Methanoic 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. In this example, methanoic acid (formic acid), HCOOH is added to distilled water. Due to its low molecular size and highly polar carboxyl group, the methanoic acid molecules readily form strong intermolecular hydrogen bonds with polar water molecules. This produces one immiscible layer as the two liquids are mixed. For the same process with butanoic acid, see K004 1189.