CHAPTER - 3
ATOMS AND MOLECULES
CLASS :- IX
MADE BY :- N K SINHA
,1) Laws of chemical combination :-
i) Law of conservation of mass :- states that ‘Mass can neither
be created nor destroyed in a chemical reaction’.
This means that during a chemical reaction the sum of the masses of
the reactants and products remain unchanged.
Activity :-
Take some sodium sulphate solution in a conical flask and some
barium chloride solution in an ignition tube. Hang the ignition tube in
the flask by a thread and pot a cork on the flask. Find the mass of the
flask on a balance. Then tilt the flask. A chemical reaction takes place
and sodium chloride and barium sulphate are formed. Then find the
mass of the flask again. It will be seen that the sum of the masses of the
reactants and products remain unchanged.
BaCl2 BaSO4 white
solution
, ii) Law of constant proportions :- states that ‘ In a chemical
compound the elements are always present in a definite proportion by
mass’.
Eg :- Water ( H2O) always contains two elements hydrogen and oxygen
combined together in the same ratio of 2:16 or 1:8 by mass. If 9 g of
water is decomposed we get 1 g of hydrogen and 8 g of oxygen.
Ammonia (NH3) always contains two elements nitrogen and
hydrogen combined together in the same ratio of 14:3 by mass.
ATOMS AND MOLECULES
CLASS :- IX
MADE BY :- N K SINHA
,1) Laws of chemical combination :-
i) Law of conservation of mass :- states that ‘Mass can neither
be created nor destroyed in a chemical reaction’.
This means that during a chemical reaction the sum of the masses of
the reactants and products remain unchanged.
Activity :-
Take some sodium sulphate solution in a conical flask and some
barium chloride solution in an ignition tube. Hang the ignition tube in
the flask by a thread and pot a cork on the flask. Find the mass of the
flask on a balance. Then tilt the flask. A chemical reaction takes place
and sodium chloride and barium sulphate are formed. Then find the
mass of the flask again. It will be seen that the sum of the masses of the
reactants and products remain unchanged.
BaCl2 BaSO4 white
solution
, ii) Law of constant proportions :- states that ‘ In a chemical
compound the elements are always present in a definite proportion by
mass’.
Eg :- Water ( H2O) always contains two elements hydrogen and oxygen
combined together in the same ratio of 2:16 or 1:8 by mass. If 9 g of
water is decomposed we get 1 g of hydrogen and 8 g of oxygen.
Ammonia (NH3) always contains two elements nitrogen and
hydrogen combined together in the same ratio of 14:3 by mass.
