CBSE Class 9 Science Chapter 2 - Is Matter Around Us Pure




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 are pure substances that are non separable by physical methods like sublimation, evaporation etc.


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 are natural elements and others are man-made, they can be broadly classified as metal, nonmetals and metalloids.


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 sonorous that means it can make a ringing sound when hit and also it can conduct heat and electricity 

  • They are lustrous 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]



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


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

Examples of metalloids are Boron, silicon and Germanium.

A chemical compound is a substance made up of numerous identical molecules (or molecular entities) made up of atoms from multiple elements held together by chemical bonds. 

A molecule composed entirely of atoms of a single element is thus not a compound.

Compounds are classified into four types based on the way their constituent atoms are held together:


  • molecules that are joined by covalent bonds
  • Certain complexes are held together by coordinate covalent bonds. 
  • Ionic compounds are held together by ionic bonds. 
  • Intermetallic compounds are held together by metallic bonds.

A chemical formula denotes the number of atoms of each element contained in a compound molecule by using the standard chemical element abbreviations and numerical subscripts. 

For instance, the formula H2O denotes two hydrogen atoms bonded to one oxygen atom. Numerous chemical compounds are assigned a unique CAS number by the Chemical Abstracts Service.

 Over 350,000 chemical compounds (including chemical mixtures) have been registered for production and use on a global scale.

A compound can be converted into another chemical substance through a chemical reaction with another substance.

In either or both of the interacting substances, bonds between atoms may be broken and new bonds formed during this process.

Compounds are chemical substances composed of two or more elements in a fixed ratio. 

Chemistry is the science that investigates the

structures, physical properties, and chemical properties of matter.

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.

In chemistry, a mixture is a substance composed of two or more distinct chemical compounds that have not been chemically combined.

A mixture is a physical combination of two or more substances that retains their identities and can take the form of solutions, suspensions, or colloids.

Mixtures are created by mechanically blending or mixing chemical substances such as elements and compounds without causing chemical bonding or other chemical changes, so that each ingredient substance retains its unique chemical properties and makeup.

Despite the fact that its constituents remain chemically unchanged, a mixture’s physical properties, such as its melting point, may differ from those of the components.

Certain mixtures can be physically separated into their constituents via mechanical or thermal means.


The term “homogeneous mixture” refers to a mixture whose composition or consistent throughout.

The salt water described above is homogeneous, as the dissolved salt is distributed uniformly throughout the sample.

Because both are uniform, it is frequently easy to confuse a homogeneous mixture and a pure substance.

The distinction is that the substance’s composition remains constant. The concentration of salt in seawater varies considerably between samples.


Because the dissolved material is present in the same amount throughout the solution, all solutions are considered homogeneous.


One property of mixtures is their ability to be broken down into their constituents.

Due to the fact that no component of the mixture has reacted with another component, the identities of the various materials remain unaltered.


A mixture where the constituents or components are mixed together uniformly and distributed throughout that is without any clear boundary of separation is called a homogeneous mixture. 

Here the constituents or components cannot be seen with naked eyes or under a microscope. 

Some examples are sugar solution, water solution, air, crude oil, biogas and alloy.

A mixture that does not have uniform composition and the constituents 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:

  1. Sugar mixed with sand 
  2. Water mixed with oil


  1. Specific elements are present in specific ratios. 
  2. Components lose their individual properties when mixed in a compound. 
  3. Components cannot be separated by simple physical methods. 
  4. Large quantities of energy are required to change the formation of the compound. 
  5. Compounds are always homogeneous in nature. 


  1. The components are mixed in random ratios. 
  2. Chemical reactions do not take place during its formation. 
  3. Components retain their individual properties. 
  4. Components can be separated by simple physical methods 
  5. Energy does not take place during the formation or decomposition of the mixture. 
  6. 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 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, if we take brass, it is a mixture of approximately 30% zinc and 70% copper

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.

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: 

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.

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 dissovein the solution, you can add as much solute as you want until you reach the saturation level of the solution


What are the different ways of expression the concentration of the solution?


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: 

  1. Water mixed with oil, 
  2. Sand mixed in water, 
  3. Chalk powder mixed with water.
  1. Suspension is a heterogeneous mixture.
  2. Different particles can be seen with naked eyes.
  3. Particles can scatter a beam of light if passed through it, making the particles visible. This effect is called 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.

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

  1.  Colloid is a heterogeneous mixture.
  2.  Size of individual particles of colloid is too small to be seen by naked eyes.
  3.  The components of colloidal solution 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.

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 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 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.

  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.

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 beads 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 gases 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 to separate pure solids 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.
  1. Impurities may remain diluted in the solution even after filtration which might contaminate the solid during evaporation. 
  1. Crystallization technique is primarily used in the purification of salt obtained from seawater.

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, gravell at the centre and the sand 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.

  1. In the chlorination tank all the bacterias are eliminated from water where it is treated with bleaching powder. Now this water is drinkable and can be consumed by humans



  • Properties which can be observed and are specified like colour,hardness, rigidity, fluidity, density and melting point are called physical properties 
  • These changes occur without a change in composition and chemical nature and these properties are called physical changes 
  • The interconversion of states of matter is a physical change that is when water changes into ice its physical shape changes but it’s chemically the same 
  • During chemical changes, one substance reacts with another substance to undergo a chemical change in composition. 
  • Chemical changes bring about a new property and a new substance is obtained. 
  • Chemical changes are also called a chemical reaction for example, water and cooking oil are both liquids but their chemical characteristics  are different in odour 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 Question Answer

Question 1.

