Is the Matter around us Pure

Matter is composed of Solids, Liquids and Gases.  For a better understanding of Matter, there is a need to differentiate it on the basis of composition.  On the basis of composition, matter can be classified as:

(i) Pure substances; and

(ii)Mixtures.   

Learn more about matter.

Chap 2 Topics

1. Pure Substance

A pure substance consists of particles of only one kind of matter and has a uniform composition throughout.

Pure substances can be classified into two categories, i.e., Elements and Compounds

1.   An element is a basic form of pure substance which cannot be broken down into simpler substances by any physical or chemical means. For e.g., hydrogen, oxygen, gold, iron, copper etc.

Elements are further classified into metals, non-metals and metalloids or semi-metals.

(a)  Metals is an element that is

(i)   malleable (can be hammered into thin sheets),

(ii)  ductile (can be drawn into wires),

(iii) sonorous (make a sound when hit),

(iv) conduct heat and electricity and

(v)   are lustrous (shine like gold and silver).

Note: Metals are solids except for Mercury (is the only metal that is liquid at room temperature).  Gallium and Caesium are metals with very low melting points remain in liquid state at a temperature slightly above room temperature.

(b)  Non- metals are neither malleable nor ductile. They are not lustrous and do not conduct of heat or electricity.  Bromine is the only non-metal that exists in liquid state at normal temperature and pressure. E.g., hydrogen, oxygen, bromine, sulphur, phosphorous etc.

(c)  Metalloids or semi-metals have properties in between those of metals and non-metals. For example:- silicon, germanium, arsenic, antimony, tellurium.

2.    A compound is a pure substance composed of two or more elements combined chemically in a fixed proportion by mass. It can be broken down into simpler substances by chemical or electrochemical methods. Properties of compounds are different from its constituent particles. For eg., water, carbon dioxide, nitrogen, methane, ammonia etc.  Water is composed of elements Hydrogen and Oxygen. Carbon dioxide is composed of elements Carbon and Oxygen. Buy Nitrogen gas is composed to two elements of Nitrogen, hence a compound.

2. Mixture

A mixture contains two or more substances (elements or compounds) which are physically mixed in any proportion. Therefore, mixtures, have a variable composition. In a mixture, no new compound is formed. A mixture shows the properties of its constituents.

Mixtures are classified as Homogeneous mixtures and Heterogeneous mixtures.

  1. A mixture is said to be homogeneous if its composition is uniform throughout. For e.g., air (free from suspended impurities), diesel, natural gas, soda-water etc.
  2. A heterogeneous mixture has a non – uniform composition. For e.g., kerosene oil and water, iron filings and powdered sulphur, sugar and sand etc.
Compound and Mixture Difference
Homogenous_Heterogeneous-Mixture

3. Solution

A homogeneous mixture   of two or more substances is called solution. A solution is sometimes also called a true solution. Salt solution, sugar solution, etc., are all examples of solutions.

Two components of a solution are: –

(a)  Solute — it is generally the component which is lesser in amount and it occupies the intermolecular space in the other component.   It’s the component which gets dissolved in the solvent.

(b)  Solvent – the major component in the solution. It dissolves the solute.

Solute and Solvent together form the solution.

Examples of Solution

(i)    Sugar Solution – Sugar is solute and water is  the solvent

(ii)   Tincture of Iodine – Iodine is the solute and alcohol is the solvent

(iii) Soda Water – Carbon Dioxide is solute and water is solvent

(iv)  Air is a gaseous solution

Properties of a Solution

Some of the important properties of a solution are: –

(i)              A solution is a homogenous mixture.

(ii)             The particles of a solution are smaller than 10-9 m (invisible to the human eye).

(iii)           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 a solution.

(iv)            The solute particles cannot be separated from the mixture by the process of filtration.

Solute-Solvent-Solution

Alloys

Alloys are mixtures of two or more metals or a metal and a non-metal and cannot be separated into their components by physical methods.  An alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition.  Eg. Brass is an alloy which is a mixture of Zinc and Copper.

Solubility of a Solution

The ability of a substance to dissolve in another substance is called solubility. Solubility of a solute in a given solvent at a particular temperature is the amount of the solute in grams that can be dissolved in 100g of the solvent at that particular temperature. Solubility of solids in liquids increases with increase in temperature and vice-versa.

(i)    Solubility of solids in liquids however remains unaffected by changes in pressure.

(ii)   Solubility of gases in liquids increases on decreasing the temperature and vice-versa.

(iii) Solubility of gases in liquids increases on increasing the pressure and vice-versa.

Concentration of solution

Concentration of a solution is the amount of solute present in a given amount (mass or volume) of solvent or solution.

Depending upon the amount of solute present in a given amount of solvent, it can be classified as under;

(i) Saturated solution: A solution in which no more amount of solvent can be dissolved at a given temperature, is called a saturated solution.   

