Abnormal Densities of Gases - QS Study
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Abnormal Densities of Gases: Molecular Association and Dissociation

The density of gases depends upon the temperature. Measurements of densities of vapours have served to determine the molecular masses of a great many substances. In most cases the results are in agreement with the values obtained from the sum of the atomic masses of elements in the molecular formula. A few cases were, however, found where the molecular masses as determined from vapour density seemed abnormal. These abnormal values were attributed to association or dissociation of the molecules in the vapour state.

High pressure means high density, cooler, sinking air. Low pressure means low density, warmer, rising air.

Two types of Abnormal Densities of Gases:

  • Molecular Association: The formation of double molecules or poly-molecules from a single species as a result of specific and moderately strong inter-molecular forces.
  • Molecular Dissociation: Dissociation is a general process in which molecules separate or split into smaller particles such as atoms, ions or radicals, usually in a reversible manner.

Example: In this case, the carbon dioxide produced by the vinegar and baking soda reaction sinks to the bottom of the beaker. In the process, the air which is less dense is pushed up and out of the beaker. As the oxygen in the air is pushed out of the beaker by the denser carbon dioxide, the candles go out one by one.