Corrosion of Metals

        Corrosion of Metals

If a metal is reactive, its surface may be attacked slowly by the air and water ( moisture) in the atmosphere. The metal reacts with the oxygen of air and water vapour of air forming Compounds on its surface. The formation of these compounds tarnishes the metals, that is ,it makes the the surface of metal appear dull. The compounds formed on the surface of metal are usually porus and gradually fall off from the surface of metal, and then the metal underneath is attacked by air and water. This process goes on and on. In this way, the action of air and water gradually eats up the whole metal. At some places ( especially in industrial areas) there are some acidic gases in the air which mix with rain water to form chemicals such as acids. These acids also attack the surface of metals and eat them up slowly.

            The eating up of metals by the action of air, moisture or a chemical ( such as an acid) on their surface is called corrosion. Most of the metals corrode when they are kept exposed to moist air.
For example- Iron metal corrodes when kept in moist air for a considerable time. When an iron object is kept in moist air for a considerable time, then a red-brown substance called rust is formed on its surface. Rust is  soft and porous, and it gradually falls off from the surface of iron object, and then the iron below starts corroding. Thus corrosion of iron is a continuous process which ultimately eats up the whole iron object.
The corrosion of iron is called rusting while other metals are said to corrode, iron metal is said to rust.

                                Rusting of Iron-

When an iron object is left in moist air (or water) for a considerable time, it gets covered with a Red- brown flaky substance called rust. This is called rusting of iron. During the rusting of iron, iron metal combines with the oxygen of air in the presence of water to form hydrated iron(|||) oxide, Fe2O3.xH2O.  This hydrated iron(|||) oxide is called rust. So rust is mainly hydrated iron(|||) oxide,Fe2O3.xH2O.   (The number of molecules of water x varies, it is not fixed). Rust is Red brown in colour. We have all seen iron nails, screws, pipes and railings covered with Red brown rust here and there. It is not only the iron which rusts, even the Steel rust on being exposed to moist air (or on being kept in water). But still rusts less readily than iron .

Conditions necessary for the rusting of iron-

Rusting of iron (or corrosion of iron) needs both air and water.  Thus two conditions are necessary for the rusting of iron to take place: 

(1)  Presence of air (or oxygen) 

(2) Presence of water (or moisture)

We know that iron rusts when placed in moist air or when placed in water. Now, moist air also contains water vapour. Thus, moist air alone supplies both the things, air and water required for the rusting of iron. Again, ordinary water has always some air  dissolved in it. So, ordinary water alone also supplies both the things, air and water needed for rusting.

Prevention of rusting-

The wasting of iron objects due to rusting causes a big loss to the country's economy. So it must be prevented. Several methods are used to protect the iron objects from rusting (or corrosion).  Most of the methods involve coating the iron object with something to keep out air and water (which cause rusting).  The various common methods of preventing the rusting of iron (or corrosion of iron) are given below-

(1) Rusting of iron can be prevented by painting- 

 The most common method of preventing the rusting of iron is to coat its surface with paint. When a coat of paint is applied to the surface of an iron object, then air and moisture cannot come in contact with the iron object and hence no rusting takes place.

(2) rusting of iron can be prevented by applying grease or oil-

When some grease or oil is applied to the surface of an iron object then their and moisture cannot come in contact with it and hence rusting is prevented.

(3) Rusting of iron can be prevented by galvanization-

The process of depositing a thin laye of zinc metal on iron object is called galvanization. Galvanisation is done by dipping and iron object in molten zinc metal. A thin layer of zinc metal is then formed all over the iron object. This thin layer of zinc metal on the surface of iron objects protects them from rusting because zinc metal does not corrode on exposure to moist air.

(4) Rusting of iron can be prevented by tin plating and chromium plating-

Tin and chromium metals are resistant to corrosion. So when a thin layer of tin metal or chromium metal is deposited on iron and Steel objects by electroplating, then the iron and steel objects are protected from rusting.

