CBSE Subject Notes
Chemistry - Atoms and Molecules
Laws of Chemical Combination:-
Antoine L. Lavoisier , known as ‘Father of Modern Chemistry’ laid the foundation of chemical science.
Law of conservation of mass
It states that ‘ mass can neither be created nor destroyed in a chemical reaction’.
Total mass of reactants must be equal to total mass of products i.e., why all chemical reactions are balanced.
E.g. C + O2 → CO2
12 + 32 = 12 + 2 x 16
Mass reactants = Mass of products
Law of constant proportion or Law of Definite proportion
It states ‘In a pure chemical substance (compound) , the elements are always present in definite proportions of mass’.
Compound prepared by any method contains the same elements in the fixed ratio by mass, e.g., H2O, the ratio of mass of hydrogen to mass of oxygen is always 1:8, whatever may be the source.
2 H2O → 2H2 + O2
If we decompose 9g of H2O , we shall get 1 g of hydrogen and 8 g of oxygen.
2H2 + O2 → 2H2O
4g of H2 reacts with 32 g of Oxygen; 1g of H2 will react with 8 g of O2.
If we react 5g of hydrogen with 8 g of oxygen, 1 g of hydrogen will react with 8 g of oxygen to form 9 g of water and 4 g of hydrogen will remain unreacted.
Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, whatever the method or the source from which it is obtained.
All matter whether element, compound or mixture is composed of small particles called ‘atoms’.
Postulates of Dalton’s atomic Theory:
Advantages of Dalton’s Atomic theory
Prepared By: Mrs. Kritika Bhola