CBSE Class 9 Science Is Matter Around Us Pure Notes Set B



homogeneous material which contains particles of only one kind and has a definite set of properties is called a pure substance.

Example : Iron, silver, oxygen, sulphur, Carbon dioxide etc., are pure substances because each of them has only one kind of particles.

(a) Characteristics of A Pure Substance :

(i) A pure substance is homogeneous in nature.

(ii) A pure substance has a definite set of properties.  These properties are different from the properties of other substances.

(iii) The composition of a pure substance cannot be altered by any physical means


(b) Elements :

A pure substance, which cannot be subdivided into two or more simpler substances by any chemical means is called an element.

(i) Example : Hydrogen, oxygen, nitrogen, copper, zinc, tin, lead, mercury, etc. are all elements as they cannot be subdivided into simpler parts by any chemical means. A substance made up of the atoms with same atomic number is called an element.

(ii) Classification of elements :

(A) On the basis of physical states, all elements can be classified into three groups :-

(1) Solids  (2) Liquids  (3) Gases

It has been found that :

  1. Two element exist as liquids at room temperature. They are mercury and bromine.
  2. Eleven elements exist as gases at room temperature. They are hydrogen, nitrogen, oxygen, fluorine, chlorine, helium, neon, argon, krypton, xenon and radon.
  3. Remaining 102 elements are solids at room temperature.

(B) Elements can be classified as metals and non - metals. There are 22 non - metals and 93 metals.

  1. amongst the metals, only mercury is liquid metal. All other metals are solids.
  2. Amongst the 22 non - metals : 10 non - metals are solids. They are boron, carbon, silicon, phosphorus, sulphur, selenium, arsenic, tellurium, iodine and astatine. 1 non-metal, bromine, is a liquid. Find non-metals, hydrogen, nitrogen, oxygen, fluorine and chlorine are chemically active gases. Six non-metals, helium, neon, argon, krypton, xenon and radon are chemically inactive gases. These are also called noble gases or rare gases.


There are a few elements which show some properties of metals and other properties of non - metals. For example they look like metals but they are brittle like non - metals. They are neither conductors of electricity like metals nor insulators like non-metals, they are semiconductor. The elements which show some properties of metals and some other properties of non-metals are called metalloids. Their properties are intermediate between the properties of metals and non-metals. Metalloids are also sometimes called semi-metals. The important examples of metalloids are : Boron (B), Silicon (SI) and Germanium (Ge).

Hydrogen is the lightest element.

(C) Elements can be classified as normal elements and radioactive elements. The elements which do not give out harmful radiations are called normal elements. Elements from atomic number 1 to atomic number 82 are normal elements. The elements which given out harmful radiation are called radioactive elements. Elements from atomic number 83 to atomic number 112 and 114, 116 and 118 are radioactive in nature.

(c) Compounds :

 A pure substance, which is composed of two or more elements, combined chemically in a definite ratio, such that it can be broken into elements only by chemical means is called compound.

The two or more elements present in a compound are called constituents or components of the compound. For example, water is a compound of hydrogen and oxygen, combined together in the ratio of 1 : 8 by weight. The water can be broken into its constituents only by electro-chemical method, i.e., by passing electric current through it.

The compounds can be further classified as acids, bases and salts. Sulphuric acid, nitric acid, hydrochloric acid, formic acid, etc. are the compounds which can be classified as acids.

Sodium hydroxide, potassium hydroxide, zinc hydroxide and calcium hydroxide can be classified as bases. Ammonium chloride, zinc sulphate, lead nitrate and calcium carbonate can be classified as salts. It must be pointed out that salts are formed by the chemical reaction between acids and bases.


Most of the materials around us are not pure substances, but contain more than one substances, elements or compounds. Such materials are called mixtures.

(a) Definition :

When two or more substances (elements, compounds or both) are mixed together in any proportion, such that they do not undergo any chemical change, but retain their individual characteristics, the resulting product is called a mixture.