(5) Rusting of iron can be prevented by alloying it to make stainless steel-

When iron is alloyed with chromium and nickel, then stainless steel is obtained. Stainless steel does not rust at all. Cooking utensils, knives, scissors and surgical instruments etc. are made of stainless steel and do not rust at all.

Chemical bonding and their types

Chemical bonding and their types-

Chemical bond -
A bond is any force which holds two atoms together. The formation of bond between two atoms is due to some redistribution or regrouping of electrons to form a more stable configuration. Several atoms are assembled and held together to form thousands of molecules which participate in the building and function of physical and biological systems.

Types of chemical bond-
The regrouping of electrons in the combining atoms may take place in either of three ways-

(1) Electrovalent or polar or ionic bond======> by a transfer of one or more electrons from one atom to another.

(2) Covalent bond =======> by a sharing of one or more pairs of electrons between the combining atoms.

(3) Co-ordinate Bond =======> by a combination of the two processes of transfer and sharing of electrons.



(1) Electrovalent or polar or ionic bond-

Ionic bond formation takes place between atoms of strongly electropositive and strongly electronegative elements. An element preceding and inert gas in the periodic table is strongly electronegative and the element immediately following the inert gas is strongly electropositive. For example- chlorine is electronegative while sodium is electropositive. According to W. kossel (1916),a transfer of electrons takes place from the outermost shell of the electropositive atom to the outermost shell of the electronegative atom, resulting in the formation of stable positive and negative ions respectively which are held together by electrostatic forces of attraction to form a molecule or more precisely and ion pair .


A   +   B ------> A+  +  B-

The atoms involved are electrically neutral before combining. The element A which has lost its electrons is known as electropositive where as element B which has gained the electrons is termed electronegative element. The compound formed by electron transfer is termed as electrovalent by Langmuir (1919) because the resulting compound is electrolyte. It is also called Polar since the molecules develops a positive and a negative pole.
               The electrovalent compounds always exist in ionic form, are hard and nonvolatile, have high melting and boiling points because of stronger nature of the bond and are soluble in polar solvents and because of the presence of ions conduct electricity in solution or in the fused state. The electrovalent compounds having identical electronic configuration exhibit the phenomenon of isomorphism.

 Na   +   Cl  ---->   Na+    +     Cl-

 Ca   +   O  -----> Ca2+   +   O2-

(2) Covalent bond (Nonpolar Bond)-

Covalent bond formation first suggested by G.N. Lewis (1916) consists in sharing or holding a pair of electrons in partnership between two combining atoms, so that the pair counts towards the electronic grouping of both atoms. By this mechanism also, the stability akin to the inert gas is attained by each atom. For each pair of electrons to be shared between two atoms in each of the constituent atom contributes one electron.

   A•  +   B• ----->  A : B

This type of linkage which is the result of equal contribution and equal sharing of electrons is known as covalent bond. The compound formed by electron sharing is termed as covalent or or nonpolar by Langmuir (1919).
              The covalent compounds always exist in molecular form, are non electrolytes or non ionizable, soluble in organic solvents and have low melting and boiling points because of weaker nature of the bond. They are usually liquids or gases and are generally soft, easily fusible and volatile. They are non conducting in the fused state or in solution. The covalent bond is rigid and directional and as such there is a  possibility of position isomerism and stereoisomerism amongst these compound.
              covalent linkage is common in organic compounds, although inorganic compounds also have it. In covalent compounds one pair of shared electrons corresponds to a single bond, two pairs to double bond, three pairs of electrons or 6 shared electrons to triple Bond.
        Some common examples from three categories are-

