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Laws Of Chemical Combination And Dalton's Atomic Theory

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Laws Of Chemical Combination And Dalton's Atomic Theory - Lesson Summary

The five basic laws of chemical combination that govern every chemical reaction are:
  •  Laws of Conservation of Mass
  •  Law of Constant Composition or Definite Proportions
  •  Law of Multiple Proportions
  •  Law of Combining Volumes (Gay-Lussac’s Law of Gaseous Volumes)
  •  Avogadro’s Law

Laws of Conservation of Mass:
In 1789, Lavoisier proposed the law of conservation of mass. It states that in all physical and chemical changes, the total mass of the reactants is equal to the total mass of the products.
Law of Constant Composition or Definite Proportions:
In 1799, Joseph Proust proposed the law of definite proportions. According to this law “a pure chemical compound always contains the same elements combined together in the same fixed ratio by mass”. Ex: Even though water obtained from different sources like river, well, lake, sea...etc, it always contains hydrogen and oxygen elements in the ratio of 1:8 by mass.

Law of Multiple Proportions:
In 1804, Dalton proposed the law of multiple proportions. It states that when two elements combine to form more than one chemical compound, then the masses of one of the elements that combine with a fixed mass of the other element, bears a simple ratio. Ex:  When the elements, Carbon and Oxygen combine, to 
Forms Carbon dioxide when 12 parts by mass of Carbon combine with 32 parts by mass of Oxygen.
Forms Carbon monoxide when 12 parts by mass of Carbon combine with 16 by mass of Oxygen.

Gay Lussac’s law of combining volumes:
In 1808, Gay Lussac proposed this law, which states that “when gases combine or are produced under similar conditions of temperature and pressure the volumes of both reactants and  products bears a simple ratio to one another. Ex: When 1 lit of hydrogen reacts with 1 lit of chlorine,  results in always 2 lit of hydrogen chloride.
Avogdro’s Law:
In 1811, Avogadro  proposed this law. According to this law “under similar conditions of temperature and pressure, equal volume of all gases contains equal number of molecules”.
Dalton’s Atomic Theory:
  • Matter consists of extremely small indivisible and indestructible particles called atoms.
  • Atoms of the same elements are identical in shape, size and mass.
  • Atoms of different elements have different sizes, masses and chemical properties.
  • Atoms of two or more elements combine in a fixed ratio to form a compound.


Atoms can neither be created nor be destroyed during chemical reaction.

Modern Atomic Theory:
An atom is no longer considered indivisible:  it is made up electrons, protons and neutrons. Atoms of the same element may have different atomic masses. Atoms of elements that possess different atomic masses are known as isotopes.


Atoms of different elements may have same atomic masses are known as isobars.

Ex: Element Name Atomic Mass Unit (amu)  Calcium  40 amu  Argon  40 amu

The ratio in which different atoms of different elements combine may be fixed and integral, but may not always be simple. The atom is the smallest particle that takes part in a chemical reaction.


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