What is meant by a pure substance?


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.

Question 2.

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.

Question 5.

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 ?


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.

Question 6.

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


  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.

Question 7.

Which of the following materials fall in the category of pure substances ?

(a) Ice

(b) Milk

(c) Iron

(d) Hydrochloric acid

(e) Calcium oxide

(f) Mercury

(g) Brick

(h) Wood

(i) Air.


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.

Question 8.

Identify the solutions among the following mixtures :

(a) Soil

(b) Sea water

(c) Air

(d) Coal

(e) Soda water.


Please keep in mind that a change that is easily reversed is a physical change, whereas a change in nature that cannot be reversed is a chemical change. According to this concept, the changes listed below may be classified as :

  1. Chemical change
  2. Physical change
  3. Chemical change
  4. Physical change
  5. Chemical change
  6. Physical change
  7. Physical change
  8. Chemical change.

Question 9.

Which of the following will show the “Tyndall effect” ?

  1. Salt solution
  2. Milk
  3. Copper sulphate solution
  4. Starch sol.


Pure substances : Distilled water, diamond, graphite, raw rubber

Mixtures : curd, ice cream, kerosene oil, cooking oil, steel, vulcanised rubber, solder wire (alloy of lead and tin)


  1. Evaporation :As the water evaporates, sodium chloride is left behind..
  2. Sublimation : Ammonium chloride will be collected as sublimate.
  3. Filtration : Filtration can be used to separate metal fragments.
  4. Chromatography : Chromatography can separate the pigments (coloured components) from flower plant extracts.
  5. Centrifugation : Centrifugation will separate the butter.
  6. Separating funnel :To separate oil from water, a separating funnel can be used.
  7. Filtration : Tea leaves will be collected on the sieve following filtration using a sieve.
  8. Magnetic separation : Iron pins, not sand particles, are attracted by a magnet.
  9. Sieving : With the aid of a sieve, it is possible to separate wheat grains from their husks.
  10. Sedimentation : Sedimentation causes mud particles to settle as precipitate. Filtration can be used to separate it later.

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.

Question 3.

Pragya tested the solubility of four different substances at different temperatures and collected the data as given below (results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution).

(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 30 grams of water at 313 K ?

(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools ? Explain.

(c) Find the solubility of each salt at 293 K. Which salt has the maximum solubility at this temperature ?

(d) What is the effect of change of temperature on the solubility of a salt ?


(a) At-313 K, in the saturated solution

(b) When a saturated solution made at 353 K is cooled to room temperature (about 298 K), potassium chloride’s solubility in water decreases. It will gradually separate at the bottom of the jar as a crystalline white residue.

(c) The solubility of the salt in a water is defined as :

the greatest amount of salt that may be dissolved in 100 g of water (or other solvent) to form a saturated solution at a specified temperature

In the light of this, at 293 K

(d) The solubility of all salts in water increases as the temperature rises. This is demonstrated by the data in the table. Similarly, as the temperature decreases, these salts’ solubility in water diminishes.

Question 4.

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 precipitate.

Question 5.

Classify each of the following as a homogeneous or heterogeneous mixture :

(a) Soda water

(b) Wood

(c) Air

(d) Soil

(e) Vinegar

(f) Filtered tea.


Homogeneous mixture : Soda water, air, vinegar, filtered tea.

Heterogeneous mixture : Wood, soil.

Air is a gaseous combination of several gases. When dust or other particles are included, however, air becomes a heterogeneous mixture

Question 6.

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.


Pure substances in the given list of materials are :

(a) Ice (compound)

(c) Iron (element)

(e) Calcium oxide (compound)

(f) Mercury (element).

Milk, like hydrochloric acid, is a homogenous solution. Please keep in mind that the acid is generated when hydrogen chloride gas vapours are transported through water.

The mixes of wood and air (which contains suspended particles) are heterogeneous. However, air that is particle-free is a homogeneous mixture.

Question 8.

Identify the solutions among the following mixtures :

(a) Soil

(b) Sea water

(c) Air

(d) Coal

(e) Soda water.


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.

Question 9.

Which of the following will show the “Tyndall effect” ?

  1. Salt solution
  2. Milk
  3. Copper sulphate solution
  4. Starch sol.


Tyndall effect is shown by colloidal sol. Since milk and starch sol are colloidal sol therefore, these will show Tyndall effect.

Question 10.

Classify the following into elements, compounds and mixtures :

  1. Sodium
  2. Soil
  3. Sugar solution
  4. Silver
  5. Calcium carbonate
  6. Tin
  7. Silicon
  8. Coal
  9. Air
  10. Soap
  11. Methane
  12. Carbon dioxide
  13. Blood.


Elements : Sodium, Silver, Tin and Silicon

Compounds : Calcium carbonate, Methane, Carbon dioxide.

Mixtures : Soil, Sugar solution, Coal (as percentage of carbon varies), Air, Blood, Soap.

Question 11.

Which of the following are chemical changes ?

  1. Growth of a plant
  2. Rusting of iron
  3. Mixing of iron filings and sand
  4. Cooking of food
  5. Digestion of food
  6. Freezing of water
  7. Burning of a candle.


Chemical changes are : Growth of a plant, Rusting of iron, Cooking of food, Digestion of food, Burning of a candle.



multiple choice questions - 1 marks


short questions - 2-3 marks


long questions - 5 marks



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