Class 9 Science Chapter 2 Notes: Pure substances and mixtures (BOSEM)

class 9 science pure substances notes

Class 9 Science Chapter 2 Notes

Class 9 Science Chapter 2 Notes is a comprehensive study guide that covers all the topics covered in your science textbook as per Board of Secondary Education Manipur(BOSEM) Chapter 2: Pure Substances and Mixtures.

The Class 9 Science Chapter 2 Notes notes are prepared for you in an easy-to-understand with animations wherever applicable. The notes provided below will help you understand the core concept of the nature of matter.

After studying the chapter you will have the concept of the following topics.

What is a pure substance?

A pure substance is a form of matter which cannot be separated into other kinds of matter by any physical process. They are composed of only one type of element or compound and have uniform and consistent properties throughout.

Examples: Gold (Au), Silver (Ag), Diamond (pure carbon), Oxygen, Hydrogen, mercury etc.

What is a Mixture?

A mixture is a combination of two or more substances mixed in any proportion. They are physically blended together but not chemically bonded.

In a mixture, each component retains its own chemical properties and can be separated from the mixture by using physical methods.

Types of mixture

Mixtures can be divided into two types

Homogeneous mixture

In a homogeneous mixture, the components are uniformly distributed throughout the mixtures and there are no distinct boundaries between them.

For example: A mixture of salt and water (salt solution), a mixture of sugar and water (sugar solution).

Heterogeneous mixture

In a heterogeneous mixture, the components are not uniformly distributed and we can see distinct boundaries or phases within the mixtures.

For example Mixture of sugar and sand, Petrol and water etc.

Heterogeneous mixtures can be classified further into colloids and suspensions.


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

Components of solution

  1. Solvent
  2. Solute

Solvent: A solvent is a substance that has the ability to dissolve other substances(solute) to form a homogeneous mixture called a solution.

Solute: The component that is dissolved in the solvent, which is present in the smaller amount is the solute.


  1. When sugar(solute) dissolves in water (solvent), a solid in liquid solution is formed.
  2. A solution of glycerin in water is an example of liquid in liquid solution.
  3. A solution of gas in gas- Air is a mixture of gases like nitrogen (78%), oxygen(21%), and other gases like carbon dioxide and water vapour etc. in small amounts, so air is a solution of gases.
  4. Solid in solid solution: Alloys are homogeneous mixture of metals and so are solid solutions. For example, Brass contains 30% of zinc and 70% copper.

Properties of solution

Solutions are very stable as the solute particles cannot be separated by filtration.

Solubility of a Solution

The solubility of a substance can be defined as the maximum number of grams of a substance that can be dissolved in 100g of a solvent at a certain temperature.

Depending upon the amount of solute present in a solution, it can be classified into:

  1. Saturated solution
  2. Unsaturated solution.

Saturated solution: Saturated solution is a kind of solution in which no more solute can be dissolved in the solution at a given temperature.

Unsaturated solution: If the amount of solute present in a solution is less than the saturation level, it is unsaturated solution.

Concentration of a solution

It is the amount of solute present in a given amount of a solution. it can also be defined as the amount of solute dissolved in a given volume of a solvent.

Concentration of a solution

The concentration of a solution may be expressed in terms of mass/mass per cent as

Or in terms of mass/volume per cent as


A suspension is a heterogeneous mixture in which particles of solid are spread throughout a liquid without dissolving in it.

Properties of suspension


A colloidal dispersion or colloid is a heterogeneous mixture in which one substance is dispersed in another in the form of tiny particles, droplets, or bubbles that are evenly distributed throughout the mixture.

Colloids consist of two-phase

  1. Dispersed phase
  2. Continuous phase

Disperse Phase: In this phase, the substance exists in the form of small particles or droplets suspended within the mixture.

These particles are typically solid, liquid, or gas, and they are often referred to as colloidal particles.

Continuous phase: This is the medium in which the dispersed phase is suspended. It is usually a liquid, but colloids can also exist in gases or even solids. The continuous phase surrounds and stabilizes the dispersed phase, preventing the particles from settling.

Colloids can be found in various forms in everyday life, including emulsions(such as mayonnaise and milk). aerosols (like fog or paint spray) etc.

When a beam of light is allowed to pass through the colloidal dispersion, the path of the light beam is illuminated.

Tyndall Effect

The scattering of light by colloidal particles is known as the Tyndall effect.

