Page 724 - 725 of textbook: When distilling mixtures of liquids, azeotropes can form. An azeotrope is a mixture of liquids with fixed composition that distills as if it was a single compound. That is, azeotropic mixtures cannot be separated by simple distillation. Examples of azeotropic mixtures are given in table 15.2 (p. 726).
In this experiment, the reaction mixture contains several liquids, namely the alkene product (b.p.= 101-102o), the alcohol starting material (b.p.= 171-173o), and water (b.p.= 100o) from the reaction and from phosphoric acid (which is sold as an 85% solution in water). Sulfuric acid is also a liquid, but its b.p. is very high. The boiling point of the product is very close to that of water, so they can codistill. Some of the alcohol can also codistill, even though its b.p. is substantially different from the others. Finally, some of the acid might codistill with the water due to formation of hydrogen bonding. A complete separation of liquids by simple distillation is almost never possible.