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Dalton's atomic theory

From time immemorial, philosophers were puzzled about the nature of matter. They were suggesting that all matter is composed of tiny, discrete , indivisible particles called atom. In 1808, John Dalton put forward the atomic theory and successfully explained the laws of chemical combination known at that time. Dalton's atomic theory may be summarized in the following postulates:

1. All matter consists of extremely small indivisible particles called atoms.

2. Atoms of same element are all alike.

3. Atoms of different elements are entirely different and have different properties.

4. Atoms cannot be destroyed, created or transformed into atoms of other elements.

5. Atoms combine together in simple whole number ratio to give compounds.

6. The relative number and kinds of atoms are constant in a given compound.

Dalton's atomic theory is the milestone in the development of modern chemistry. Dalton's original statement has undergone expansion, modification and clarification to a great extent. According to dalton atoms unite together to form chemical compounds. The quantitative weight relationship in a chemical reaction is very important. Stoichiometry is the branch of chemistry which deals with the weight relationship in chemical reaction and weight relationship that prevails in a chemical compound. This type of study was initiated by Antoine Lavoisier. Lavoisier was the man who introduced chemical balance in chemistry laboratory. The quantitative relationship in chemical combination is governed by the following laws of chemical combination or stoichiometry.

1. The law of conservation of mass

2. The law of definite proportions

3. The law of multiple proportions

4. The law of reciprocal proportions

5. The law of gaseous volumes

In the above five laws , the fifth law is not concerned with the weight relationship, rather it deals with volume relationship in a chemical reaction.

The law of conservation of mass:

The law of conservation of mass was probably first stated by russian chemist M.V. Lomonosov in 1756. This law was again independently restated by the French chemist Antoine Lavoister in 1774. The law of conservation of mass may be stated as

Mass is neither gained nor lost during a chemical reaction or in a chemical reaction the total mass of the reactants consumed is equal to the total mass of products formed.

This law is also known as the law of indestructabilityof matter . The truth of this law had been firmly established by German scientist H.Landolt by performing a series of experiments during the period of 1900-1908. Landolt found no deviation from the law within a possible error of 1 part in 10,000,000.

The law of definite proportions:

The law of definite proportions is also known as law of constant proportion. This law first enunciated by French chemist Joseph Louis Proust in 1799 which states that:

The same chemical compound always contains same elements combined together in definite proportion by weight regardless of the origin or mode of formation of the compound.

This law may be illustrated by taking the following examples.In certain compounds , the law of definite proportion is violated. For example, the metal zinc forms only one normal oxide, namely zinc oxide, but under certain conditions this compound is found to have less than stoichiometry proportions of oxygen, so that formula for zinc oxide  becomes different.

 The law of multiple proportions:

When one element combines with another element to give only one compound., then these elements combine in definite proportions. Sometimes, two elements combine to give two or more compounds. In such cases the proportions of the two elements will be naturally different in two different compounds. For example, carbon combines with oxygen to give two compounds, namely carbon carbon dioxide and carbon monoxide. The ratio by weights of carbon and oxygen in these two compounds are different, but there is a definite relationship in the weights of oxygen which combine with the constant weight of carbon. After studying composition of innumerable compounds formed by the same two elements, John Dalton in 1803 put forth the following law known as the  law of multiple proportions which may be stated as:

When one element combine with another element to formtwo or more dufferent compounds , then the weights of one of the elements which combine with theconstant weught of the other bear  a simple whole number ratio to one another.

The law of reciprocal proportions:

The law of reciprocal proportions tells us that there is a definite relationship between the weights of elements in any given compound. Now the question is," Is there any simple relationship between the weights of two elements each of which combines with the fixed weight of third element?" To answer this question, let us study the combine proportions of the elements in two compounds water and hydrogen chloride.

Such relation was first noticed by Richter . After studying such reciprocal relationship that existed in a large number of compounds . Richter in 1792 stated the following law known as The law of reciprocal proportions. This law states that:

When two different elements combine separately with the same weight of third element , the ratio in which they do so will be the same or some simple multiple of the ratio in which they unite with each other.

The law of gaseous volumes:

This law deals with the relationship between the volumes of gaseous reactants and products. Gay Lussac had stated this law in 1805. It states that:

Whenever gases react, they do so in volumes which bear a simple ratio to one another, and to the volumes of the products if these are also gases provided all measurements are made under similar conditions of temperature and pressure.

It should be borne in mind that all the laws of chemical combination given above were established in the second half of the eighteenth century and the early years of the nineteenth century before John Dalton gave his atomic theory. These law were formulated purely on the basis of experimentations on the chemical analysis and synthesis of compounds.