(b) Types of Mixture :

(i) Homogeneous Mixture : A mixture in which different constituents are mixed uniformly is called a homogeneous mixture.

Examples : All solutions, such as solutions of common salt, copper sulpahte, sugar etc. are examples of homogeneous mixtures. Similarly, alloys such as brass, bronze etc. are homogeneous solid solutions of metals.      

(ii) Heterogeneous Mixture : A mixture in which different constituents are not mixed uniformly is called a heterogeneous mixture.

Example : A mixture of sand and salt, iron powder and sulphur powder, soil etc. are examples of heterogeneous mixtures.




(i) Composition of air is not same at all places. The percentage of oxygen decreases in the air at higher altitudes. Similarly, the air in industrial areas has more amount of carbon dioxide gas and other polluting gases as compared to air in the countryside.

(ii) The main constituents of air can be separated by physical methods, such as liquefaction and fractional distillation.

(iii) No chemical action takes place when the constituents of air, i.e., oxygen, nitrogen, water vapour and carbon dioxide are mixed. Thus, no heat or light energy is evolved or absorbed, when its constituents are mixed.

(iv) Each of the constituent of air retains its physical and chemical properties. For example, oxygen helps in combustion, carbon dioxide lowly turns limewater milky, etc. 


(i) The composition of pure water is same throughout. It always contains one part of hydrogen and eight parts of oxygen by weight.

(ii) The constituents of water cannot be separated by physical means. However, by electrochemical means water can be decomposed into hydrogen and oxygen. 

(iii) Chemical reaction takes place with the liberation of heat and light energy when one part of hydrogen combines with the eight parts of oxygen by weight.

(iv) The properties of water are entirely different from the properties of its constituents. For example, hydrogen is a combustible gas, oxygen in supporter of combustion, but their compound water extinguishes fire. 


Though the alloys are homogeneous mixture of metals which cannot be separated by any physical means, yet they are regarded mixtures on account of the following reasons :

(i) The composition of constituent metals can be varied in an alloy. For example, brass is an alloy of copper and zinc. If an alloy has 60% of copper and rest of zinc or 58% of copper and rest of zinc, in either case it is brass.

(ii) The individual metals in any alloy retain their chemical and physical properties. For example, if brass is treated with dilute sulphuric acid, then zinc in it reacts to form zinc sulphate and hydrogen, but copper does not react


A homogeneous mixtures of two or more substance is called a solution. Usually we think of a solution as a liquid that contains either a solid or a liquid or a gas dissolved in it. However, this is not true. We can also have a solid have a solid solution as in the case of alloys.

(a) Components of a Solution :

The substances present in a homogeneous solution are called components of the solution. A solution basically has two components, i.e., a solvent and a solute.

(i) Solvent : The component of a solution which is present in large proportion, is called solvent.

Usually, a solvent is the LARGER component of the solution.

For example : In the solution of copper sulphate in water, water is the solvent. Similarly, in paints, turpentine oil is the solvent.

(ii) Solute : The component of the solution which is present is small proportion is called solute.

For example : In the solution of common salt in water, the common salt is solute. Similarly, in carbonated drinks (soda water), carbon dioxide gas in the solute. 

Usually, solute is the SMALLER component of the solution.

(b) Examples of Solutions :

(i) Solid - Solid solutions : All alloys are solid solutions of metals. Brass is a solid solution of approximately 30% of zinc and 70% of copper. In this solid solution, copper (larger component) is solvent and zinc (smaller component) is solute. Similarly, Bell Metal is a solid solution of 80% of copper and 20% of tin, in which copper is the solvent and tin is the solute.

(ii) Solid - Liquid solutions : Sugar solution is an example, in which sugar is the solute and water is the solvent. Similarly, common salt solution is an example, in which common salt is the solute and water is the solvent. In case of tincture of iodine, iodine is the solute and ethyl alcohol is the solvent.

