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Laws of Chemical Combination - Law of Constant Proportions

notes

Law of constant proportions:
This law was first stated by Proust in 1797. According to the law “In a chemical substance the elements are always present in definite proportions by mass”.” e.g. the ratio of hydrogen and oxygen in pure water is always 1: 8 by weight. So if we decompose 9g of water we will obtain 1g of hydrogen and 8g of oxygen.

law of definite proportions:
According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory may be stated as follows:

  • All matter is made of very tiny particles called atoms, which participate in chemical reactions.
  • Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
  • Atoms of a given element are identical in mass and chemical properties.
  • Atoms of different elements have different masses and chemical properties.
  • Atoms combine in the ratio of small whole numbers to form compounds.
  • The relative number and kinds of atoms are constant in a given compound.

notes

Law of constant proportions:

  • "In a chemical substance, the elements are always present in definite proportions by mass".
  • For example, no matter that water is from the sea or ocean, its chemical formula is H2O everywhere.

Dalton’s atomic theory:

  • All matter is made of very tiny particles called atoms.

  • Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

  • Atoms of a given element are identical in mass and chemical properties.

  • Atoms of different elements have different masses and chemical properties.

  • Atoms combine in the ratio of small whole numbers to form compounds.

  • The relative number and kinds of atoms are constant in a given compound.

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