A. Classification based on observation of a phase change

1. Gas evolution: A gas is evolved from a solid or a solution.
   Examples:
   
   Sodium carbonate dissolves in dilute aqueous acid.
   
   Decomposition of hydrogen peroxide.

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2. Precipitation: A solid (the precipitate) is produced from solution.
   Examples:
   
   Insoluble silver chloride forms when aqueous solutions of two soluble salts, sodium chloride and silver nitrate, are mixed.

   
   A precipitate forms when carbon dioxide is bubbled through "limewater".

3. Dissolution: A solid dissolves on addition of a reactant or a solvent.
   Examples:
   
   Calcium carbonate (limestone) dissolves in dilute aqueous acid.

   
   Calcium phosphate dissolves in aqueous acid.

   
   Common salt dissolves in water. This is not really regarded as a chemical reaction, rather a physical change.