(iii) Liquid - Liquid solutions : In case of an alcoholic drink, ethyl alcohol is solute and water is solvent. Similarly, in case of vinegar, acetic acid is solute and water is solvent.

(iv) Liquid - Gas solutions : In case of aerated drinks (soda water), carbon dioxide is the solute and water is the solvent.

(v) Gas - Gas solutions : Air is a homogeneous mixtures of two main gases, i.e., 78% of nitrogen and 21% of oxygen. In this mixture, nitrogen is solvent and oxygen is solute. Similarly, the petrol fed into the engines of automobiles is a mixture of petrol vapour and air.

(c) Types of Solution :

(i) Saturated solution : A solution, which at a given temperature dissolves as much solute as it is capable of dissolving, is said to be a saturated solution.

(ii) Unsaturated solution : When the amount of solute contained in a solution is less than the saturation level, the solution is said to be an unsaturated solution.

(iii) Super saturated solution : A solution, which contains more of the solute than required to make a saturated solution, is called a super saturated solution.


A solution in which particles of the solute are broken down to such a fine state, that they cannot be seen under a powerful microscope is called a true solution.

(a) Characteristics of a True Solution :

(i) A true solution is always clear and transparent, i.e., light can easily pass through it without scattering.

(ii) The particles of a solute break down to almost molecular size and their diameter is of the order of 1 nm (10-9 m) or less.

(iii) A true solution can completely pass through a filter paper as particle size of solute is far smaller than the size of pores of filter paper.

(iv) A true solution is homogeneous in nature.

(v) In a true solution, the particles of solute do not settle down, provided temperature is constant.

(vi) From a true solution, the solute can easily be recovered by evaporation or crystallisation.

(b) Concentration of a Solution :

It is defined as the amount of solute present in a given quantity of the solution. The most common method for expressing the concentration of a solution is called percentage method. The concentration of solution refers to the percentage of solute present in solution. Furthermore, the percentage of solute can be expressed in terms of :

(i) mass of the solute

(ii) volume of the solute.

(i) Concentration of a solution in terms of mass percentage of solute : If a solution is formed by dissolving a solid solute in a liquid solvent then the concentration of solution is expressed in terms of mass percentage of solute and is defined as under :

The concentration of solution is the mass of the solute is grams, which is present in 100 g of a solution. 

It is very important to keep in mind that the percentage concentration of a solution refers to mass of solute in 100 g of solution and not 100g of solvent, i.e., water.

The concentration of a solution in terms of mass percentage of solute is calculated by the formula given below :   

Concentration of solution       


(ii) Concentration of a solution in terms of volume percentage of solute : If a solution is formed by dissolving a liquid solute in a liquid solvent, then the concentration of the solution is expressed in terms of volume percentage of solute. The concentration of a solution is the volume of the solute in milliners, which is present in 100 milliliters of a solution.

It is very important to keep in mind that the percentage concentration of solution refers to volume of solute in 100 ml of solution and not 100 ml of solvent, i.e., water.

 The concentration of a solution in terms of volume percentage of the solute is calculated by the formula given below :

Concentration of solution


The concentration of a solution is a pure percentage number and has NO UNITS.

(c) Examples :

1. What is the meaning of 15% solution of NaCl ?

Sol. 15% solution of NaCl is a solution 100 g of which contains 15 g of NaCl and 85 g of water.

2.Calculate the amount of glucose required to prepare 250 g of 5% solution of glucose by mass.


 3.A solution contains 50 mL of alcohol mixed with 150 mL of water. Calculate concentration of this solution.

Sol.   This solution contains a liquid solute (alcohol) mixed with a liquid solvent (water), so we have to calculate the concentration of this solution in terms of volume percentage of solute (alcohol). Now, we know that :


 4.How much water should be added to 16 ml acetone to make its concentration 48% ?



A heterogeneous mixture of insoluble particles of solute, spread throughout a solvent, is called a suspension. The particle size (diameter) in a suspension is more than 10-5 cm. The particles has a tendency to settle down at the bottom of the vessel and can be filtered out, because their size is bigger than the size of the pores of the filter paper.