Methods of bringing a chemical reaction

                  When a chemical change occurs a chemical reaction is said to have taken place. A chemical reaction can be represented with the help of symbols and formula which is known as chemical equation. The details about chemical equation and its significances are discussed in language of chemistry.
                    A chemical reaction can be induced by various techniques. Some chemical reaction
occurs as soon as the reactants comes into contact while other may require a drastic condition to bring about a chemical reaction. The different methods of bringing a chemical reaction are briefly described below.

1. By simple contact:
                      A contact of the reactants is the essential condition for any chemical reactions. Reactants whatsoever reactive may be, they will not react unless they come into contact. For example, phosphorus and iodine can be kept side by side without any reaction but as soon as they some into contact thry react together to give phosphorus tri-iodide.

2. By contact through solution:
                        Certain reaction does not occur by simple contact in solid but they have to be brought into contact through solution. For example, tartarie acid and sodium bicarbonate powder can be kept together without any reaction .but when they are brought together  through the solution they react together vigorously  with effervescence evolving carbon dioxide gas.similarly
silver nitrate and sodium chloride do not react  in solid state but they immediately react together
giving precipitate of silver chloride when they are brought  together in solution.

3. By application of heat:
                            
                           Generally heat accelerates a chemical reaction. sometimes application of heat is essential to bring about some chemical reaction.for example copper turning and conc. sulphuric acid do not react in cold but when heated they react together producing sulphur dioxide gas.some of the example are decomposition of potassium chlorate,mercuric oxide by heat. such reaction are called thermal decomposition.

4. By application of pressure:
                         
                              Certian reaction occurs when pressure is applied.foe example potassium chlorade and sulphur react together producing a loud noise when rubbed in mortar by pestle with pressure.usually gaseous reaction are carried out under pressure.for example in the synthesis of ammonia by haber process as high as 400 atmosphere is used.

5. By application of electricity:
                          
                               Electricity can bring about many important chemical reaction. for example, when eletricity is passed through acidulated water, water splits into hydrogen and oxygen.
  sodium metal is prepared be eletrolysis of molten sodium chloride and many metals are purified be eletriolysis process.

6.  By irradiation of light:
                            
                                Certian reaction occurs  in presence of light only. Such reactions are called phototchemical reactions. Photosynthesis is a very important reaction occuring in nature which is brought by light . In photosynthesis reaction, carbon dioxide and water combine in green plant in presence of light. The photography is also based on the reaction brought about by light . Methane combines with chlorine in diffuse sunlight to givessubstitution product.

7. By use of catalyst:
                                  Many industrially important reactions are carried out in presence of catalyst. Catalyst increases the speed of a reaction. For example, in the manufacture of ammonia by Haber process , iron is used as catalyst. Similarly. in the manufacture of vegetable ghee , nickel is used as catalyst. Other important catalyst are platinum, rhodium, etc. We will hardly find any industrial process where some kind or other kind of catalyst is not used. During combustion of petrol  in automobiles some amount of poisonous gas like carbon monoxide comes out in automobiles exhaust which pollutes the atmosphere. Owing to this, catalyst is incorporated in the exhaust system in automobiles in advanced countries to convert carbon monoxide to carbon dioxide. Hydrogen peroxide decomposes slowly on its own giving oxygen and water. In presence of manganese dioxide this reaction occurs with greater speed.
                     

Separation of mixture

           In the nature matter seldom occurs as pure substances, rather they occur as mixtures. Mixture can be separated into individuals components by different processes. Methods of separation depend upon the physical properties of individual components of mixture. Some commonly used laboratory processes for separation of mixtures of substances are described here. 

    

Evaporation:

 
                   

 If a little salt is added to the water and shaken well, the salt will dissolve and a homogeneous mixture of salt and water will be formed which is known as solution. The salt cannot be separated from he solution by filtration; both water and salt passed straight through the filter paper. To obtain the salt , the solution must be heated so that the water escape out as vapour leaving behind the salt. For this the solution is taken in the procelain basin and graduallyt heated to dryness over a Bunsen flame.

Filtration:

                     Filtration is the process of separating an insoluble solid substance from liquid by passsing through filter.Suppose we have turbid water. From this clear water can be obtained by filtration. For this a circular filter paper is folded twice and then opened to form a cone. This is then fitted into the  funnel. 

Pour turbid water to the filter paper in the funnel. Water penetrates through the pores of the filter paper and is collected in the beaker whgich is known as filtrate. Suspended particles remain in the filter paper which is known as residue. By this process any insoluble substance existing as suspension can be separated from the liquid. In municipal supply of water suspended particles in water are removed by filtering water through a layer of sand and gravel in a filter bed. The dust particles from air can be separated by filtering through the filter pad.

Distillation:

                     In the separation of salt from solution by evaporation the solvent water escapes out to the atmosphere. If the solvent is to be recovered, distillation should be used.Distillation is the process in which liquid is comverted to vapour by heating and the vapour is condensed back to liquid by cooling. Distillation is used for the purificxation of liquid as well as separation of two miscible liquids. For this, liquid to be distilled is taken in thedistiullation flask fitted with Leibug's condenser.