 H •  +  H•  ------> H-H

   :            :
 :O:   +   :O:   --------> O=O

                                    _
 N:.  +   .:N   ------>  N=N
                                 

(3) Co-ordinate Bond (Semipolar or Dative Bond)-

Co-ordinate bond is also formed by mutual sharing of electrons but in this case the two electrons that are shared come from the same atom. The shared pair of electrons is called lone pair. The atom which provides the pair of electrons is called the donor and the atom accepting this pair is called the acceptor. After the formation of the bond, the lone pair of electrons is held in common. This sort of bonding is called co-ordinate (Sidgwick) or dative (Menzies). In this mechanism although the sharing is equitable the contribution is one sided and therefore a slight polarity develops in the molecule. For this reason this bond is also called semipolar (Sugden). This type of linkage is represented by an arrow pointing away from the donor atom (or pointing towards the acceptor atom). Usually the donor is an atom which has already acquired stable electronic configuration and the acceptor is generally two short of the stable configuration.

    :         :                  :   :
  :A:  +  B:  ------>   :A:B:
    :         :                  :   :
In this case atom A is dinor and atom B is acceptor because atom A is given a lone pair of electrons to atom B.

For example- In O3(Ozone) form a co- ordinate bond.

   :         :        :              :    :      :
 :O:  + :O: + :O: -----> :O::O:->O:
                                                  :

Trichloromethane (chloroform)

Trichloromethane  (chloroform)      CHCl3

 Trichloromethane is a trichloro derivative of methane. It is commonly called as chloroform.

Methods of preparation-

 It is prepared by the chlorination of methane in the presence of sunlight.
         Sunlight
CH4 -------------------> CH3Cl + HCl
           +Cl2


             Sunlight
CH3Cl ---------------> CH2Cl2+ HCl
                +Cl2


             Sunlight
CH2Cl2 ---------------> CHCl3+ HCl
                  +Cl2           

Properties-

(1) colourless heavy liquid

(2) sweet taste 

(3) it is oxidised in the sunlight and form a harmful gas carbonyl chloride which is commonly called as phosgene.

                    Sunlight
2CHCl3+O2 ---------> 2COCl2+HCl                                           

Phosgene is a very poisonous gas. So it is not used in the anaesthetic purpose.

Process of storage for chloroform-

(1) For the prevent from sunlight chloroform is stored in coloured bottle.
(2) for the prevent from air chloroform is fully filled in the bottle.
(3) before the storage of chloroform in the bottle it is mixed with ethanol 0.6 to 1%. Ethanol reacts with phosgene and form non poisonous diethyl carbonate.

2C2H5OH + COCl2 ------->(C2H5)2CO3  +  2HCl


Uses of chloroform-

(1) it is used as a anaesthetic but its heavy dose is very lethal.
(2) it is used as a solvent for oil,fat ,rubber,raisin etc.
(3) it is used for preparation of chloropicrin ,chloretone etc.
(4) it is used for conservation of animals in the laboratory.
(5) it is used for making medicines.
(6) it is used as a laboratory reagent in the laboratory.
(7) in the present time chloroform is used for making freon refrigerant R-22.

Alcohols

                    Alcohols

Hydrocarbons are the parents of other organic compounds. When One or more hydrogen atoms are replaced by the different types of atoms or group of atoms from hydrocarbons then different types of organic compounds are formed .
           When hydrogen atoms are replaced from saturated aliphatic hydrocarbons by the hydroxyl group (-OH) ,then Alcohols are formed.
  For example-

                -H
  R-H ------------->    R-OH                                +OH

Where R is the alkAl group.

                   -H
 CH3-H ------------->     CH3-OH                         +OH              (Methanol)


So, Hydroxy derivatives of the aliphatic hydrocarbons are known as Alcohols.



Classification of Alcohols-

Hydroxy derivatives of the aliphatic hydrocarbons are known as Alcohols.
The general formula of the alcohols are R-OH. Where R is the Alkyl group.

Alcohols are classified on the basis of the number of the Hydroxyl groups (-OH) attached to the alkAl group.


