Is matter around us pure class 9 science notes

Solubility, Suspension, colloids, Properties of colloids, Tyndall Effect, centrifugation, Electrophoresis, much more… Is matter around us pure class 9 science notes


Solubility of solids in liquids: When a saturated solution at a particular temperature (say room temperature) is heated, it becomes unsaturated. This is because the solubility of a substance generally increases with increase in temperature and hence more solute can be dissolved on increasing the temperature. If a saturated solution, at a particular temperature is cooled, then some of the dissolved solute will separate out in form of crystals. This is because solubility of solute in the solution decreases on cooling.

Solubility of gases in liquids: The solubility of gases in liquids increases on decreasing the temperature or decreases on increasing the temperature. For example, water contains dissolved oxygen. When water is boiled, the solubility of oxygen in water decreases and the excess oxygen escapes in form of bubbles.


  • An increase in pressure increases the solubility of a gas. Example: aerated water bottles contain carbon dioxide gas under pressure.


  • Increase in temperature increases the solubility. Example: it is easier to dissolve sugar in hot milk than in cold milk.


A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. The particles in suspensions are bigger than 100 nm to 200 nm across. The particles of a suspension may not be visible to the naked eye but are visible under a microscope. Such as: Milk of magnesia is a suspension of magnesium hydroxide in water.


  • Suspensions are heterogeneous systems.
  • They stay only for a limited period. i.e. these are not stable as the particles have a tendency to settle down under the influence of gravity.
  • The particles of a suspension can neither pass through ordinary filter paper nor through animal membranes. Examples of suspensions are sodium chloride in benzene, turmeric in water, silver chloride, barium sulphate or sand in water.


Solution in which the size of the particles lies in b/w those of true solutions and suspensions are called colloidal solutions or colloids.

Colloidal solution, represent an intermediate kind of a mixture between true solution and suspension. The size of a colloidal particle lies roughly between 1-100 nm. Colloids are also a two-phase heterogeneous system consisting of the dispersed phase and dispersion medium.

  • Colloidal sols in which both the dispersed phase and the dispersion medium are liquids are called emulsions.
  • Colloids can pass through ordinary filter paper but do not pass through animal membranes.
  • True solution = less then 1nm, colloidal solution = between 10 to 100nm and suspension = greater than 100nm. 


 Dispersed phase Dispersed medium Colloidal system Examples
 Solid Solid Solid solsgem stones, pearls, some alloys
 Solid Liquid Sols Paints, sulphur sol, starch, proteins
 Liquid Solid Gels Jellies, cheese butter, hoot polish
 Liquid Liquid Emulsion Milk, hair cream, medicines
 Solid Gas Aerosols of solids Smoke, dust in air, smog
 Liquid Gas Aerosols of Liquids Fog, clouds, insecticide sprays
 Gas Solid Solid foam Foam, pumice stone, ice-cream, rubber
 Gas Liquid Foam, Froth Soda water, whipped cream, etc.

Properties of colloids


A colloidal solution is heterogeneous in nature consisting of two phases called the dispersed phase (discontinuous phase) and the dispersion medium (continuous phase).

Stable nature

Colloidal solutions are quite stable. The colloidal particles do not settle at the bottom under the influence of gravity. This is because of the constant motion of colloidal particles.


Colloidal particles do not pass through ultra filter papers, animal and vegetable membranes.


In colloidal systems the number of colloidal particles per litre of the sol is relatively much smaller than solute particles in a true solution. Colloidal particles are aggregates of simple molecules and colligative properties such elevation in boiling point, depression in freezing point and lowering of vapour pressure depend upon the number of colloid particles present in system and not on the nature of the particle. The values of colligative properties are consequently much smaller as compared to true solutions.

Brownian movement

When colloidal solutions have been observed through ultra microscope, the colloidal particles are seen in constant and rapid zigzag motion called Brownian movement. Sir Robert Brown first observed the phenomenon in 1827. Suspensions and true solutions do not exhibit Brownian movement.

Tyndall Effect

The phenomenon of scattering of light by sol particles to form illuminated beam is called Tyndall effect or Tyndall beam or Tyndall cone.

Tyndall effect is not shown by true solutions because the ions or solute molecules are of such minute sizes that they cannot reflect light.

The Tyndall effect can therefore be used to distinguish between a true solution and a colloidal solution. Blue colour of sky and seawater, twinkling of stars and visibility of tails of comets are also due to scattering of light of Tyndall effect.


The process by which small colloidal particles lose their change and combine together to form big sized particles which ultimately settle down is called coagulation.

Applications of Colloids

  1. Medicines.
  2. Cleansing action of soap.
  3. Smoke precipitator.
  4. Sewage disposal.


Colloidal particles of a solution either carry positive or negative charge. Sols in which the colloidal particles carry positive charge are called positive sols. When colloidal particles carry negative charge, the sols are called negative sols.

The existence of charge on the colloidal particles can be demonstrated by a phenomenon called electrophoresis. Sols of basic dye stuffs, ferric hydroxide, aluminium hydroxide etc., are some common examples of positive sols. Colloidal solutions of gums, starch, soap solution, metals (Ag, Cu, Au, Pt etc.), metal sulphides, and some acid dyestuffs are the examples of negative sols.

Separation of a mixture of two miscible liquids -Distillation

Distillation involves conversion of a liquid into vapours by heating followed by condensation of the vapours thus produced by cooling.

