Laws of Chemical Combination

Atom and Molecule of Class 9

It was the Englishman Robert Boyle (17th century) who, through research on the behaviour of gases, provided clear evidence for the atomic makeup of matter. He was the first to define an element as a substance that cannot be chemically broken down further. He believed that a number of different elements might exist in nature.There are two important laws of chemical combination. These are:

  • Law of conservation of mass 
  • Law of constant proportions


In 1774, Joseph Priestley isolated the gas oxygen by heating mercuric oxide. Soon thereafter, Antoine Lavoisier claimed that oxygen is the key substance involved in combustion (burning). He also demonstrated that when combustion is carried out in a closed container, the mass of the final products of combustion exactly equals the mass of the starting reactants. This led to the statement of the Law of Conservation of Mass:

law of Conservation of Mass:

Mass is neither created nor destroyed in chemical reactions.

Suppose we carry out a chemical reaction between A and B and if the products formed are C and D then,

A + B C + D

Suppose ‘a’ g of A and ‘b’ g ob B react to produce ‘c’ g of C and ‘d’ g of D. Then, according to the law of conservation of mass, we have,

a + b = c + d

e.g. In an experiment, 63.5g of copper combines with 16g of oxygen to give 79.5g of cupric oxide (a black oxide of copper). This is in agreement with the law of conservation of mass.

Science today knows that matter can be converted into energy (and vice-versa). Hence, during all chemical and physical changes, the total mass + energy before the change is equal to the total mass + energy after the change. Still, as there is no detectable change in mass in an ordinary chemical reaction, the law of conservation of mass is still valid.

Ex. 1 Silicon dioxide, made up of elements silicon and oxygen, contains 46.7% by mass of silicon. With what mass of oxygen will 10g of silicon combine?

Solution:  100g of silicon dioxide contains: 46.7g of silicon, or : (100 – 46.7) i.e. 53.3g of oxygen.

∴ 10g of silicon will contain 10100 × 53.3 = 5.33g of oxygen.


Lavoisier, the French chemist Joseph Proust formulated a second fundamental law of chemical science – the Law of Definite Proportions.

law of Definite Proportions (Law of Constant Composition):

In a given compound, the constituent elements are always combined in the same proportions by mass, regardless of the origin or mode of preparation of the compound.

According to this law, when elements react chemically, they combine in specific proportions, not in random proportions.

e.g. A sample of pure water, whatever the source, always contains 88.9% by mass of oxygen and11.1% by mass of hydrogen.

e.g. The compound cupric oxide may be prepared by any one of the following methods –

  • Heating copper in oxygen.
  •  Dissolving copper in nitric acid and igniting the cupric nitrate formed.
  • Dissolving copper in nitric acid, precipitating cupric hydroxide, and strongly heating thecupric hydroxide. – and in each case, the ratio copper:oxygen by mass is always constant.

Ex. 2 2.16g of mercuric oxide gave on decomposition 0.16g of oxygen. In another experiment 16g of mercury was obtained by the decomposition of 17.28g of mercuric oxide. Show that these data conform to the law of definite proportions.

Solution: . Experiment 1:

Mass of mercuric oxide = 2.16g

Mass of oxygen evolved from it = 0.16g

∴ Mass of silicon in the compound = 2.16 - 0.16 = 2.00

∴ silicon:oxygen ratio = 2.000.16 = 12.5 : 1

Experiment 2:

Mass of mercuric oxide = 17.28g

Mass of silicon in it = 16.00g ∴ Mass of oxygen in the compound = 17.28 - 16.00 = 1.28g

∴ silicon : oxygen ratio = 16.001.28 = 12.5 : 1

“In both cases, the silicon to oxygen ratio is the same, thus conforming to the law of definite proportions.”

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