Class 9th Science: Chapter 2 Is Matter Around Us Pure


Types of Substances

Types of substances:  Substances are classified into two types. They are,
Pure Substance:  A substance which is made of one kind of particle.
Examples - iron, aluminum, silver and gold.

Mixtures:  A substance which contains two or more different particles is known as mixtures.
Example: Salt solution is made up of two components, salt and water. Therefore, salt solution is a mixture.

Class 9th Science: Chapter 2 Is Matter Around Us Pure, Types of Substances, Types of Mixtures, Methods of Separation of Mixtures, NCERT CBSE Solved Quetion Answers, KEY NOTES, NCERT Revision Notes, Free NCERT Solutions Online

Pure Substances: Types
On the basis of their chemical composition, pure substances are classified as elements and compounds.
Element:  A pure substance which is made up of only one kind of atom and cannot be broken into two or more simpler substances by physical or chemical means is referred to as an element.
Characteristics of elements are:
  • An element is homogeneous in nature; it is a pure substance, made up of only one kind of atoms. For example, iron and silver are made of only iron and silver atoms.
  • An element cannot be broken down into simpler substances by any physical or chemical methods such as heat, light electricity, or chemical reactions with other substances. For example, if you were to smash a piece of iron into smaller pieces or heat it, the piece still remains as the element iron.
  • An atom is the smallest unit that shows all the properties of an element. For example, an atom of iron shows all the properties of that metal.
  • Elements have sharp melting and boiling points.

Elements are classified into metals, non-metals and metalloids.

Metals:  Metals are the elements that readily loose an electron to form a positive ion or cation.
Example: Gold, silver, copper, iron, potassium etc.
Properties of Metals:
  • Metals have lustre. When freshly cut, they show metallic lustre. Example: Gold.
  • Metals are good conductors of heat and electricity. As metals have free electrons in them, they are able to conduct heat and electricity. Example:  Copper
  • Metals are malleable, meaning they can be hammered into thin sheets. Example: Aluminum
  • Metals are ductile, which means they can be drawn into wires.
  • Metals are sonorous. They give a ringing sound when they are hit by a hard iron rod. Example: copper.
  • Almost all metals are solids at room temperature.

Exceptions:
  • Sodium and potassium are soft metals.
  • Mercury is liquid at room temperature.
  • Zinc is brittle in nature, it is non-ductile and non-malleable.
  • Tungsten is a poor conductor of electricity

Non - metals:  Non - metals are those elements that readily gain an electron(s) to form a negative ion or anion.
Example: Hydrogen, Oxygen, Iodine etc.
Properties of Non-metals:
  • Non-metals exist as solids, liquids and gases.
  • Example: Silicon and carbon are solids; bromine is a liquid; chlorine, fluorine and oxygen are gases.
  • Non-metals are non-lustrous, that is, they have a dull appearance.
  • Example: The surfaces of sulphur and phosphorus do not shine.
  • Most non-metals have very low density.    
  • Example: Oxygen and nitrogen are lighter than air.
  • Exception is diamond, a form of carbon. Diamond is one of the strongest known substances.
  • Non-metals are not malleable.  
  • Example: Sulphur and iodine cannot be hammered into sheets.
  • Non-metals, except for carbon fibres, are not ductile.                                                      
  • Example: Phosphorous and Bromine cannot be drawn into wires.
  • Non-metals are bad conductors of heat and electricity.
  • Example: Nitrogen and Oxygen cannot conduct electricity.
  • Exception is graphite, a form of carbon which is a good conductor of electricity.
  • Non-metals have low melting and boiling points.
  • Example: Sulphur and Phosphorus have

Metalloids:  The elements which have intermediate properties between those of metals and non-metals are called metalloids.
  • They are amphoteric in nature.
  • Metalloids react both with acids and bases.
  • For example, boron, silicon and germanium.

Compound:  A pure substance composed of two or more elements that is chemically combined in a fixed proportion is called a compound.
Example: Water
Properties of a compound:
  • A compound is homogeneous in nature, made up of the same type of molecules.
  • For example, water contains only molecules of water.
  • A compound can be broken into its constituents.                                                           
  • For example, water can be broken into constituent elements hydrogen and oxygen by the process of electrolysis.
  • A compound has a fixed composition.                                                                          
  • For example, a water molecule is always composed of two hydrogen atoms and one oxygen atom.
  • A compound has a distinct set of properties which is not similar with the properties of its constituent elements.                                                                                                  
  • For example, sodium chloride table salt is a harmless substance which is a white crystalline solid. On the other hand, its constituents sodium is a greyish white solid and chlorine is a greenish yellow gas that are potentially dangerous.
  • A compound has a sharp melting and boiling point.                                                       
  • For example,  water has a boiling point of 1000C, and a melting point of 0 0C

