CBSE Class 9 Answered

The law of definite proportion is also known as the law of constant proportion. This law states that in a compound, the elements are always present in definite proportions by mass’.

The postulates of Dalton’s atomic theory are as follows:

• All matter is made up of very tiny particles called atoms.
• Atoms are indivisible. i.e, an atom cannot be divided further.
• Atoms can neither be created nor destroyed in a chemical reaction.
• All atoms of an element are identical in all respects, including the mass, chemical properties, etc.
• Atoms of different elements have different masses and chemical properties.
• Atoms of different elements combine in small whole number ratios to form compounds.
• In a given compound, the relative number and types of atoms are constant.

So the last postulate of Dalton's theory, which states that - 'In a given compound, the relative number and types of atoms are constant' explains the law of definite proportion.