(ii) Unsaturated solution: If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution

Concentration of a Solution

4. Suspension

A suspension is a heterogeneous mixture in which the solute particles do not dissolve, but remain suspended throughout the bulk of the medium, e.g. a mixture of chalk powder in water, smoke coming out of a chimney of a factory.

(i)           The component of a colloid present in small amount is called dispersed phase.

(ii)          The medium in which colloidal particles are dispersed or suspend themselves is called dispersion medium.

(iii)         In a colloidal system, particle are always suspended and do not settle down. This constant colliding of particles in continuous motion is called Brownian motion/ movement.

(iv)         Scattering of a beam of light on passing through a colloidal solution, making the path of light visible is called Tyndall effect.

(v)          Colloids are classified according to the state (solid, liquid or gas) of the dispersed medium or dispersing medium and the dispersed phase.

(vi)         Colloids in which dispersed medium is a liquid and dispersed phase is solid, it is called sol.

(vii)        Colloids in which dispersed medium and dispersed phase are in a liquid state, it is called an emulsion.

(viii)      Colloids in which dispersed phase is either liquid or a solid and dispersed medium is a gas is called an aerosol.

Properties of Suspension

(i)              Suspension is a heterogeneous mixture.

(ii)             Its particles can be seen by naked eyes.

(iii)           Its particles scatter a beam of light passing through it and make its path visible (Tyndall effect).

(iv)            It is unstable, i.e. the solute particles settle down when suspension is left undisturbed. They can be separated by the process of filtration. When the particles settle down, the suspension breaks and it does not scatter light any more.

5. Colloidal Solution

A colloid (or colloidal solution) is a mixture that is actually heterogeneous  but  appears  to  be  homogeneous as  the particles are uniformly spread throughout the solution, e.g. milk, shaving cream, cheese, ere. Colloidal solutions are also called colloidal sols.

Examples of Colloids

Properties of a Colloid

(i)              A colloid is a heterogeneous mixture.

(ii)             The size of particles of a colloid is too small to be individually seen by naked eyes.

(ii)             Colloids are big enough to scatter a beam of light passing through it and make its path visible.

(iv)            The colloids are quite stable. Particles do not settle down when a colloid is left undisturbed.

(v)             Particles of colloid can pass through filter paper, therefore a colloid cannot be separated by filtration.  However, they get  separated by a special technique called centrifugation.

Tyndall Effect

The scattering of light by colloidal particles is known as Tyndall effect.  Tyndall effect can also be observed in the following situations:

(i)  When a fine beam of light enters a room through a small hole (due to scattering of beam of light by the particles of dust and smoke in air).

(ii)  When sunlight passes through the canopy of a dense forest (as the mist containing tiny droplets of water scatter it).

 

Tyndall-Effect

6. Different methods of Separation

In order to separate components of  a  mixture,   single  or  a  combination  of methods  are used. Selection of method depends upon the nature of the components present in the mixture.

 Heterogeneous mixtures can be separated  into  their respective  constituents  by  simple  physical  methods  like handpicking, sieving,  filtration,   etc.,  that  we  use in  our day-to-day life. Sometimes, special techniques are adopted for    the    separation     of    components   of    a    mixture, e.g. separation of coloured component (dye) from ink.

 1. Evaporation (Separation of Volatile Component from Non-Volatile Component)

(i)    The  process  of  conversion  of  a substance from a liquid state to a gaseous state is called evaporation and the substance is said to be volatile.

(ii)   This method  can be used to separate a volatile component (sovent)  from  a non-volatile component (solute) of a mixture. On  heating  the mixture,   the volatile component evaporates leaving behind the non-volatile component, e.g. common salt from sea water can be separated by this method. The heat of sun gradually evaporates water in the shallow lakes and common salt is left behind as a solid.

Evaporation

2.  Centrifugation (Separation of Colloidal Particles from Solution)

(i)    Two components having difference in densities can be separated by centrifugation.  This method is based on the principle that, the denser particles are forced to the bottom and the lighter particles stay at the top   when   spun rapidly. A device used to separate liquids from solids by spinning is called centrifuge.  The machine can be rotated either by hand, i.e. manually or electrically.

 (ii)   Applications

(a)             diagnostic laboratories for blood and urine tests.

(b)             washing machines to squeeze out water from wet clothes.

(c)             dairies and homes to separate butter from cream.

Centrifuge

3.  Separation using Separating Funnel (Separation of Mixture of Two Immiscible Liquids)

(i)    This  method  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.

(ii)   Applications Separating funnel is used in

(a)             separation of mixture of oil and water.

(b)             during  extraction of iron  from  its ore, the lighter slag is removed from molten iron.

