Why is sodium benzoate soluble in cold water whilst benzoic acid is relatively insoluble?
Jenkin says Sodium benzoate is a salt and so it is completely ionised, the ions being Na+ and C6H5COO-. The hydration of these ions will be an exothermic process as bonds are formed between the ions and water molecules. The energy released will be sufficient to overcome the hydrogen bonds between the water molecules. Benzoic acid, on the other hand, is not significantly ionised: it is a weak acid. There will be hydrogen bonds between the benzoic acid molecules and there will also be strong van der Waals forces as the molecules contain a large number of electrons. All these intermolecular forces, as well as the hydrogen bonds between water molecules have to be overcome if the acid is to dissolve. This can only happen if the attractive forces between acid and water molecules release sufficient energy and this is not the case. Risk assessment Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess th
