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In chemistry, we consider a substance to be pure when it is made up of one type of constituent particle; in other words, a substance is a pure single form of a matter.

Depending upon the chemical composition, matter can be classified into elements, compounds that are Pure substances that are non-separable by physical and mixtures separable by physical methods like sublimation, evaporation etc.

Pure Substance

A substance that consists of only a single type of constituent particles is called a pure substance.

Based upon the nature of constituent particles a pure substance can be classified into two types: elements and compounds.


The term element was first used by Robert Boyle in the year 1661. According to French chemist Lavoisier,an element is a basic form of matter that cannot be broken down into a simpler substance by chemical reaction’.

Till now 118 elements have been discovered out of these 92 natural elements and others are man-made, they can be broadly classified as metal, nonmetals and metalloids. 

  • Metals: Metal is an element that is malleable which means it can be hammered into thin sheets, it is also ductile which means it can be drawn into wires and also so nervous that means it can make a ringing sound when hit and also it can conduct heat and electricity
  • They are lost which means they are Shiny and have a unique colour.
  • Mercury is the only metal that is liquid at room temperature.
  • Gallium and caesium, because of their very low melting points, remain in a liquid state at a temperature slightly above room temperature [303k].
  • Non-Metals: A non-metal is an element that is neither malleable nor ductile and does not conduct heat or electricity. They display various colours

Examples of non-metals are hydrogen, oxygen, iodine, carbon, and bromine.

  • Metalloids: Elements having intermediate properties between those of metal and nonmetals are called metalloids.

Examples of metalloids are Boron, silicon and Germanium.


A compound is a substance composed of two or more elements chemically combined with each other in a fixed proportion.

Examples of compounds are water- H2O, methane CH4, carbon dioxide CO2, Ammonia NH3.


Mixtures are composed of more than one type of pure form known as a substance. Most of the matter around us exists in the form of a mixture where two or more pure components are mixed together.

Examples of mixtures are water, soil, salt and pepper.


  1. Homogeneous Mixture : A mixture where the constituents are the components are mixed together uniformly and distributed throughout that is without any clear boundary of separation is called a homogeneous mixture. Here are the constituents of the components that cannot be seen with naked eyes or under a microscope. Some examples are sugar solution, water solution, air, crude oil, biogas and alloy.
  2. Heterogeneous Mixture : A mixture that does not have uniform composition and the constituencies can be seen as separate entities mixed together is called a heterogeneous mixture. The components can be seen with naked eyes and under a microscope.

Examples of heterogeneous mixtures are: Sugar mixed with sand Water mixed with oil

Differences between compound and mixtures 


Specific elements are present in specific ratios.

Components lose their individual properties when mixed in a compound.

Components cannot be separated by simple physical methods.

Large quantities of energy are required to change the formation of the compound.

Compounds are always homogeneous in nature.


The components are mixed in random ratios.

Chemical reactions do not take place during its formation.

Components retain their individual properties.

Components can be separated by simple physical methods

Energy does not take place during the formation or decomposition of the mixture.

A mixture can be homogeneous and heterogeneous in nature.


A homogenous mixture of two or more substances is called a solution. A solution might be called a true solution.

Soda water, salt solution, and sugar solution are all examples of solutions.

In solution, there is homogeneity at particle level that means that particles of the substance are dissolved together and evenly distributed in the solution and hence they cannot be distinguished from each other.

There are two main components of a solution

  1. Solvent – (Dissolving face) The component which is usually present in a larger amount of the solution that dissolves the other component within it is called the solvent
  2. Solute (dissolved phase) The component which is present in lesser quantity of the solution and which gets dissolved in the solvent is called the solute

Alloys : Alloys are mixtures of two or more metals or a metal and non-metal and they cannot be separated into their components by physical methods.

Still, an alloy is also considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, when we take brass is a mixture of approximately 30% zinc and 70% copper

Properties Of A Solution 

Some of the important properties of a solution are as follows:

  1. Solution is a homogeneous mixture.
  2. The particles of a solution are less than 1 NM.
  3. Due to very small particles they do not scatter a beam of light passing through the solution so the path of light is not visible in the solution.
  4. A solution is stable if the solute particle does not settle down when the solution is left undisturbed. The solute particles cannot be separated from the mixture by the process of filtration.

