CBSE Class 9 Science Atoms and Molecules Exam Notes

CBSE Class 9 Science Atoms and Molecules Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.


Matter is called ‘padarth’ in Hindi. Kanad was one of the first persons to propose that matter (or padarth) is made

up of very small particles called ‘parmanu’. John Dalton called these particles by the name of atom.  The word

‘atom’ means ‘indivisible’ something which cannot be divided further. The particles of matter (atoms or parmanu)

normally exist in a combined form. This combined form of atoms is now called ‘molecules’. All matter is made up of

small particles called atoms and molecules. Different kinds of atoms and molecules have different properties due to

which different kinds of matter also show different properties.

Laws of Chemical Combination

The laws of chemical combination played a significant role in the development of Dalton’s atomic theory of matter.

There are two important laws of chemical combination.

(A) Law of conservation of mass

  * Law of conservation of mass was given by Lavoisier in 1774. According to the law of conservation of mass :

Matter is neither created nor destroyed in a chemical reaction. The substances which combine together in a

chemical reaction are known as ‘reactants’ whereas the new substances formed as a result of chemical reaction

are called ‘products’. The law of conservation of mass means that in a chemical reaction, the total mass of products

is equal to the total mass of reactants. There is no change in mass during a chemical reaction.

Ex. 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                           92.6 g                    7.4 g

Sol. Mass of the reactant = 100 g

        Mass of the products = 92.6 + 7.4 g = 100.0 g

                                            = 100 g

Since the total mass of the products formed is equal to the total mass of the reactants undergoing reaction, so the data is in agreement with low of conservation of mass.

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