model for solvents with a density greater than water. This type has a tube at the bottom of the collection vessel to allow the organic solvent at the bottom to flow back into the reaction vessel. The water generated during the reaction floats on top of the organic phase. The entire volume must be drained to remove the water and the organic layer can be separated to be returned to the system. This piece of equipment is usually used in azeotropic distillations. A common example is the removal of water generated during a reaction in boiling toluene. An azeotropic mixture of toluene and water distills out of the reaction, but only the toluene (density 0.865g/ml) returns, since it floats on top of the water (density 0.998g/ml), which collects in the trap. Some high-boiling liquids that have an azeotrope with water can be dried by adding toluene or another azeotrope-breaking solvent to allow the extraction of water. The Dean–Stark method is commonly used to measure moisture content of items such as bread in the food industry. This equipment can be used in cases other than simple removal of water. One example is the esterification of butanol with acetic acid catalyzed by sulfuric acid. The vapor contains 63% ester, 29% water and 8% alcohol at reflux temperature and the organic layer in the trap contains 86% ester, 11% alcohol and 3% water which is reintroduced. The water layer is 97% pure. Another example is the esterification of benzoic acid and n-butanol where the ester product is trapped and the butanol, immiscible with the water, flows back into the reactor. Removing water in the course of these esterifications shifts the chemical equilibrium in favor of ester formation, in accordance with Le Chatelier's principle. References Laboratory equipment Laboratory glassware