Concentration Of A Solution 

The concentration of a solution is equal to the amount of solute present in a given amount of mass or volume of solution for the amount of solute dissolved in a given mass or volume of solvent. In solution, the relative proportion of solute and solvent does vary.

Depending upon the amount of solute present in a given amount of solvent it can be classified as follows:

  • Saturated solution : A saturated solution is a solution where no more solvent can be dissolved at a given temperature. The amount of solute present in the saturated solution at a given temperature is called solubility.
  • Unsaturated solution : An unsaturated solution is a solution where the amount of solute contained in a solution is less than the saturation level. In this solution you can add more solute which can dilute in the solution, you can add as much solute as you want until you reach the saturation level of the solution

Expressing The Concentration Of A Solution 

The method by which the concentration of a solution can be expressed are


Suspension is a heterogeneous mixture in which the solute particles do not completely dissolve but remain suspended throughout the bulk of the medium.

Examples of suspension are: 

Water mixed with oil, Sand mixed in water, Chalk powder mixed with water.

Properties Of Suspension 

  1. Suspension is a heterogeneous mixture.
  2. Different particles can be seen with naked eyes.
  3. If particles scatter, a beam of light passes through making the particles visible and this effect is called the Tyndall effect.
  4. Suspension is unstable in nature, which means the solute particles settle down when the suspension is left undisturbed.
  5. The constituents of suspension can be separated by the process of filtration.

Colloidal Solution

A colloid or a colloidal solution is a mixture that is actually heterogeneous but appears to be homogenous as particles are uniformly spread throughout the solution.

Examples of Colloidal Solutions are  milk, Shaving cream, Cheese, Dahi.

Properties Of A Colloid 

  1. Colloid is a heterogeneous mixture.
  2. Size of individual particles of colloid is too small to be seen by naked eyes.
  3. Colloidal solution components are big enough to scatter a beam of light passing through it, making its path visible.
  4. Colloidal Solutions are quite stable and particles do not settle down when the colloid is left undisturbed.
  5. Particles of colloid can pass through a filter paper therefore a colloidal solution cannot be separated by the process of filtration. But they can be separated by the process called centrifugation.

Common Examples Of Colloid 

Colloidal Solutions are classified into three states: solid liquid or gas

Separating the Components of a Mixture

Many of the natural substances are not chemically pure and can be separated by using different methods of separation to get individual components from the mixture.

In order to separate the components from a mixture, many methods can be utilised; the nature of the methods utilised to separate the mixture depends on the nature of components present in the mixture.

Heterogeneous mixtures can be separated into their components by simple physical methods like handpicking filtration at 17 that we use in our day-to-day life. Sometimes special techniques might be used to separate the components of mixtures.


Evaporation is a process of conversion of a substance from a liquid state to a gaseous state and the substance is said to be volatile if it gets evaporated.

This method is primarily used to separate volatile components from non-volatile components of a mixture. On heating, the mixture of the volatile component evaporates, leaving behind non-volatile components and hence gets separated.


Centrifugation is a process in which two components having differences in densities can be separated, this method is based on the principle that denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.

A device used to separate liquid from solid by spinning is called a centrifuge.

Centrifugal machines are commonly used for this method; the machine can be rotated by hand or using a machine.

  • Centrifugal machines are primarily used to separate ghee from milk
  • Washing machine to remove and squeeze out water from wet clothes

Separation By Using Separating Funnel 

Separation is a method that is used to separate a mixture of two immiscible liquids.

This method is based upon the principle that immiscible liquids separate out in layers depending on their densities.

Separating funnels are used in the separation of a mixture of oil and water

And it is also used during the extraction of iron from its iron ore


Sublimation is a process in which some solids which have the tendency to sublime on heating convert directly from solid to gaseous vapour state without passing through the liquid state.