On heating, the liquids get vapourized and passes through the inner tube of 
condenser.In the outer jacket cold water  is circulated so that the vapour condenses back to the liquid which is collected in the receiver.This liquid is known as distillate. Distillated water for lead acid accumulator as well as for injection is produced by distillation.The distillation is in practice from cennturies for the preparation of spirit from the fermented wine.Two miscible liquids can also be separated from each  other if their boiling points are quite different. For better separation of liquid mixture fractional distillation  with fractioniating column should be used. Petrol , kerosene, diesel, mobil are obtained byfractional distillation of crude oilextracted from the mines.Alchemists had used a special types of glass apparatus for distillation propose which is known as retort.


 Sublimation:

                        
                      If we heat the shining iodine crystal, it will not meilt but it gives violet vapour and if this vapour is cooled it will convert back to iodine crystal. This process is known as sublimation.
Sublimation is the process of converting solid to vapor by heating and condensing vapour back to solid by cooling. Sublimation is used for the purification of substance which sublimes. This process can be used for separation of two substances in which one sublimes and the other does not. For Sublimation, the mixture is taken in a procelain basin  and a funnel is inverted over and gently heated by keeping it over a sand bath. The inverted funeel is cooled by putting a wet filter paper. The volatile solid will sublime into the funnel and is knwon as sublimate.For practice in the laboratory , amixture of sand and camphor can be separated by Sublimation process.


Crystallization:

                         For the purification of impure solid substances ,Crystallization is frequently used.
This process is most often used for the purification of crude organic compounds prepared in the laboratory. Pure crystals of copper sulphate  may be prepared from bazaar copper sulphate by Crystallization.


  


For this a saturated solution of bazaar copper sulphate is prepared and filtered. The filtrate is taken in the procelain basin and evaporated to the Crystallization point. Then this hot solution is cooled whereby pure crystals of copper sulphate separate out.
 

chemistry

There are different substances around us having different properties.Why do they have different properties?For example, why does petrol burn but water does not?What is the difference in composition of petrol and water? Matter undergoes varieties of changes. Why do such changes occur?For example,why does iron rust?Why does milk turn sour?all substances around us do not occur in nature.For example, plastic does not occur in nature but it is prepared by a chemical process.Iron does not occur in free state but it is extracted form the iron compounds that occur in mines. Washing soda is manufactured by a chemical process. If we want to learn all about them, than we should study chemistry.

Chemistry is a branch of science which deals with the composition, properties and transformation of matter.

For convenience chemistry may be sub-divided into the following major branches.

1. Inorgani chemistry

2. Organic chemistry

3. Physical chemistry

4. Analytical chemistry

5. Biochemistry

6. Industrial chemistry

7. Nuclear chemistry

Since the dawn of civilization man has been curious to know about the things around them .If we look around us we will find many things.All these things are nothing but matter.

Classification of matter

           Matter cab be classified in many different ways .One way of classification of matter is shown in the following diagram:

Matter:

1. Physical Classification

a.solid

b.liquid

c.gas

2.Chemical classification

A. Pure substance

a. Element

b. Compound

B. Mixture

Physical classification

Depending upon the conditions of temperature and pressure,matter may exist in the following three physical states.

1. SOLID

2. LIQUID

3. GAS

SOLID is that substance which at room temperature and pressure has definite shape and volume . In solid state  , the molecules constituting the solid are closly packed and there is very little space between them.The

molecules in the solids are tightly held by stroing force of attraction .So solid has definite shape and volume.

LIQUID is that  substance which at room temperature has definite volume but does not have definite shape.In liquid state , the molecules are closly packed but not tightly held like in solid. Mplecules can slide one over another easily in liquid state.Therefore , liquid has definite volume but does not have definite shape.

GAS is that  substance which has neither definite volume nor definite shape.In gaseous state ,molecules are vary far apart and free to move in all possible directions. In gaseous state the intermolecular forces of attraction is very weak. Hence gas has  definite volume nor definite shape.

Molecules

               A molecule is the smallest unit of element or a compound which is capable of independent existence.A molecule of an element will contain only one kind of atoms while a molecule of compound will contain at least two kimd of atoms. For example , a molecule of hydrogen contains two atoms of hydrogen.Hence hydrogen is an element. On the other hand

a molecule of water contains two atoms of hydrogen and one atom of oxygen. Hence water is a compound. A molecule of ozone contains three atoms of oxygen and a molecule of helium contains one atoms of helium. Are ozone and helium an element or compound? What is the difference between helium atom and helium molecule?

Molecular weight

                  Molecular mass or weight is the mass of the molecule in the atomic mass unit.The molecular mass is calculated by adding the atomic masses of constituent atoms in the molecule. The molecular mass of water is 18. This is obtained by adding the mass of two hydrogen and one oxygen.