Mixtures:  Matter that consists of two or more substances which may be elements, compounds or both mixed together physically in any proportion, but not chemically combined.
Properties of mixture:
  • There is no definite proportion in which the constituents of a mixture combine.
  • For example, the mixture of salt and sand can be in any ratio.
  • The parts of a mixture can be separated by physical means.
  • For example a mixture of iron filings and sulphur can be separated by using a magnet.
  • When a mixture is created, no new substance is formed; each part of a mixture retains its own properties
  • For example, we could mix various proportions of hydrogen and oxygen gas, as long as you do not ignite the mixture, the combination will form a mixture that can be separated.
  • Energy is neither given out nor absorbed in the preparation of a mixture
  • For example no heat or light energy is liberated or absorbed when iron filings and sulphur are mixed together.
  • A mixture does not have a sharp melting or boiling point.
  • For example sugar solution does not boil at a fixed temperature.

Mixture: Classification:  Mixtures are classified into homogeneous and heterogeneous types.
Homogeneous mixture:  A mixture in which various constituents are mixed uniformly is called homogeneous mixture.
Example: Air, Alloy, Solution of salt in water.
Heterogeneous mixture:  A mixture that is not uniform throughout is called a heterogeneous mixture.
Example: A mixture of sugar and salt is a heterogeneous mixture.
Differences between a mixture and a compound:
                          Mixture
                          Compound
It consists of two or more substances which may be elements, compounds or both mixed together physically in any proportion, but not chemically combined.
It consists of two or more elements that are chemically combined in a fixed proportion.
It may be homogeneous or heterogeneous
It is always homogeneous
The parts of a mixture can be separated by physical means.
The components of a compound cannot be separated by physical methods but they can be separated by chemical and electrochemical methods
Each component of a mixture retains its individual properties.
A compound has a distinct set of properties which is not similar with the properties of its constituent elements.   
Energy is neither given out nor absorbed in the preparation of a mixture.
Heat is taken in or given out in the preparation of a compound.
A mixture does not have a sharp melting or boiling point.
A compound has a sharp melting and boiling point.  

Types of Mixtures

A solution is a homogeneous mixture of two or more substances.
Component of the solution:  A solution has two components. The component of a solution that is present in larger quantity is called the solvent and the component present in smaller quantity is called as solute.
Solvent +Solute = Solution
Example: Tincture Iodine. In tincture of iodine, the iodine is the solute and alcohol is the solvent.
Types of solution:  Depending on the physical state of solute and solvent, different types of solutions can be formed.
Solid in solid solution:  When solid solute is mixed in solid solvent solid-solid solution will formed.
Example: Alloys.

Solid in liquid solution:  When solid solute is mixed in liquid solvent solid-liquid solution will formed.
Example: Tincture of iodine
Gas in liquid solution:  When gas solute is mixed in liquid solvent gas-liquid solution will formed.
Example: Aerated Drinks
Gas in Gas solution:  When gas solute is mixed in gas solvent gas-gas solution will formed.
Example: Air
Properties of a solution:
  • Homogeneous mixture of two or more substances.
  • Particles of a solution are smaller than 1 nm in diameter. Hence they cannot be seen with the naked eye.
  • When a beam of light directed through a solution it is not visible. Since the particles in a solution are very small, they do not scatter a beam of light passing through the solution.
  • The solute particles cannot be separated from the mixture by filtration or by any physical means.
  • Solute particles in a solution do not settle down when left undisturbed.

Concentration of a solution:  The amount of solute present per unit volume of the solution is known as concentration of a solution.
Dilute solution:  A solution that contains a relatively smaller quantity of solute as compared to the solvent is known as a dilute solution.
Concentrated solution:  A solution that contains a relatively larger quantity of solute as compared to the solvent is known as a concentrated solution.
The concentration of a solution can be expressed using two methods
Mass by mass percentage of a solution:
Mass by mass % of a solution refers to the mass of solute present per 100 g of the solution.
Mass by mass % of a solution = Mass of solute / Mass of solution x 100.