(a) Example :

(i) Muddy water, in which particles of sand and clay are suspended in water.

(ii) Slaked lime suspensions used for white - washing has particles of slaked lime suspended in water.

(iii) Paints in which the particles of dyes are suspended in turpentine oil.

(b) Characteristics of Suspensions :

(i) The size of particles is more than 10-5 cm in diameter.

(ii) The particles of suspension can be separated from solvent by the process of filtration.

(iii) The particles of suspensions settle down, when the suspension is kept undisturbed.

(iv) A suspension is heterogeneous in nature.

(v) More scattering takes place in suspensions, because of bigger size of particles.

The process of settling of suspended particles under the action of gravity is called sedimentation.


A heterogeneous solution in which the particle size is in between 10‑ cm to 10-5 cm, such that the solute particles neither dissolve nor settle down in a solvent is called colloidal solution.

In a colloidal solution, relatively large suspended particles are called dispersed phase and the solvent in which the colloidal particles are suspended in called continuous phase or dispersing medium.

(a) Examples of Colloidal Solutions :

blood                         Milk                            Writing ink

Jelly                         Starch solution           Gum solution

Tooth paste               Soap solution             Liquid detergents

Mist and fog.

(b) Characteristics of Colloidal Solutions :

(i) The size of colloidal particles is in between 10-7 cm and 10-5 cm.

(ii) The particles of a colloidal solution are visible under a powerful microscope.

(iii) The particles of a colloidal solution do not settle down with the passage of time.

(iv) The particles of a colloidal solution can easily pass through filter paper.

(v) The particles of a colloidal solution scatter light, i.e., when strong beam of light is passed through the colloidal solution, the path of beam becomes visible.

(vi) Colloidal solutions are not transparent, but translucent in nature.

(vii) The particles of a colloidal solution are electrically charged.

(viii) The colloidal solutions are heterogeneous in nature.

TYNDALL EFFECT :The phenomenon in which light is scattered by colloidal particles and path of light becomes visible as a Tyndall cone is called Tyndall effect.           

Experiment :Take a wooden box, which is fitted with a convex lens on one side and a microscope on the other side, such that convex lens and the objective lens of microscope face each other as shown in the figure.

Place a beaker containing soap solution inside the wooden box. Place a powerful bulb on the side of the convex lens and move it backward or forward till a narrow parallel beam of light is formed. Looking through the microscope. We will observe individual colloidal particles, surrounded by a cone of bluish light. The bluish cone of light is called Tyndall cone.


(c) Classification of Colloids :

The colloids are classified according to the state of dispersed phase (solid, liquid or gas) and the state of dispersing medium. A few examples are shown in the table :


Colloidal solutions can be separated by the process of CENTRIFUGATION.






Heterogeneous mixtures can be separated into their respective components by simple physical methods such as handpicking, sieving, filtration.

Generally following physical properties are considered in the separation of the constituents of a mixture.

(i) Density of the constituents of the mixture.     

 (ii) Melting points and boiling points of the constituents of the mixture.

(iii) Property of volatility of one or more constituents of the mixture.

(iv)  Solubility of the constituents of the mixture in different solvents.

(v) Ability of the constituents of the mixture of sublime.

(vi) Ability of the constituents of the mixture to  diffuse.

However, for separating homogenous mixtures special techniques are employed depending upon the difference in our or more physical properties of the constituents of the mixture.


(a) By Evaporation :

(i) Separation of coloured components (due) from blue black ink : The process of evaporation is suitable for the separation of non-volatile soluble solid (dye) from its liquid solvent (water).

(ii) Method :

  1. Heat sand in an iron vessel by placing it over a tripod stand. This arrangement is called sand bath.
  2. Place a china dish on the sand bath. Pour about 5 cc of the ink into the china dish.
  3. l Heat gently evaporates water from the ink such that it does not boil. In a few minutes the water evaporates leaving behind dry blue black ink. Method of evaporation is suitable for the following solid-liquid mixtures.


