Atoms and Molecules

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02 Laws of Chemical Combination

Law of Conservation of Mass:

  • The Law of conservation of mass was discovered by Lavoisier in 1774.
  • The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction.
  • According to this law, “In a chemical substance the elements are always present in definite proportions by mass”.
  • It means that, in a chemical reaction, the total mass of reactants is equal to the total mass of products and there is no change in the mass during a chemical reaction.
  • Many compounds were observed to contain two or more elements since earlier times and each element was present in definite proportion irrespective of its source.
  • For e.g. River and ocean water, both have oxygen and hydrogen in the same proportion.
  • Example:
    Every 17 gram of ammonia contains 14 gram of nitrogen and 3 grams of hydrogen irrespective of source. 34 grams of ammonia will contain 28 gram of nitrogen and 6 grams of hydrogen. So, the ration by mass of ammonia, nitrogen and hydrogen is always 17:14:3., Although this law of chemical combination is very obvious today with the concept of atom, molecular mass and molar mass explained, it was a major breakthrough in the early age of chemistry. Based on this theory, Dalton’s gave his postulates about its atomic theory which form base for understanding of chemistry.

Properties of atoms and molecules

  • The law of constant proportions was formulated and proven by Joseph Louis Proust in 1799.
  • This law states that, when a compound is broken, the masses of the constituent elements remain in the same proportion.
  • This means that, a chemical compound always consists of the same elements combined together in the same proportion by mass.

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