(1) Monohydric Alcohols -

In the monohydric alcohols only one Hydroxyl group (-OH) attached to the alkAl group. The general formula of the monohydric alcohols are CnH2n+1OH or ROH.
   These are further classified as follows-

(A) Primary alcohols-

Those alcohols in which -OH group is attached to the 1° carbon atom is called primary alcohols.

For example-

           H

           |1°

      H-C-OH    (Methyl alcohol)

          |
          H

               H 

               |1°

      CH3-C-OH    (Ethyl alcohol)

               |
               H

(B) Secondary alcohols-

Those alcohols in which -OH group is attached to the 2° carbon atom is called secondary alcohols.

For example-

           CH3

           |2°

  CH3-C-OH    (Isopropyl alcohol)

           |
           H

  (C) Tertiary alcohols-

Those alcohols in which -OH group is attached to the 3° carbon atom is called tertiary alcohols.

For example-

           CH3

           |3°

  CH3-C-OH    (tert.butyl alcohol)

           |
           CH3

     

(2) Dihydric alcohols-

These are generally called as Glycol . The general formula of the Dihydric alcohols are (CH2)n(OH)2 , where n= 2,3,4 etc.
In this type of the alcohols 2--OH groups are attached to the two different carbon atoms.
For example-


  HO-CH2CH2-OH
 (Ethane-1,2-diol)


   CH3-CHOH-CH2-OH
  (Propane-1,2-diol)

(3) Trihydric alcohols-

In this type of the alcohols 3--OH groups are attached to the three different carbon atoms. In I.U.P.A.C. system these are known as alkanetriol.
For example-

     CH2-OH
     |
     CH-OH
     |
     CH2-OH
   
( Propane-1,2,3-triol)
Or Glycerol

Nomenclature of Haloalkanes

Nomenclature of Haloalkanes

The name of the haloalkanes are given by two different ways-

(1)Trivial or common system-

In this system haloalkanes are known as Alkyl halide.

       There nomenclature are performed by the adding halide word in the given alkyl group. In this system the name of the compound is written in the two different words.
   For example-
    CH3-Cl          (Methyl chloride)
CH3-CH2-Br     (ethyl bromide)                    

Prifixes of the different types of alkyl group-

(a) prefix- n

The means of prefix-n is normal. This is used in the straight chain of the Alkyl group.
 For example-

 CH3-CH2-CH2-Br                          (n-propyl bromide)


(b) prefix- iso

Prefix-iso is used for those Alkyl group in which methyl (--CH3) branch is present in the last of the chain.
For example-

CH3-CH-Br

         |

         CH3
  (Isopropyl bromide)


(c) prefix- neo


Prefix-neo is used for those Alkyl group in which two methyl (--CH3) group is attached in the last of the chain by a single carbon atom.
For example-

         CH3
         |
CH3-C-Br

         |

         CH3
  (neo butyl bromide)

(2) I.U.P.A.C. system-

In this system the monohalogen derivatives of the alkane is written as haloalkane.
For the naming of haloalkanes, fluoro, chloro, bromo or iodo prefix is writes before the longest carbon chain.
   The numbering is starts from the nearest halogen atom of the chain.
   The I.U.P.A.C. name of the haloalkanes is always writes in a single word.
  For example-

  CH3-CH-CH2-Br
           |
           CH3
   (1-bromo-2-methylpropane)



         CH3
         |
CH3-C-CH2-CH2-I
         |
         CH3
   (1-iodo-3,3-dimethylbutane)

Haloalkanes and Haloarenes

 Haloalkanes and Haloarenes

Haloalkanes- 
The word Haloalkane is made up by the two words, -
         Halo = halogen
         Alkane= Aliphatic hydrocarbon (containing single bond only)

So, Haloalkanes are those compounds which is formed when a hydrogen atom replaced by any halogen atom from the alkanes.

