methods
No energy change during formation	Energy is absorbed or released

Multiple Choice Questions (MCQ)

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Subjective Questions

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