Separating Funnel

4.  Sublimation (Separation of a Solid Volatile Component from Mixture)

(i)    Some solids have a tendency to sublime on heating, i.e. they convert directly from solid to  gaseous/ vapour phase on heating without  passing through  the liquid phase. A mixture containing such solid with any other (normal) solid can be separated by sublimation. Examples of solids that sublime are camphor,  ammonium chloride, naphthalene,  iodine,  anthracene, etc.

(ii)   Hence, sublimation is used to separate such mixtures that contain a sublimable volatile component from a non-sublimable  impurity,  e.g.  a mixture of common salt and ammonium chloride can be separated by sublimation.

Sublimation

5.  Chromatography (Separation of Components with the Help of Same Solvent)

(i)    The term chromatography is based on Greek word kroma means colour.  This technique was first used for the separation of colour. Chromatography is the technique used for the separation of those solutes that dissolve in the same solvent. The separation of different components of a mixture is depend upon their different solubilities in the same solvent, e.g. separation of dyes present in ink by paper chromatography. 

(ii)   The ink that we use has water as the solvent and the dye is soluble in it. As the water rises on the filter paper it takes along with it the dye particles. Usually, a dye is a mixture of two or more colours.  The coloured  component  that  is more  soluble  in  water,  rises  faster  and  in  this  way  the colours get separated.

 (iii) Applications Chromatography is used to separate

(a)             drugs from blood.

(b)             pigments from natural colours.

(c)             colours in a dye.

Chromatography

6.  Distillation (Separation of Two Miscible Liquids)

(i)    If liquids in a mixture are miscible, boil without decomposition and possess different boiling points, then they can be separated by distillation.

(ii)   Distillation involves the conversion of a liquid into vapour followed by the condensation of vapour back int the liquid.  Distillation is used only if the liquids have a difference in boiling points of more than 25K.

(iii) Application

(a) preparation of distilled water

(b) volatile and aromatic water are prepared

(c) non-volatile are separate from volatile liquid

7.  Fractional Distillation

To separate a mixture of two or more miscible liquids for which the differences in boiling points is less than 25 K, fractional distillation process is used, e.g. separation of different gases from the air, different fraction from petroleum products, etc.

(i)    The apparatus is similar to that for simple distillation, except that a fractionating column is fitted in between the distillation flask and the condenser.

(ii)   A simple fractionating column is a tube packed with glass beads. The beads provide the surface for the vapours to cool and condense repeatedly.

Fractional Distillation

8.  Separation of Different Gases Present in Air

Air is a homogeneous mixture and can be separated into its components by fractional distillation. For this purpose, the air is compressed by increasing the pressure and cooled to a very low temperature. Thus, liquid air is obtained. This liquid air is allowed to warm up slowly in a fractional distillation column, where   gases get   separated at   different    heights depending upon their boiling points.

Separation of Gases

9.  Crystallisation (Separation. of Pure Substance from its Impure Form)

(i)    Crystallisation  is a process that separates a pure solid in the form of its crystals from a solution. This method is used to purify solids, e.g.  the salt we get from  sea water  can  have many  impurities  in it. To  remove  these impurities, the  process of crystallisation is used.

(ii)   Crystallisation technique is better than simple evaporation  technique as

(a)   Some solids decompose or some, like sugar, may get charred during heating to dryness.

(b)   Some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.

(iii) Applications

(a) purification of salt obtained from sea water.

(b) separation of crystals of alum (phitkari) from impure samples.

Crystallisation

Purification of Drinking Water:

The  various processes used in water works  for purification are:                                      

 (i)    Sedimentation to remove suspended solids (water is allowed  to stand  for sometime so that suspended impurities settle down).

 (ii)   Loading with alum to remove small particles like clay present  in the colloidal  state.  Particles  like clay being negatively charged are neutralised by alum and are coagulated at the bottom  of tank.

 (iii) Filtration to remove dissolved solids by passing the water through filtration tank which  possess three layers. Coarse gravel at the bottom, fine gravel in the centre and fine sand at the top acts as filters. Impure water is introduced from the  bottom  such  that  impurities are retained  in  three  layers of gravel and  pure  water  goes to the  top  and  sent  to chlorination tank.

 (iv)  Chlorination to kill bacteria.  Filtered water is treated  with  bleaching powder  to kill bacteria.

Purification of Water

7. Physical and Chemical Changes

Physical Change

When there is change in shape, size, appearance or state of any substance  but its chemical composition remains the same and no new substance is formed is called physical change. These changes are reversible. For example:- the interconversion of the states of matter(i.e., ice to water or water to steam) is a physical change.

Chemical Change

A chemical change brings about changes in the chemical properties of the substance, as a result the original substances lose their identity to form new substances. These changes are irreversible and are usually accompanied with heat exchanges. For example:-corrosion of iron, burning of magnesium ribbon etc.

Physical and Chemical Change
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