               - H
   R-H  -------------->      R-X
                +X

Where ,   R  =  Alkyl group
              X  = Halogen ( F,Cl, Br, I)
For example-
   
                           - H
         CH3-H  --------------> CH3-Cl
                           +Cl


Haloarenes- 
The word Haloarene is made up by the two words, -
         Halo = halogen
   Arene= Aromatic hydrocarbon

So, Haloarenes are those compounds which is formed when a hydrogen atom replaced by any halogen atom from the arenes.

               - H
   Ar-H  -------------->      Ar-X
                +X

Where ,  Ar  =  Aromatic hydrocarbon
              X  = Halogen ( F,Cl, Br, I)
For example-
   
                           - H
         C6H5-H  --------------> C6H5-Cl
                           +Cl


Classification of Halogen derivatives of Hydrocarbon -

(A) On the basis of Nature of carbon chain - 
On this basis the Halogen derivatives of Hydrocarbons are of two types :-
(1) Aliphatic Halogen Compounds  -

Aliphatic Halogen Compounds are divided into three groups-

(a) Haloalkanes - The Halogen derivatives of the alkanes are called Haloalkanes.
These are called mono, di ,tri, tetra or polyhaloalkane on the number of hydrogen atom replaced by a halogen atom.
 For example-

         H
         |
    H-C-Cl     ( ChloroMethane)
         |
         H


         H
         |
    H-C-Cl     ( diChloroMethane)
         |
         Cl


         Cl
         |
    H-C-Cl     (tri ChloroMethane)
         |
         Cl

         Cl

         |
    Cl-C-Cl  ( tetra ChloroMethane)
         |
         Cl

Mono Halogen derivatives are also called alkyl halides or Haloalkanes.

Classification of alkyl halides-

(|) Primary (1°) alkyl halide - 

When Halogen atom is attached by 1° carbon atom is known as primary alkyl halide.
   For example-


         H
         |
    H-C-Cl     ( ChloroMethane)
         |
         H

(||) Secondary (2°) alkyl halide - 

When Halogen atom is attached by 2° carbon atom is known as secondary alkyl halide.
   For example-


             H
             |
    CH3-C-Cl  ( 2-ChloroPropane)
             |
             CH3

(|||) Tertiary (3°) alkyl halide - 

When Halogen atom is attached by 3° carbon atom is known as tertiary alkyl halide.
   For example-


             C H3
             |
    CH3-C-Cl  ( 2-Chloro 2- methyl
             |            Propane)
             CH3

(b) Haloalkenes - The Halogen derivatives of the alkenes are called Haloalkenes. 

For example-

CH2=CH-Cl    ( Chloroethene)

(c) Haloalkynes - The Halogen derivatives of the alkynes are called Haloalkynes. 

For example-

      _ 

CH=C-Cl    ( Chloroethyne)

(2) AromaticHalogen Compounds  -

Aromatic Halogen Compounds are divided into two groups-

(a) Nuclear Halogen derivatives-

When one or more hydrogen atom are replaced by one or more Halogen atom from an aromatic ring then formed compound is called nuclear halogen derivative or Haloarene or aryl halide.
For example-
  C6H5-Cl     ( chlorobenzene)


(b) Side chain  Halogen derivatives-

When one or more hydrogen atom are replaced by one or more Halogen atom from a side chain of aromatic ring then formed compound is called side chain halogen derivative or  aryl alkyl halide.
For example-
       C6H5CH2-Cl           
  ( Phenyl chloro methane )



(B) On the basis of hybridisation of the C -X bond-   




(a) Csp3--- bond containing compounds-


In this type of monohalogen Compounds , the halogen atom is attached to the directly to the sp3 hybridised carbon atom.

(1) Haloalkane or alkyl halide -
In Haloalkane , Halogen atom is attached to the alkyl group (R) . These are further divided into primary (1°), secondary (2°) , or tertiary (3°) .