(b) By Centrifugation :

The method of separating finely suspended particles in a liquid, by whirling the liquid at a very high speed is called centrifugation.

(i) Separation of cream from milk : The process of centrifuging in employed in separating cream from milk. This process is generally employed in separating colloidal solutions which easily pass through the filter paper.

(ii) Principle of centrifugation : It is based on the principle that when a very fine suspension or a colloidal solution is whirled rapidly, then the heavier particles are forced towards the bottom of liquid and the lighter stay at the top


(ii) Method :

  1. Pour full cream milk in the test tube with a pivot in your laboratory centrifuge.
  2. Shut the lid of the centrifuge and switch on the current. When the centrifuge starts working, the tub containing milk swings out in the horizontal position and whirls around its axis at a high speed.
  3. The centrifuge pull (the outward pull) pushes the heavier particles outward, i.e., towards the bottom of the mixture. Thus, the heavier particles of the proteins, carbohydrates, etc. are pushed towards the bottom of the tube, but the lighter particles of the fat stay near the top of the tube and hence separate.

(iv) Applications of centrifugation :

  1. It is employed in milk dairies to separate cream from the milk.
  2. It is employed in diagnostic laboratories in testing urine samples.
  3. It is employed in blood banks to separate different constituents of blood.
  4. It is used in drying machines to squeeze out water from the wet clothes.

(c) By Separating Funnel :

(i) Separation of mixture of two immiscible liquids : The separation of two immiscible liquid is based on the difference in their densities. The apparatus used for separation is separating funnel. It is a long glass tube provided with a tap at is bottom. The tale bellow shows different immiscible liquids which can be separated by separating funnel.


(ii) Method :

  1. Close the tap of separating funnel and clamp it in a vertical position in an iron stand.
  2. Pour the immiscible liquid mixture (say benzene-water mixture) in the separating funnel. Allow the mixture to stand for half an hour or more.
  3. The immiscible components of the mixture, i.e., benzene and water separate out into two distinct layers. The benzene forms the lighter layer on the top and the water forms the heavier layer at the bottom.
  4. Place a conical flask or a beaker under the nozzle of the separating funnel. Turn the tap gently so that the water trickles in the flask or the beaker drop by drop. Once the water is drained out, close the tap.
  5. Now place another conical flask or a beaker under the nozzle of separating funnel. Open the to drain out benzene.


(iii) Applications :

This method is used for separating any two immiscible liquids.

This method is used in separation of stag (a waste material) form the molten metals during their extraction. For example, during the extraction of iron from its ore, the molten iron and slag collect at the base of blast furnace. The slag being less dense floats up the surface of molten iron. They are topped out from two different outlets.

(d) By Sublimation :

The changing of slid directly into vapours on heating and of vapours into directly solid on cooling is known as sublimation.

(i) Separation of a mixture of common salt and ammonium chloride : This method is used in the separation of such solid-solid mixtures where one of the components sublimes on heating. However, it is useful only if the components of the mixture do not react chemically on heating. The tale shows the list of mixtures which can be separated by the process of sublimation.


(ii) Method :

  1. Place the mixture of common slat and ammonium chloride in a china dish and heat it over a low Bunsen flame.
  2. Place a clean glass funnel in an inverted position in the china dish and close the mouth of its stem with cotton wool.
  3. The ammonium chloride in the mixture sublimes to form dense white fumes. These fumes condense on the cooler sides of the funnel in the form of fine white powder.
  4. When the mixture gives off no more white fumes, lift the funnel, scrap the fine white powder from its sides on a piece of paper. This is pure ammonium chloride. The residue left behind in the funnel is sodium chloride.


Dry ice (solid CO2), naphthalene, Anthracene, Iodine etc. are sublimable  solids.