Separation of liquid mixture into individual components can be achieved at their respective boiling points. More volatile component distils first while the less volatile component distils after.


  • A mixture of ether (b.p.308 k) and toluene (b.p.384 k)
  • A mixture of hexane (b.p 342k) and toluene (b.p. 384k)
  • A mixture of benzene (b.p. 353k)and aniline (b.p. 457k) or nitrobenzene (b.p. 483k)


Distillation is employed to separate two or more liquids that are miscible. The boiling points of these liquids should be fairly far apart.


The process of fractional distillation has been used to separate crude oil in petroleum industry into various useful fractions such as gasoline, kerosene oil, diesel oil, lubricating oil etc.

  1. Fractional distillation has been used to separate a mixture of acetone (b.p. 329k) and methyl alcohol (b.p. 338k).

Limitations: The components of constant boiling mixture called azeotropes cannot be separated by fractional distillation. For example, rectified spirit consists of 95% alcohol (b.p. 780C or 351k) and 5% water (b.p. 1000C or 373k).


This technique is based upon the principle that when a mixture is rotated at a high speed, the lighter particles stay on the surface of the liquid while the heavier particles are forced to the bottom of the liquid.


  1. The technique of centrifugation is used in diagnostic laboratories.
    1. It is used in dairies and homes to separate butter from cream.
    1. Centrifugation technique is also used in washing machines to squeeze out water from wet clothes.


This method is used to separate a solid compound in pure and geometrical form. A nearly saturated solution of an impure substance is prepared in a hot solvent. The prepared solution is quickly filtered and the filtrate is then allowed to cool slowly in a china dish. The resulting pure crystals that form are removed with the help of a spatula. They are dried by pressing them between the folds of filter papers and finally put into desiccators. Crystals are the purest form of a substance having definite geometrical shapes. The process by which an impure compound is converted into its crystals is known as crystallisation.


  1. The salt that we get from sea water contains a number of impurities. To remove these impurities, the process of crystallisation is used.
  2. Crystallisation can also be used to obtain pure alum (phitkari), nitre (potassium nitrate),etc. from impure samples.


Chromatography is the most modern and versatile method used for the separation, purification and testing the purity of inorganic and organic compounds.


Chromatography is an important and powerful tool for chemical analysis. It is used;

  1. to separate coloured substances present in days and natural pigments.
  2. to separate and identify the amino acids obtained by hydrolysis of proteins.
  3. to detect and identify drugs present in the blood of criminals in forensic science. to monitor the progress of a reaction
  4. to separate small amounts of different products of a chemical reaction.


It is defined as a substance that cannot be further reduced to simpler substances by ordinary processes. Elements are made up of particles/atoms of only one kind. There are 114 elements known. Out of these 92 of them occur in nature.

For example: Hydrogen and oxygen


All elements except hydrogen, which form positive ions by losing electrons during chemical reactions, are called metals.

Characteristics of Metals

  • Metals possess a peculiar shine on their surface, called the metallic lustre. They can take good polish.
  • Metals have high density and are hard substances.
  • Metals have high melting and boiling properties.
  • Metals are good conductors of heat.
  • Metals have high electric conductivity which decreases s with temperature.
  • Metals are malleable and ductile, i.e; they can hammered into sheets and can be drawn into sheets and drawn into thin wires.
  • Metals have high elasticity, i.e., they can withstand high stress.
  • Metals are opaque to light.
  • Metals are generally electropositive elements.


Non-metal is a term used in chemistry when classifying the chemical elements. With the help of the general physical and chemical properties, every element in the periodic table can be named either a metal or a non-metal. A few elements with intermediate properties are referred to as metalloids. The elements regarded as Non-metals are:

  • Hydrogen
  • Carbon from group 14
  • Nitrogen, Phosphorous in group 15
  • Oxygen, sulphur, selenium in group 16
  • Halogens in group 17
  • Nobel gases from group 17

Physical Properties of Non Metals

  • Conduction: They are poor conductor of heat and electricity.
  • Physical State: Most of the non-metals exist in two of the three states of matter at room temperature like gases (oxygen) and solids (carbon). These have no metallic lustre, and do not reflect light.
  • Nature: Non-metals are very brittle and also cannot be rolled out into wires or pounded into sheets.


Elements that behave like both metals and non-metals are called metalloids. Common metal, non-metals and metalloids

 Metals Non-metals Metalloids
 Gold Oxygen Silicon
 Silver Carbon Boron
 Copper Hydrogen Arsenic
 Iron Nitrogen Antimony
 Mercury Sulphur Germanium
 Zinc Phosphorus 

Sedimentation & Decantation

Particles of the solid being heavier than the liquid (usually water), settle down due to gravity. The clear upper layer of the liquid is then gently poured out into another container. Settling down of the coarse particles due to the effect of gravity is called sedimentation. The mechanical transfer of the clear upper liquid without disturbing the settled solid particles is called decantation.

Separation of the coarse particles of a solid from a liquid by sedimentation.

Saturated, Unsaturated and Supersaturated Solutions

  1. A solution which contains the maximum amount of solute dissolved in a given quantity of the solvent at the given temperature and which cannot dissolve any more solute at that temperature is called a saturated solution. A saturated solution, however becomes unsaturated either on heating or on dilution
  2. A solution that can dissolve more solute in it at the given temperature is called an unsaturated solution.

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