Is Matter Around Us Pure?

Interactive Notes on Pure Substances and Mixtures

Pure Substances

The word "pure" means that there is no mixing in a substance. However, according to scientific language, most things are mixtures of many substances, not single ones. That is why they are not considered pure.

Pure substances have all elements with the same chemical properties.
A pure substance is made up of only one kind of element or compound.

Substance: A substance is a kind of matter that cannot be separated into other kinds of matter by any physical process.

What is a Mixture?

A mixture is a substance in which two or more substances are simply mixed together without any chemical bonding.

Example: Air is a mixture of oxygen, nitrogen, CO₂, and water vapor.

Types of Mixtures

Homogeneous mixture
Heterogeneous mixture

Homogeneous Mixtures

A homogeneous mixture is one in which the composition is uniform throughout.

Example: Sugar in water. It has uniform composition.

Heterogeneous Mixtures

A heterogeneous mixture is one in which the composition is not uniform throughout the mixture. It consists of two or more phases.

Example: Mixture of oil and water. They do not mix thoroughly but instead form two separate layers.

What is a Solution?

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

Examples: Lemon water (nimbu pani), soda water, etc.

Solution Example: Sugar + water → Sugar solution

Types of Mixtures

Property	True Solution	Colloidal	Suspension
1. Size of solute particles	Smallest (< 10⁻⁹ m)	Bigger than true but smaller than suspension (10⁻⁹ to 10⁻⁶ m)	Biggest (> 10⁻⁶ m)
2. Visibility of particles	Can't be seen with naked eye	Can't be seen with naked eye	Can be seen with naked eye
3. Nature of mixture	Homogeneous	Seems homogeneous but actually heterogeneous	Heterogeneous
4. Separation by filtration	Particles can't be separated	Particles can't be separated	Can be separated by filtration
5. Appearance	Transparent	Translucent	Opaque
6. Stability	Stable (solute particles don't settle)	Stable	Unstable (solute particles settle)
7. Tyndall effect	Don't show	Show	May or may not show
8. Diffusion through paper	Diffuse rapidly through filter and parchment paper	Pass through filter but not parchment paper	Don't pass through either
9. Examples	Sugar in water	Milk, blood	Sand/mud in water

Common Examples of Colloids

No.	Dispersed Phase (Solute)	Dispersion Medium (Solvent)	Type	Example
1.	Liquid	Gas	Aerosol	Fog, cloud
2.	Solid	Gas	Aerosol	Smoke
3.	Gas	Liquid	Foam	Shaving cream
4.	Liquid	Liquid	Emulsion	Milk, face cream, emulsion paint
5.	Solid	Liquid	Sol	Mud, Digene
6.	Gas	Solid	Foam	Foam rubber, sponge
7.	Liquid	Solid	Gel	Jelly, cheese
8.	Solid	Solid	Solid sol	Colored gemstones, glass (milky, colored)

Note: Gas in gas is not a colloidal solution - it is called a mixture.

Concentration of Solution

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

Mass by Volume percentage = (volume of solute / volume of solution) × 100

Methods of Separation

1. Evaporation

Used when one component of a mixture can evaporate (has lower boiling point) while the other has a higher boiling point.

Example: Mixture of dye and water. Water evaporates but dye is left behind as it has higher boiling point.

2. Centrifugation

Separation of particles/substance based on their density when the mixture is rotated very fast. Denser particles are forced to the bottom and lighter particles stay above.

Example: Separating cream from milk.

Applications:

Used in diagnostic labs for blood and urine tests
Used in dairies and homes to separate butter from cream
Used in washing machine dryers to squeeze out water from clothes

3. Separating Funnel

Two immiscible liquids (which do not dissolve in each other) can be easily separated by putting in a separating funnel.

Applications:

Separation of oil from water
Extraction of iron from its ore (lighter slag is removed from above the molten iron)

4. Sublimation

Used when one component sublimes (directly converts to gas from solid) while the other does not.

Example: NH₄Cl (ammonium chloride) and NaCl (salt) mixture can be easily separated by heating so that NH₄Cl sublimes but common salt remains behind.

