There are three important laws of chemical combination
based on the Daltonian atomic concept as below:
1. Law of Conservation of Matter (or Mass): According
to this law “Matter can neither be created nor
destroyed in a chemical reaction”. Thus there is no
change in mass in a chemical reaction. For example,
the reaction between Sodium Chloride and water
can be written as :
NaCl + H2O ® NaOH + HCl
We see that the total no. of atoms before and after the
reaction remains the same. Hence mass or matter is
conserved in this reaction.
2. Law of Constant Composition or Law of Definite
Proportion: This law states that “the composition of a
pure compound is always the same.” Compound is
always made up of the same elements in the same
percentage. This law can also be explained on the
basis of Daltonian theory : “The no. and kind of atoms
in a given compound is fixed.” H2O molecule always
contains 2 atoms of hydrogen and 1 atom of Oxygen.
3. Law of Multiple Proportion : This law states that when
two elements combine to form two or more than two
compounds, a simple ratio of weight of both elements is
maintained. For example, consider the formation of
Carbon dioxide (CO2. and Carbon monoxide (CO) by
the combination of Carbon and Oxygen atoms.
In Carbon dioxide one Carbon atom combines with
two Oxygen atoms. In Carbon monoxide (CO) one
Carbon atom combines with one Oxygen atom.
Hence the ratio of Oxygen combining with the fixed
amount of Carbon is 2 : 1 between CO2 and CO. This
is also the ratio of their weights.