A mixture containing such a solid with any other normal solid can be separated by the process of sublimation.

For example, solids that are sublime are camphor, naphthalene, and iodine.


The term chromatography is based on the Greek word Chroma which means colour, this technique is used for the separation of colour.  Chromatography is the process used for the separation of those solutes that are dissolved in the same solvent. The separation of different components of a mixture is dependent upon their different solubilities in the same solvent.

The ink we use has water as a solvent and the dye is soluble in it as the water rises on the filter paper it takes the dye particles with it.

Usually, dye is a mixture of two or more different colours. The coloured component that is most soluble in the water rises faster in this way the colour is separated from its solvent.

Applications Of Chromatography:

  1. It helps in separating drugs from the blood.
  2. It helps to separate pigments from natural colours.
  3. It helps to separate colours that are present in the dye.


Distillation is a process of separating two or more miscible liquids which boil  without decomposition and have different boiling points.

Distillation involves the conversion of a liquid into vapour followed by condensation of vapour back into the form of liquid.

Distillation is a process that is used only if the liquids have differences in their boiling points of more than 25 Kelvin.

Fractional Distillation

Where the differences in the boiling points are less than 25 Kelvin the process of fractional distillation is used which means.

For example separation of different types of gases from the air

Separation of different types of petroleum products from crude oil.

The Apparatus used for fractional distillation is similar to that of simple distillation except that a fractionating column is installed between the two distillation flask and condenser.

A simple fractionating column is a kind of tube that is packed with glass beads. The beach provides a larger surface for vapours to cool down and condense repeatedly at a faster speed.


Air is a homogenous mixture that can be separated into its components by fractional distillation. For this specific purpose, the air is compressed by increasing the pressure and it is given time to cool down to a very low temperature which causes the air to become liquid.

This liquid air is allowed to warm up slowly and gradually in a fractional distillation column where different gasses present in the air get separated at different heights depending upon their boiling point.


Crystallization is a process where the separation of pure substances from their impure form is done.

Crystallization is a process that helps a separate pure solid in the form of its crystal from a solution and this process is primarily used to purify solids for example the salt we get from seawater can have a lot of impurities but we use the process of crystallization to remove the impurities.

Crystallization technique is better compared to simple evaporation technique because:

  1. Some solids decompose or may get charged during heating from liquid to two solid-state.
  2. Impurities may remain diluted in the solution even after filtration which might contaminate the solid during evaporation.

What Is The Application Of Crystallization? 

  1. Crystallization technique is primarily used in the purification of salt obtained from seawater.
  2.  Crystallization technique is primarily used to separate crystals of alum from impure samples.


The process of purification of drinking water is done on a large scale to provide Potable water to the human population residing in a country. A large number of impurities are therein in water which is taken from the Reservoir and it requires purification before it is drinkable.


  1. Sedimentation is a process that helps us to remove suspended solids where the water is allowed to stand for some time so the impurities that settle down at the bottom or float on the surface of the water.
  2. Loading with alum helps us to remove small particles like clay soil which is present in the colloidal state. Alum helps to neutralize the clay particles and coagulate clay at the bottom of the tank.
  3. Filtration helps us to remove dissolved solids in water through a filtration tank which comprises three layers.  Coarse gravel at the bottom, Global at the centre and the sun at the top act as a filter. Impure water is introduced from the bottom so that it is retained in these three layers of gravel pure water then goes to the top and is sent to the chlorination tank.
  4. In the chlorination tank all the bacterias are element the water where the water is treated with bleaching powder now this water is drinkable and can be consumed by humans

Physical and Chemical Changes

Physical Changes 

  • Properties which can be observed and I are specified like colour hardness rigidity fluidity density melting point are called physical properties
  • These changes occur without a change in composition and in chemical nature and these properties are called physical changes
  • The interconversion of states of physical change that is when water changes into ice its physical shape changes but it’s chemically the same

Chemical Changes 

  • During chemical changes, one substance reacts with another substance to undergo a chemical change in composition.
  • Chemical changes bring along a new property and a new substance is obtained.
  • Chemical changes are also called a chemical reaction for example when water and cooking oil are both liquids their chemical characteristics are different in order and inflammability.