(e) By Chromatography :The process of separation of different dissolved constituents of a mixture by absorbing them over an appropriate absorbent material is called chromatography.

  1. The adsorbent medium is generally magnesium oxide, alumina or filter paper. The solvent generally  used for dissolving a mixture of two or more constituents is water or alcohol. The different constituents of a mixture get absorbed differently on the same absorbent material, because they have different rates of movement. The rate of movement of each adsorbed material depends upon :
  2. The relative solubility of the constituents of mixture in a given solvent.
  3. The relative affinity of the constituents of mixture for the adsorbent medium. If a filter paper is used as an adsorbent material for the separation of various constituents of a mixture, then this method of separation of mixture is called paper chromatography. Paper chromatography is very useful in separating various constituents of coloured solutes present in a mixture of lime, ink, dyes etc.

Kroma means colour in Greek language and technique of chromatography was first applied for the separation of colours, so this name was given.

 (i) Separation of coloured constituents present in a mixture of ink and water.

(ii) Method : 

  1. Take a filter paper 22 cm long, 5 cm broad and stick its smaller end to a glass rod with the help of gum. On the other end, measure a distance of 2 cm from lower end and mark a small point. On this point pour one or two drop of the ink.
  2. Suspend this filter paper in a wide and tall cylinder as shown in Figure. Gradually, pour water into the cylinder till the lower end of filter paper slightly dips in the water. Cover the cylinder with a glass lid to prevent any evaporation and leave the apparatus undisturbed for an hour. The water rises up the filter paper and reaches the ink mark. This water then dissolves various constituents of the ink, gets absorbed by the filter paper in different amounts. More the constituent gets absorbed, the lesser it moves upward and vice versa.
  3. When the solvent (water) reaches near the top of filter paper, the filter paper is removed from water and dried. On the filter paper will be seen a band of colours, of various constituents.
  4. A filter paper with separated bands of various constituents of a coloured substance is called chromatogram.


(iii) Advantages :

  1. It can be carried out with a very small amount of material.
  2. The substances under investigation do not get wasted in chromatographic separation.

 (iv) Applications :

  1. It is used to separate colours from dye.
  2. It is used in the separation of amino acids.
  3. It is used in the separation of sugar from urine.
  4. It is used in the separation of drugs from the samples of blood.

(f) By Distillation :

Distillation is the process of heating a liquid to form vapour and then cooling the vapour to get the back liquid.      

Distillation can be represented as :


The liquid obtained by condensing the vapour in the process of distillation is called DISTILLATE.

(i) Liebig condenser : Liebig condenser is a water condenser. It is a long glass tube surrounded by a wider glass tube (called water jacket) having an inlet and outlet for water. During distillation, cold water form tape is circulated through the outer tube of condenser. This water takes away heat from the hot vapour passing through the inner tube of condenser and causes it condensation.

Process of simple distillation is used to recover both salt as well as water, from a salt - water mixture (or salt solution) and to separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling points.

(ii) Fractional distillation : Separation of mixture of two miscible liquids for which the difference in the boiling points in less : In case of two liquids which have very close boiling points, both the liquids tends to distil over in different proportions. It means lesser the boiling point of a liquid, more is the proportion of it distilling over.

The above problem can be avoided by using a fractionating column. it gives the effect of repeated distillation by offering resistance to the passage of vapour. The process of separation of two miscible liquids by the process of distillation, making use of their difference in boiling points, is called fractional distillation.


The process of fractional distillation is useful only, if the difference in the boiling points of the two miscible liquids is less than 100C.

(A) Method :

  1. The process of fractional distillation is similar to the process of distillation, except that a fractionating column is attached.
  2. The design of a fractionating column is such that the vapours of one liquid (with a higher boiling point) are preferentially condensed a s compared to the vapours of the other liquid (with lower boiling point).
  3. Thus, the vapours of the liquid with low boiling point, pass on to the Leibig’s  condenser where they condense. The liquid so formed is collected in receiver.
  4. The thermometer shows a constant reading as long as the vapour of one liquid are passing to Liebig’s condenser. As soon s the temperature starts rising, the receiver is replaced by another receiver to collect second liquid.



