An Azeotrope is a 'constant boiling mixture' that has the same composition in the liquid and vapor phases.

• Ethanol and water form a Minimum Boiling Azeotrope at $95.6\%$ ethanol and $4.4\%$ water by mass.
• This mixture boils at $78.17^{\circ }C$ (often rounded to $78.5^{\circ }C$), which is lower than the boiling points of both pure ethanol ($78.3^{\circ }C$) and pure water ($100^{\circ }C$). Because the composition does not change during boiling, they cannot be further separated by simple fractional distillation.