Reflux apparatus is when a distillation tube is surrounded by an apparatus that pumps cold water around it in order to condense its compounds in the mixture
As ethanol has a low boiling point, it evaporates fairly quickly.This means some of the ethanol may escape before it has been fully oxidised ino a carboxylic acid. Reflux therefore allows ethanol to be condensed back into liquid form to ensure all of the reactants are used up in the reaction. This will help increase the yield of ethanoic acid.