Butanoic acid in water. Carboxylic acids are categorised by their functional -COOH (carboxyl) group. This functional group is highly polar and allows the carboxylic acid to form strong intermolecular forces with other polar molecules. However, as the molecular weight of the carboxylic acid molecule increases, the number of non-polar C-C and C-H bonds increases proportionally. Both C-C and C-H bonds are non-polar and these limit the solubility of the carboxylic acid in a polar substance such as water. In this clip, butanoic acid (butyric acid) is mixed with water. Larger carboxylic acids such as hexanoic acid are solids at room temperature. For the same process with methanoic acid, see K004 1187.