In order to separate the major components of air, it is first purified, then liquefied and finally fractionally distilled. The steps involved in the process are as follows -

(a) Purification of Air :

(i) Air generally contains carbon dioxide gas, hydrogen sulphide gas and sulphur dioxide gas as impurities. In addition to it there are dust particles also.

(ii) First of all air is washed by passing it through water, where the dust particles are removed.

(iii) The washed air is passed through dilute caustic soda solution, where the gases like carbon dioxide, sulphur dioxide and hydrogen sulphide are removed.

(iv) The purified air, however, contains moisture. The moist air is passed through pipes, maintained at a temperature below - 200 C, where vapour present in it freezes and hence, air becomes dry.

(v) The air leaving the cooling pipes is free from all impurities.    

(b) Liquefaction of Air :

(i) The cool air, free from all impurities is compressed to a pressure 200 times more than the atmospheric pressure. The compression raises the temperature of the air.

(ii) The hot compressed air is then passed through cooling tank in which cold water enters from one end and warm water leaves from the other end.

(iii) The compressed and cooled air is passed through a spiral pipe, placed in a vacuum flask. The end of spiral pipe is provided with a fine jet.

(vi) When compressed air suddenly escapes from the jet, its pressure suddenly falls. Thus, its molecules move wide apart. When the molecules move wide apart, they need energy. This energy is taken by the molecules from themselves and hence their temperature drops.

(v) The air so cooled, is now at a pressure equal to that of atmosphere. This cooled air rises up and in the process further cools the incoming compressed air in spiral tube. The air is then sucked again by the compression pump and the cycle is repeated. With every cycle, the temperature of air drops, till it liquefies.

(c) Fractional Distillation of Air :

(i) The liquid air mainly of nitrogen and oxygen, and is at a temperature of - 2000C.

(ii) The boiling point of liquid nitrogen is - 1950C and that of liquid oxygen is - 1830 C.


(iii) The liquid is gradually warmed to -1950 C, when nitrogen starts boiling off from the liquid air. The nitrogen gas so formed, is compressed and filled is steel cylinders.

(iv) The liquefied oxygen left behind, is also changed to gas and then filled in compressed state in steel cylinders.


River water is normally used to supply drinking water to big cities. This water is unfit for drinking purposes as it contains a large amount of suspended impurities and harmful micro-organisms, such as bacteria and germs. The river water is purified in the following stages -

(a) Sedimentation :

The water is allowed to stand in big tanks where heavier suspended impurities settle down. To increase the rate of sedimentation, alum is added to it (loading). The impurities settle down at the bottom.           

(b) Filtration :

The semi - clear water is allowed to pass through beds of sand, charcoal and gravel to remove suspended impurities. After that water is passed through sand filter.

(c) Removal of harmful Micro-Organism or Sterilisation :

The harmful bacteria in filtered water can cause very serious diseases such as typhoid, cholera, jaundice, dysentery, etc. Thus, to the filtered water bleaching powder or chlorine gas is added. This kills the micro-organisms and hence the water becomes fit for drinking. This water is directly pumped into overhead tanks for supply to a city. 



Some kind of change always takes place in the matter when it is subjected to every changes. Almost all the changes (except nuclear changes) taking place in the matter can be classified under two heading, these are as follows -

(a) Physical Changes :

Definition : A change which alters some specific physical property of the matter, like its state, texture, magnetic or electrical conditions or its colour, without causing any change in the composition of its molecules, is called physical change, provided it get reversed, if the cause producing the change is removed.

Following points need special consideration :

(i) Now new or different product is formed : The composition of molecules of the substance remains unaltered.