For example-


         H
         |
    H-C-Cl     ( ChloroMethane)                   (1°)
         |
         H

             H
             |
    CH3-C-Cl  ( 2-ChloroPropane)               (2°)
             |

             CH3

             C H3

             |

    CH3-C-Cl  ( 2-Chloro 2- methyl             

             |            Propane)                             (3°)

             CH3

(2) Alylic halide-
In this type of halides, the halogen atom is attached to the next carbon ( sp3) of carbon - carbon double bond ( C= C).
For example -
   CH2 = CH- CH2-X

(3) Benzylic halide-

In this type of halides, the halogen atom is attached to the next carbon ( sp3) of  aromatic ring.
For example
 C6H5-CH2-Cl

(b) Csp2--- bond containing compounds-

In this type of monohalogen Compounds , the halogen atom is attached to the directly to the sp2  hybridised carbon atom(C=C).

(1) Vinylic halide-
In this type of halides, the halogen atom is attached to the sp2 hybridised carbon - carbon double bond ( C= C).
For example -
   CH2 = CH-X

(2) Aryl halide-
In this type of halides, the halogen atom is attached to the directly aromatic ring.
For example -

C6H5-Cl

(c) Csp--- bond containing compounds-

In this type of monohalogen Compounds , the halogen atom is attached to the directly to the sp  hybridised carbon atom(C-C triple bond).

For example-

       _

H-C=C-Cl

Allotropes of carbon

           Allotropes of carbon

 The various physical forms in which an element can exist are called allotropes of the element. The carbon elements exists in three solid forms called allotropes. The three allotropes of the carbon are -
(1) Diamond
(2) Graphite
(3) Buckminster fullerene

              (1) Diamond

Diamond is a colourless transparent substance having extraordinary shining. Diamond is quite heavy. Diamond is extremely hard. It is the hardest natural substance known. Diamond does not cunduct electricity. Diamond burns on strong heating to form carbon dioxide. If we burn diamond in Oxygen , then only CO2 gas is formed and nothing is left behind. This shows that diamond is made up of carbon only.

Structure of Diamond - 

A Diamond crystal is a very big molecule of carbon atoms. Each carbon atom in the diamond crystal is linked to four other carbon atoms by strong covalent bonds. The four surrounding carbon atoms are at the four vertices of a regular tetrahedron.

        The diamond crystal is, therefore, made up of carbon atoms which are powerfully bonded to one another by a network of covalent bonds. Due to this, diamond structure is very rigid. The rigid structure of Diamond makes it a very hard substance. The compact and rigid three dimensional arrangements of carbon atoms in diamond gives it a high density. The melting point of diamond is also very high. Diamond is a non conductor of electricity.

Uses of Diamond - 
(1) Diamonds are used in cutting instruments like glass cutters, saw for cutting marble and in rock drilling equipment.
(2) Diamonds are used for making jewellery.
(3) Sharp edged diamonds are used by eye surgeons as a tool to remove cataract from eyes with a great precision.

             (2) Graphite

Graphite is a greyish - black opaque substance. Graphite is lighter than diamond. Graphite is soft and slippery to touch. Graphite conductes electricity. Graphite burns on strong heating to form carbon dioxide. If we burn graphite in Oxygen,then only carbon dioxide gas is formed and nothing is left behind. This shows that graphite is made up of carbon only.

Structure of graphite - 


The structure of graphite is very different from that of diamond. A graphite crystal consists of layers of carbon atoms or sheets of carbon atoms.

          Each carbon atom in a graphite layer is joined to three other carbon atoms by strong covalent bonds to form flat hexagonal rings. The various layers of carbon atoms in graphite are quite far apart so that no covalent bonds can exist between them. The various layers of carbon atoms in graphite are held together by weak van der Waals forces. Due to the sheet like structure, graphite is a comparatively soft substance. Graphite is a good conductor of electricity , due to the presence of free electrons .