Types of colloidal system

There are eight types of colloidal systems as given below:

Disperse phaseDispersion mediumTypeExamples
SolidLiquidSolMud, Starch solution
SolidGasSolid aerosolSmoke, Dust
SolidSolidSolid solMilky glass,
Coloured Gemstone
GasLiquidFoamSoap seeds,
Whipped cream
GasSolidSolid foamPumice, Sponge
LiquidGasLiquid aerosolFog, Clouds
LiquidLiquidEmulsionMilk, Face cream
LiquidSolidGelJelly, Cheese

Separation Techniques/Methods


Centrifugation is the technique of separating particles from a solution using centrifugal force according to the size, density, and shape of the medium. The principle of the technique is that when the mixture is rotated rapidly the heavier particles are forced to the bottom and the lighter particles stay at the top. It is done by a machine called Centrifuge which can rotate at high speed.

Application of centrifugation

  1. It is used in diagnostic laboratories for blood and urine tests.
  2. It is used to separate butter from milk at home or in dairy industries.
  3. Used in drying clothes in washing machines.


Sublimation can also be used to separate volatile substances from its mixture containing non-volatile components.

sublimation can be used to separate ammonium chloride, naphthalene, camphor and anthracene from their mixtures containing salt, sand and earthly materials.

Figure: Separation of salt and ammonia by sublimation


Chromatography is a technique used for the identification and separation of solutes present in a solution in small quantities.

Figure: Separation of dyes in ink by paper chromatography

Application of chromatography

  1. It is used in the separation of colour substances (dyes and pigments) in solution.
  2. Used in forensic science to detect and identify trace amounts of drugs in the urinary bladder or stomach and blood.
  3. To separate small amounts of products from chemical reactions.


Crystallization is a technique of separating a pure solid in the form of a crystal from a solution.


Separation of potash alum from an impure sample

  1. The impure solid is dissolved in minimum quantity of water.
  2. The impurities are filtered out.
  3. The clear solution is heated gently to get a saturated solution in a china disc.
  4. The hot-saturated solution is cooled down.
  5. The crystal of pure solid can be separated by filtration and dried.

Separation and purification of liquid

Immiscible liquids

Liquids which do not mix each other and form separate layers when put it in a container are called immiscible liquids.

Example, mustard oil and water, kerosene and water etc.

A separating funnel can be used to separate immiscible liquids as shown below

Figure: Separating Funnel

Simple Distillation

This technique can be used t purify liquids which boil under ordinary pressure without decomposition. Alcohol and water are imiscible and the mixture of these two can be separated b distilllation as alcohol boils at 78.5°C and water boils at 100°C.

Figure: Simple Distillation

Fractional Distillation

Fractional distillation is a method employed to separate a mixture of two or more miscible liquids which have a boiling point difference of less than 30°C.

It is used in the separation of different fractions from petroleum. It is also used for separation of different gases from air.

In fractional distillation, a fractionating column is fitted over the distilling flask. This fractionating column provides a temperature graidient i.e. higher temperature at the bottom and lower temperature at the top. Fractional distillation of crude petroleum results to products such as- Petroleum gas, petrol, kerosene, diesel, lubricating oil, asphalt etc.

Figure: Fractonal Distillation

Physical and chemical change

Physiccal changes are the changes in the physical properties such as texture, shape, size and the state of substances. THe changes are usually reversible.

e.g, Dissolving sugar in water, freezing of water etc.

Chemical changes are the changes that results in the formation of new substances due to changes in the internal comosition of the original substances. These changes are irreversible in nature. e.g. Burning of insence stick, rusting of iron etc.

Types of pure substances

  1. Elements
  2. Compounds
A pure substance which cannot be further splitted up into any substancesA compound is a pure substance form by combination of two or more elements in a fixed proportion
Examples Sodium (Na), Potassium (K) etcExamples are Water (H2O), Carbon dioxide (CO2) etc
Classification of elements
  1. Metals
  2. Non-Metals
  3. Metalloids

Some of the properties of metals are:

  • They show a shining appearance called metallic lustre.
  • They are good conductors of heat and electricity.
  • They are malleable that is they can be beaten into sheets.
  • They are ductile.
  • They have high melting and boiling points.
  • They are sonorous.

Some properties of non metals are

  1. They are not lustrous, sonorous and malleable.
  2. They are non-ductile.
  3. They are poor conductor of heat and electricity.
  4. They have low melting and boiling points. Examples are Hydrogen, Oxygen, Nitrogen etc

Metalloids: Elements that have intermediate properties between those of metals and non metals are called metalloids.

Examples: Arsenic (As), Antimny (Sb), Silicon (Si), etc.

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