Solubility of apsirin. Aspirin is a non-polar molecule which is insoluble in water in its molecular form. The molecular form of aspirin reacts as a carboxylic acid, and will form a water-soluble salt upon its reaction with sodium carbonate. The sodium salt of aspirin is water-soluble as it contains a deprotonated and thus charged acid group, along with a sodium ion. Soluble aspirin tablets contain sodium carbonate mostly to produce this reaction and solubility. The production of carbon dioxide from the tablet also helps, by breaking the tablet apart and increasing its surface area.