Uses of Graphite- 
(1) Due to its softness, powdered graphite is used as a lubricant for the fast moving parts of the machinery.
(2) Graphite is used for making carbon electrodes in dry cells and electric arcs.
(3) Graphite is used for making the cores of our pencils called pencil leads and black paints.


       (3) Buckminster fullerene

Buckminster fullerene is an allotrope of carbon Containing clusters of 60 carbon atoms joined together to form spherical molecules. Since there are 60 carbon atoms in a molecule of Buckminster fullerene ,so its formula is C60
( C- sixty ) .
Structure of Buckminster fullerene - 

Buckminster fullerene is a football shaped spherical molecule in which 60 carbon atoms are arranged in interlocking hexagonal and pentagonal rings of carbon atoms. There are 20 hexagons and 12 pentagons of carbon atoms in one molecule of Buckminster fullerene .

    This allotrope was named Buckminster fullerene after the American architect Buckminster Fuller because its structure resembled the framework of domeshaped halls designed by Fuller for large international exhibitions .

Buckminster fullerene is a dark solid at room temperature. Just like diamond and graphite, Buckminster fullerene also burns on heating to form carbon dioxide. If we burn Buckminster fullerene in Oxygen then only carbon dioxide is formed and nothing is left behind. This shows that buckmbuckmi is made up of carbon only. Buckminster fullerene is neither very hard nor soft.
 Other properties of Buckminster fullerene are still being investigated.

Acids

      Acids

*Acids are those compounds 

  which gives Hydrogen ions (H+ ion) in aqueous solutions.

For example-

       HCl --------> H+    +  Cl`

       (aq)            (aq)       (aq)

* Acids are those compounds which makes hydronium ions in aqueous solutions.

For example-

          HCl --------> H+    +  Cl`

       (aq)            (aq)       (aq)

H+  +   H2O ------->  H3O+

                           (Hydronium ion)

* Acids are those compounds which is reacts with bases and form salts and water.

For example-

NaOH  +  HCl -------> NaCl +H2O

(Base)     (Acid)      (Salt)   (Water)

* Acids are those compounds which are sour  in taste.

  For example- citric acid , lactic acid etc.

*Organic acids and mineral acids-

(1)Organic acids-  The acids present in plant materials and animals are called organic acids.

 For example- Acetic acid, formic acid etc.

(2)Inorganic acids-  The acids prepared from the minerals of the earth are called mineral acids.

 For example- HCl,  H2SO4 , HNO3 etc.

*Strong acids and weak acids-

(1) Strong acids-   The acids which gives maximum number of the Hydrogen ions in the aqueous solutions are called strong acids. 

All the mineral acids are strong acids.( Exception Carbonic acid)

For example- HCl,  H2SO4 , HNO3 etc.

 (2)Weak acids-    The acids which gives minimum number of the Hydrogen ions in the aqueous solutions are called weak acids. 

All the Organic acids are weak acids.

For example- citric acid, lactic acid etc.

*Concentrated acids and Dilute acids-

(1) Concentrated acids-  A concentrated acid is one which contains the minimum possible amount of water in it.

(2) Dilute acids-  A dilute acid is one which contains much more of water in it.

Properties of acids-

(1) Acids have a sour taste.

(2) Acids turn blue litmus to red.

(3) Acid solutions conduct electricity.

(4) Acids reacts with metals to form hydrogen gas.

    Zn  +  H2SO4 ------> ZnSO4 + H2

(5) Acids reacts with metal carbonates and metal hydrogen carbonates to form carbon dioxide ( CO2) gas.

  Na2CO3 + 2HCl ------> 2NaCl + CO2 + H2O 

(6) Acids reacts with bases to form salts and water.

  NaOH + HCl ------> NaCl + H2O 

(7) Acids reacts with metal oxides to form salts and water.

  CuO + 2HCl ------> CuCl2 + H2O

(8) Acids have corrosive nature.