Mass by volume percentage of a solution:

Mass by volume % of a solution refers to the mass of solute present per 100 mL of the solution.
Mass by volume % of a solution = Mass of solute / Volume of solution x 100.
Depending on the amount of solute present in the solution, solutions can be classified into saturated, unsaturated or super saturated.
Saturated solution:  A solution which will not dissolve anymore solute is called a saturated solution.
Unsaturated solution:  A solution in which more of the solute will dissolve is called an unsaturated solution.
Super saturated solution:  A solution which contains more dissolved solute than could be dissolved by the solvent under normal conditions is called super saturated solution.
Solubility:  Solubility can be defined as the maximum amount of solute by weight in grams dissolved in 100 grams of solvent at constant temperature.
Suspensions:  A suspension is a heterogeneous mixture in which the solute particles do not dissolve and remain suspended throughout the solvent. The solute particles can be seen with the naked eye.
Example: milk of magnesia, sand in water and flour in water.
Properties of a Suspension:
  • A suspension is a heterogeneous mixture.
  • The particles of a suspension are quite large,it is about 10 -7m and they scatter a beam of light passing through the suspension and make its path visible.
  • A suspension is unstable. When a suspension is left undisturbed, its solute particles settle down at the bottom of the container.
  • The solute in a suspension can be separated from the mixture by the process of filtration.

Colloidal solution:  A colloid is a heterogeneous mixture in which the particles cannot be seen with the naked eye.
Properties of a Colloid:
  • A colloid seems to be homogeneous. But actually it is a heterogeneous mixture.
  • The particles of a colloid are very small and cannot be seen with the naked eye.
  • The particles in a colloid do not settle down when left undisturbed. This shows the stable nature of the colloid.
  • Colloids can scatter a beam of light and make its path visible.
  • Particles in a colloid cannot be separated from the mixture by the process of filtration.

Components of a colloidal solution:  A colloidal solution has two components the dispersed phase and the dispersing medium.
Dispersed phase:  The solute-like component or the dispersed particles in a colloid form the dispersed phase.
Dispersing medium:  The component in which the dispersed phase is suspended is known as the dispersing medium.
We can classify a colloid according to the state of the dispersing medium and the dispersed phase.
Colloidal solution: Types
 Dispersed Phase
 Dispersing Medium
      Type
         Example
Liquid
Gas
Aerosol
Fog, Clouds, Mist
Solid
Gas
Aerosol
Smoke, Automobile Exhaust
Gas
Liquid
Foam
Shaving Cream
Gas
Solid
Foam
Foam Rubber, Sponge, Pumice Stone
Liquid
Liquid
Emulsion
Milk, Face Cream, Cod Liver Oil
Solid
Liquid
Sol
Starch Solution, Gold Sol, Milk of Magnesia, Mud
Solid
Solid
Solid Sol
Ruby Glass
Liquid
Solid
Gel
Jelly, Curd, Cheese, Butter

Tyndall effect:  When a beam of light passes through a true solution, there is no scattering, and the path of light cannot be traced.
When a beam of light is allowed to pass through a colloid, it gets scattered by the colloidal particles, and the path of the light can be traced.
The path of light gets illuminated with a bluish light. This phenomenon of scattering of light by colloidal particles is called the Tyndall effect. The illuminated, bright cone of light is called the Tyndall cone.

Brownian movement: The continuous zig-zag movement of colloidal particles in a dispersion medium is called Brownian movement.

Methods of Separation of Mixtures
Separation process:  The process of separating the constituent substances of a mixture by physical methods, taking advantage of the differences in their physical properties is called separation process.
Commonly used separation methods are
Separation using magnets:  This method is used when one of the components is magnetic.
Example: The mixture of iron filings and sulphur powder can be separated by using magnets.
Evaporation:  Evaporation is the process of vaporizing the solvent to obtain the solute. Evaporation is used to separate a mixture containing a non-volatile, soluble solid from its volatile, liquid solvent.
We can separate salt from a solution by evaporating the water from the solution.
Filtration:  Filtration is a process by which insoluble solids can be removed from a liquid by using a filter paper.
A filter paper is a special type of paper which has pores that are tiny enough to let only liquids pass through it. If you pass a solution through filter paper, any undissolved solid particles will get left behind on the paper whereas the liquid will filter through.
The liquid that passes through is called the filtrate and the undissolved solid particles are called residue.
Example: A mixture of chalk powder and water can be separated by this method.
Centrifugation:  If the solid particles are very small and pass through a filter paper, then centrifugation  process is used for the separation of insoluble solid particles from a solid-liquid mixture.
Principle involved in centrifugation:  The principle is that when the liquid is spun rapidly, the denser particles are forced to the bottom and the lighter particles stay at the top.
Example: Centrifugation is used for blood and urine testing in diagnostic laboratories, in dairies to separate butter from cream, and in washing machines to squeeze out water from clothes.
Separating funnel:  When two liquids do not mix, they form two separate layers and are known as immiscible liquids. These two liquids can be separated by using a separating funnel.
A separating funnel is a special type of glass funnel, which has a stop-cock in its stem to regulate the flow of liquid. It will separate the immiscible liquids into two distinct layers depending on their densities. The heavier liquid forms the lower layer while the lighter one forms the upper layer. Remove the stopper and open the tap to run the lower layer into a beaker. You will be left behind with just the upper layer in the funnel. Collect this liquid into another beaker.
Examples: Kerosene and water mixture is separated by using separating funnel method.
This method is also used to extract iron from its ore.


