NCERT 9TH CLASS SCIENCE CHAPTER ATOMS AND MOLECULES

INTRODUCTION

    The structure of matter has been a subject of speculation from very early times. According to greek philosopher Aemocritus, if we
    go on dividing matter into smaller parts, a stage would be reached when particles obtained cannot be divided further. He called these
    particles 'atoms' meaning indivisible.
    Conclusion :  All matter is made up of small particles called atoms. Different kinds of atoms and molecules have different properties
    due to which different kinds of matter also show different properties.
    

Laws of chemical combination

    By studying the result of quantitative measurement of many reactions it was observed that whenever substances react, they follow certain
    laws. These laws are called the law of chemical combination.
    (a) Law of conservation of mass.        (b) Law of constant proportions.    (c) Law of multiple proportions. 

(a)     Law of conservation of mass ; 
    This law was given by the french chemist A. Lavoisier in 1774. This law states that in every chemical reaction, the total mass before and
    after the reaction remains constant.
    "That is mass can neither be created nor destroyed in a chemical reaction". Lavoisier showed that when mercuric oxide was heated, it produced
    free mercury and oxygen. The sum of masses of mercury and oxygen was found to be equal to the mass of mercuric oxide.
    Mercuric oxide  ®   Mercury      +    Oxygen
    100 g            92.69 g        7.49 g
    Activity :  Demonstration of law of conservation of mass.  

*  Prepare separately a 5% solution of barium chloride and a 5%        
    solution of sodium sulphate.
*   Take about 20ml of barium chloride solution in a conical flask.
*   Take sodium sulphate solution in a small test tube. Hang the 
    test tube in the conical flask with the help of a thread.
    Close the mouth of the flask with cork.
*   Weigh the flask along with its contents.
*   Now tilt the flask so that the two solution get mixed.

*    Weigh the flask again along with its contents. What do you observe ? It is observed that on mixing the two solution a chemical reaction takes
      place which is indicated by the formation of a white precipitate
    Barium chloride + Sodium sulphate ® Barium sulphate (white ppt) + Sodium Chloride 
*  The mass of the flask and its contents remains constant. Thus, during a chemical reaction mass is neither created nor destroyed.
   &nb
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