Petroleum Oil burns in the air where water extinguishes fire which means that the chemical properties of oil and water are completely different.

NCERT questions & answers from Is Matter Around Us Pure

What is meant by a pure substance?

Answer: In scientific terms, a pure substance is any substance or matter that cannot be separated into other types of matter through any physical process. Each and every pure element and compound is a pure substance.

List points of differences between homogeneous and heterogeneous mixtures.


Homogeneous mixture:

  • The constituents are uniformly mixed and lack distinct boundaries.
  • The constituents are not readily visible.
  • The constituents are inseparable. For this purpose, specialised methods are required.
  • Air is a homogeneous mixture.

Heterogeneous mixture:

  • The constituents may not be uniformly mixed and may exhibit distinct separation boundaries.
  • The constituents are clearly visible.
  • Separation of the constituents is straightforward.
  • An example of a heterogeneous mixture is a mixture of iron filings and powdered sulphur.

How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25°C) which are miscible with each other ?

Answer: Separation can be accomplished through the use of a simple distillation process. Both liquids are miscible. Because the difference in boiling points is less than 25 K, fractional distillation can be used to separate the components. Petrol with a lower boiling point distils first, leaving kerosene behind in the flask distillation.

Name the technique to separate
1.Butter From Curd
2.Salt From Sea Water
3.Camphor From Salt.


  1. Centrifugation in a centrifugal machine can be used to separate the components.
  2. Separation can be accomplished through crystallisation or evaporation.
  3. Sublimation is used to separate camphor from salt. Camphor is sublimated.

What types of mixtures can be separated by technique known as crystallisation ?

Answer: Solid mixtures in which one component or impurity is more soluble in one solvent than another. This method can be used to purify impure samples of copper sulphate, potassium nitrate, and potash alum, among others.

Write the steps you would use for making tea. Use the words-solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.


Tea can be prepared in the steps given ahead :

  1. In a saucepan, heat approximately two to three cups of water (solvent) over a gas burner.
  2. Once the water begins to boil, add the necessary amount of milk and sugar (both are solutes).
  3. Now, using a spoon, stir. As a result, sugar dissolves and milk becomes water miscible. A solution will materialise.
  4. Further boil the solution for a few minutes to ensure that the sugar dissolves fully.
  5. Now, in the pan, add the needed amount of tea leaves (solute). Re-boil and strain through a strainer. Tea will be collected in the form of filtrate. As a residual, tea leaves will be collected on a sieve.

Explain the following, giving examples :
(a) Saturated solution
(b) Pure substance
(c) Colloid
(d) Suspension.


(a) Saturated solution: At a particular temperature, a solution becomes saturated when the solute begins to separate at the bottom of the container in which it is made. When heated, a saturated solution often turns unsaturated.

(b) Pure substance: A pure substance is one that cannot be divided into other types of matter through any physical process.

(c) Colloid: Colloidal solutions, like suspensions, are heterogeneous in nature, but their particles are smaller in size. It is in the range of 1 nm to 100 nm, i.e., between real solution and suspension particle sizes.

(d) Suspension: Suspension is a heterogeneous combination in which the solid particles are distributed evenly throughout the liquid without dissolving in it. If the suspension is left undisturbed for an extended period of time, they settle as a precipitate.

How would you confirm that the colourless liquid given to you is pure water ?


This can be confirmed by the following experiments :

  1. Using a very fine filter paper, filter the colourless liquid. If no residue remains on the filter paper, the liquid is pure and free of suspended particles.
  2. In a china dish or beaker, evaporate the colourless liquid. If no residue remains, the water is pure and free of dissolved contaminants.
  3. Calculate the boiling point of pure water. If it is close to 373 K (100°C), the pure liquid is water.

Identify the solutions among the following mixtures :

(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water.

Answer: A solution or homogeneous mixture is defined as the combination of two or more non-reacting components in a single phase. In light of this, the following options are given. :

(b) Sea water

(c) Air

(e) Soda water.

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