Dalton's atomic theory

Dalton's atomic theory 

The theory that all matters is made up of very tiny indivisible particles (atoms) is called atomic theory of matter. Dalton put forward his atomic theory of matter in 1808.

      The various postulates or assumptions of Dalton's atomic theory of matter are as follows-

(1) All the matter is made up of very tiny particles called atoms.

(2) Atoms cannot be divided.

(3) Atoms can neither be created nor be destroyed.

(4) Atoms are of various kinds. There are as many kinds of atoms as are elements.

(5) All the atoms of a given element are identical in every respect, having the same mass, size and chemical properties.

(6) Atoms of different elements differ in mass, size and chemical properties.

(7) Chemical combination between two or more elements consists in the joining together of atoms of these elements to form molecules of compounds.

(8) The numbers and kind of atoms in a given compound is fixed.

(9) During chemical combination,atom a of different elements combine in small whole numbers to form compounds.

(10) Atoms of same elements can combine in more than one ratio to form more than one compound.

           Dalton's atomic theory was the first modern attempt to describe the behaviour of matter in terms of atoms. This theory was also used to explain the laws of chemical combination.

Drawbacks of Dalton's atomic theory-

 It is now known that some of the statement of  Dalton's atomic theory of matter are not exactly correct. Some of the drawbacks of the Dalton's atomic theory of matter are given below-

(1) one of the major drawback of Dalton's atomic theory  of matter is that atoms were thought to be indivisible ( which cannot be divided). We know that under special circumstances, atoms can be further divided into still smaller particles called electrons , protons and neutrons.

(2) Dalton's atomic theory says that all the atoms of an element have exactly the same mass. It is, however, now known that atoms of the same element can have slightly different masses.

(3) Dalton's atomic theory says that atoms of different elements have different masses. It is however now known that even atoms of different elements can have the same mass.

                John Dalton

 John Dalton was born in 1766 in a poor weaver's family in England. He received his early education from his father and at the village school. Dalton started teaching in the village at the age of 12 . In 1793 , Dalton left for Manchester to teach physics,chemistry and mathematics in a college. In 1794 , he described colour blindness. In 1808 , Dalton presented his atomic theory to explain the properties of matter. Dalton was the first to calculate the masses of the atoms of several elements. He died in 1844.

The Gas laws

  The Gas laws 

(A) Boyle's law -

 This law is given by the scientist Robert Boyle in 1662.

According to this law-

 The volume of a given mass of a gas is inversely proportional to its pressure provided the temperature remains constant.

   

                                                                           1                                           v     =    -----                 ( at constant                 p                temperature)  

                          

Or,     pv =  k = constant

Let the pressure and volume of any  gas is p   and V  and the 

                     1            1

 pressure and volume    of the same amount of the same gas be p  and v      then -

  2          2  

    p   v    =   p    v  = constant

      1    1         2    2

 (B) Charles' law-

   This law is given by the scientist J. Charles in 1787.

According to this law-

At constant pressure, the volume of a given mass of a gas is directly proportional to its absolute temperature.

     V   ~  T  (at constant pressure)

Or,    V = kT

               V 

 Or,       ----  = k

               T 

                V         V 

                  1          2

Or,        ------  =  -----

                T           T 

                  1            2

 (C) Gay Lussac's law-

 This law is given by the scientist Gay Lussac in 1802.

According to this law-

     At constant volume, the pressure of a given mass of a gas is directly proportional to its absolute temperature.

   P ~  T ( at constant volume)

Or,    P = kT

               P 

 Or,       ----  = k

               T 

                P          P

                  1          2

Or,        ------  =  -----

                T           T 

                   1           2

 (D) Avogadro's law-

This law is given by the scientist Amedeo Avogadro in 1811.

According to this law-

     Equal volume of all gasses contain equal number of molecules under similar conditions of temperature and pressure.

  

   V   ~  n

                V         V 

                  1          2

Or,        ------  =  -----

                n           n

                  1            2