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• Weigh the flask with its contents conservation of mass and the law of
carefully. definite proportions.
• Now tilt and swirl the flask, so that the
solutions X and Y get mixed. John Dalton was born in
• Weigh again. a poor weaver’s family in
• What happens in the reaction flask? 1766 in England. He
• Do you think that a chemical reaction began his career as a
has taken place? teacher at the age of
• Why should we put a cork on the mouth twelve. Seven years later
of the flask? he became a school
• Does the mass of the flask and its
principal. In 1793, Dalton
contents change?
left for Manchester to
teach mathematics, John Dalton
Law of conservation of mass states that physics and chemistry in
mass can neither be created nor destroyed in a college. He spent most of his life there
a chemical reaction.
teaching and researching. In 1808, he
presented his atomic theory which was a
3.1.2 LAW OF CONSTANT PROPORTIONS
turning point in the study of matter.
Lavoisier, along with other scientists, noted
that many compounds were composed of two According to Dalton’s atomic theory, all
or more elements and each such compound matter, whether an element, a compound or
had the same elements in the same a mixture is composed of small particles called
proportions, irrespective of where the atoms. The postulates of this theory may be
compound came from or who prepared it. stated as follows:
In a compound such as water, the ratio of (i) All matter is made of very tiny particles
the mass of hydrogen to the mass of oxygen is called atoms, which participate in
always 1:8, whatever the source of water. Thus, chemical reactions.
if 9 g of water is decomposed, 1 g of hydrogen (ii) Atoms are indivisible particles, which
and 8 g of oxygen are always obtained. cannot be created or destroyed in a
Similarly in ammonia, nitrogen and hydrogen chemical reaction.
are always present in the ratio 14:3 by mass, (iii) Atoms of a given element are identical
whatever the method or the source from which in mass and chemical properties.
it is obtained. (iv) Atoms of different elements have
This led to the law of constant proportions different masses and chemical
which is also known as the law of definite properties.
proportions. This law was stated by Proust as (v) Atoms combine in the ratio of small
“In a chemical substance the elements are whole numbers to form compounds.
always present in definite proportions by (vi) The relative number and kinds of
mass”. atoms are constant in a given
The next problem faced by scientists was compound.
You will study in the next chapter that all
to give appropriate explanations of these laws. atoms are made up of still smaller particles.
British chemist John Dalton provided the
basic theory about the nature of matter.
Dalton picked up the idea of divisibility of uestions
matter, which was till then just a philosophy. 1. In a reaction, 5.3 g of sodium
He took the name ‘atoms’ as given by the Q carbonate reacted with 6 g of
Greeks and said that the smallest particles of acetic acid. The products were
matter are atoms. His theory was based on the 2.2 g of carbon dioxide, 0.9 g
laws of chemical combination. Dalton’s atomic water and 8.2 g of sodium
theory provided an explanation for the law of acetate. Show that these
ATOMS AND MOLECULES 27
Rationalised 2023-24