Example : ice melts to form water. In this example only the appearance (state) of matter has changed from solid to liquid. However, the composition of the molecules of ice or water remains same, i.e., for every 1 g of hydrogen there is 8 g of oxygen required. Thus, only a physical change has occurred.

(ii) The change is temporary and reversible : It means the change can be reversed by altering the causes which produce the change.

Example : The water formed from ice can be changed back to ice by placing it in a freezing mixture (a mixture of ice and common salt). 

On altering the experimental conditions, the change which gets reversed, is a physical change.

(iii) There is no net gain or loss of energy : The amount of energy required to bring about a physical change is generally equal to the amount of energy required to reverse the change. Thus, there is not net energy change involved.

Example : If 1 g of water of 1000 C on changing into steam needs 2260 J of heat energy, then 1 g of steam at 1000 c on changing into water at 1000 C, gives out 2260 J of h eat energy. Thus, the net energy change is zero.

(iv) There is no change in the weight of substance : During a physical change it is only the energy which is added or removed. No matter is added during a physical change. Similarly, no matter is removed during a physical change. Therefore, mass of the substance remains same. 

Some Examples Involving Physical Changes : 


Some Common Examples of Physical Changes :

  1. Formation of dew.
  2. Evaporation of water.
  3. Crystallisation of sugar from its solution.
  4. Ringing of an electric bell.
  5. Breaking of a glass pane.
  6. Making of ice cream.
  7. A rock rolling down a hill.
  8.  Bending of a glass tube by heating.
  9. Melting of wax.
  10. Sublimation of camphor.

(b) Chemical Change :

Definition : A change which alters the specifies properties of a material by bringing about a change in its molecular composition, followed by a change in state, is called a chemical change.

Following points needs special consideration :

(i) A chemical change results in the formation of one or more new products : The products formed have different properties than the original substance. Thus, the composition of the molecules of products is different from the original substance.

Example : Heating of sugar

When sugar is gently heated in a test tube, it melts. It gradually changes to brown colour, giving a large amount of steamy fumes. In the end a black mass is left which consists of carbon. Thus, new substances, viz. carbon and water (steam)m are formed. In this change, the arrangement between the molecules of carbon, hydrogen and oxygen breaks. The hydrogen and oxygen atoms separate from carbon atoms and join together to form water. The carbon atoms are set free and are left as black residue.

 Sugar  heat →carbon + Steam

(ii) The weight of the substance undergoing chemical change usually changes :

Example : During the heating of sugar, the weight of the black residue is far less than the actual weight of the sugar. However, this is an apparent change in weight. If we take the weight of steam into account and add to it the weight of carbon, then total weight will be equal to the weight of sugar crystals. Thus, strictly speaking, total weight of substances taking part in a chemical change remains constant.

(iii) The chemical change is permanent and irreversible : It means the change will not reverse by altering the experimental conditions.

Example : The sugar, which has decomposed on heating to form carbon and steam will not change to sugar on cooling.

(iv) During chemical change energy is either absorbed or given out : The various atoms in a chemical compound are joined by attractive forces commonly called bonds. The making or breaking of the bonds always requires exchange of energy. Thus, some amount of heat is either absorbed or given out during a chemical change.

Some Examples Involving Chemical Changes :


Some Common Examples  of Chemical Changes :

  1. Burning of wood or charcoal                               
  2. Burning of candle
  3. Digestion of food                                                   
  4. Curdling of milk
  5. Formation of biogas (Gobar gas)               
  6. Burning of petrol or diesel
  7. Smoking of cigarette                                             
  8. Drying of paint
  9. Rusting of iron                                                     
  10.  Ripening of fruit
  11. Clotting of blood                                                   
  12. Fading of the colour of a dyed cloth
  13. Baking of cake                                                     
  14. Photosynthesis
  15. Formation of wine                                                 
  16. Butter turning rancid
  17. Decomposition of water into hydrogen and oxygen.
  18. Formation of water from hydrogen and oxygen

(c) Difference Between Physical and Chemical Changes




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