The addition of a metal ion, or cation, to a carboxylate anion forms a salt. When carboxylic acids dissolve in water, they form the carboxylate anion and hydrogen cations, which lower the pH of the solution.
When metal ions combine with short chain carboxylate anions, the product is often soluble in water. To recover the carboxylate salt, one must evaporate the water to cause the salt to precipitate from the solution. Longer chain molecules are less soluble in water, so these molecules precipitate upon formation.
The addition of sodium or potassium ions to the carboxylate anion of fatty acids forms soaps. However, when soaps dissolve in hard water, the calcium anions join with the carboxylate anion to form an insoluble precipitate, which makes them less effective at improving the cleaning ability of water.