Applications:

Camphor, naphthalene, anthracene can sublime

5. Chromatography

Basic Principle:

Colored components of a mixture can be separated by using an absorbent on which they are absorbed at different rates.

Chromatography Process

Ink Spot (mixture of colors)

Chromatography Paper

When water/solvent moves up, the colors separate because they are absorbed at different speeds

Diagram showing chromatography process

Applications:

To separate colors of a dye
To separate pigments from natural colors like chlorophyll
To separate drugs from blood

Did you know? Chromatography is used when athletes undergo doping tests for their blood.

6. Distillation

Basic Principle:

Based on separating mixture of miscible liquids that have different boiling points, followed by condensation. The component with lower boiling point vaporizes first and is then condensed back to liquid.

Distillation Apparatus

Thermometer

Distillation flask

Mixture of acetone and water

Condenser

Cold water in

Acetone collected

Diagram of distillation setup

Example: When a mixture of acetone and water is heated, acetone (having lower boiling point) boils and moves to the delivery tube, where it condenses back to liquid with the help of a condenser. Thus, acetone is separated out in a beaker and water is left in the distillation flask.

Note: For mixtures with more than two components (liquids with different boiling points), we use fractional distillation with a fractionating column to separate all components. This process is used for air, petroleum, etc.

Fractional Distillation of Air:

Air → Liquid air → Allowed to warm slowly in a fractionating column → Gases separate at different heights

Applications of Fractional Distillation:

In petroleum refineries, petrochemical and chemical plants, natural gas processing and cryogenic air separation plants
In oil refineries to separate crude oil into useful substances (or fractions)
In the process of organic juice production
In the separation of oxygen, liquid nitrogen and argon from air

7. Crystallization

Basic Principle:

To remove impurities from a mixture by first dissolving in a suitable solvent and then crystallizing out one component.

Example: Copper sulphate crystals (impure) are first dissolved in sulphuric acid and then heated to make a saturated solution. This solution is left overnight, resulting in only pure copper sulphate crystals forming on the filter paper.

Why is crystallization better than evaporation?

Some solids decompose or get charred upon heating to dryness during evaporation (e.g., sugar).
Some impurities remain dissolved in solution after filtration. On evaporation, these impurities don't evaporate and remain with the mixture.

Applications:

Purification of salt from sea water
Separation of crystals (e.g., alum (phitkari), copper sulphate) from their impure forms

Water Purification in Water Treatment Plants

Water Treatment Process

Reservoir
(Impure water)

Sedimentation tank
(Solids settle down)

Loading tank
(Using alum)

Filtration tank
(All impurities filtered)

Chlorination tank
(Adding chlorine)

Water supply
to homes

Water treatment process diagram

Physical vs Chemical Changes

Chemical Changes	Physical Changes
Not easily reversible	Easily reversible
New product(s) formed	No new products
Reactants used up	Often just a state change
Often heat/light/sound/fizzing occurs
Electricity may be produced
A precipitate may form
Example: Wood burning	Example: Ice melting

Elements

Elements are made of the same type of atoms.

S.No.	Metals	Non-metals	Metalloids
1.	Lustrous	Non-lustrous	Metalloids have intermediate properties between metals and non-metals. Examples: Boron, Germanium, Silicon
2.	Malleable, ductile	Non-malleable, non-ductile
3.	Sonorous	Non-sonorous
4.	Good conductors of heat & electricity	Bad conductors
5.	Examples: Gold, iron, etc.	Examples: Oxygen, phosphorus

Mixture vs Compound

Mixture	Compound
1. Elements or compounds are simply mixed so no new substance is formed.	1. Substances are reacted together with each other to make a new substance.
2. Elements do not combine in a fixed ratio.	2. Composition of the components is fixed (they combine in a fixed ratio according to their masses).
3. A mixture shows the properties of its components.	3. Compound doesn't show the properties of component elements.
4. Components can be easily separated by suitable mechanical methods.	4. Components can't be separated from each other by simple mechanical methods.
Example: Mixture of iron and sulphur.	Example: Iron and sulphur react to form iron sulphide.