Sublimation:  Sublimation is the process in which solid directly changes to gaseous state.
Example: Salt and a sublimable solid such as ammonium chloride, can be separated by the process of sublimation.
Chromatography:  Chromatography is a method used to separate mixture that comprises solutes that dissolve in the same solvent.This method gets its name from the Greek word for colour —Kroma, as it was first used for separating colours.
Principle: Chromatography is based on differential affinities of compounds towards two phases, i.e stationary and mobile phase.
The fraction with greater affinity towards stationary phase travels shorter distance while the fraction with less affinity towards stationary phase travels longer distance.
Chromatography is used for separating colors in a dye, pigments from natural colors and drugs from blood.
Based on nature of stationary and mobile phases chromatography is classified into following types
     •  Paper chromatography
     •  Column chromatography
     •  Thin layer chromatography
     •  Gas chromatography
Paper chromatography: In paper chromatography the stationary phase is paper and the mobile phase is any suitable liquid.
Seperation of components of ink:
     •  First take a thin, long strip of filter paper. Use a pencil to draw a line on it, about 3 cm above the lower edge. Then, put a small drop of black ink.
     •  On the filter paper in the centre of the line and allow it to dry.
     •  Finally, lower the filter paper into a jar containing water so that the drop of ink on the paper is just above the water level. Don’t disturb the jar.
     •  After some time you will observe different coloured spots on the paper.
The ink has water as the solvent and the dye is soluble in it. As the water rises, it takes the particles of dye along with it. Since a dye is made of two or more colours, the colour which is the most soluble rises faster and higher. This is why there are differently coloured spots on the paper.

Distillation:  This method is used for the separation of a mixture containing two miscible liquids that boil without decomposing and have a large difference between their boiling points.
Process of conversion of a liquid into vapour by boiling, and then recondensing the vapour into liquid is called distillation.
Apparatus:  Distillation process requires a distillation flask, thermometer, heating assembly, a receiver flask and condenser as the apparatus. A distillation flask is a round-bottomed flask with a tube at its neck.  This tube is attached to a Leibig condenser. The Leibig condenser is a long glass tube within a glass jacket, with an inlet and outlet for water. The open end of the flask is fitted with a one-holed rubber cork through which a thermometer is introduced.


Principle:  Distillation process involves heating a liquid to its boiling point such that the liquid passes into its vapour state. The vapour's are condensed in a condenser and transformed into liquid form. The pure liquid is collected from the condenser in a receiver.
Example: A mixture of acetone and water can be separated by the process of distillation.
Separation of mixture of water and acetone:  Put the mixture into a distillation flask. Heat the mixture. You will see that the acetone, which has a lower boiling point, vaporizes first and then condenses in the condenser. It can be collected from the condenser outlet. Water gets left behind in the flask.
Fractional distillation method:  In case the difference in the boiling points of the liquids is less than 25K temperature, we use the fractional distillation method.
The apparatus is almost the same as used in distillation. The only difference is that a fractioning column is fitted in between the distillation flask and the condenser. A simple fractioning column is made up of a tube packed with glass beads. The beads provide the surface for the vapours to cool and condense again and again. The fractioning columns obstruct the smooth upward flow of vapours.
Example: A mixture of n-hexane and n-heptane can be separated through the process of fractional distillation.
Put the mixture into a distillation flask. Heat the mixture. The vapours of, n-hexane has a lower boiling point pass through and get condensed in the condenser. n-heptane, which has a higher boiling point, condenses and flows back into the distillation flask.


The gases in the air are separated from one another by the fractional distillation of liquid air.
Air is made up of different gases like nitrogen, oxygen and carbon dioxide. These gases are separated from one another by the fractional distillation of liquid air.
Steps involved are,
Air is compressed in the compressor and cooled in the refrigeration unit. Thus, the air gets liquefied.
The liquid air is passed through a filter to remove impurities and then fed into a tall fractional distillation column.
On warming, liquid nitrogen distils first because it has the lowest boiling point of -196 0C. Liquid argon has a slightly higher boiling point of -186 0C, so it distils next. Liquid oxygen has the highest boiling point of -1830C, it left behind.


Crystallisation: Crystallisation is a separation and purification method which involves the precipitating of solid crystals from its saturated solution on cooling.
In this process the impure sample is dissolved in minimum amount of suitable solvent. The formed solution is heated to get a saturated solution. On cooling, this saturated solution produce pure crystals of the sample.
Crystallisation is used for:  Purification of salt that we get from sea water and separation of crystals of alum from impure samples.

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