Azeotropes or constant boiling mixtures is a mixture of two or more liquid which cannot be separated by simple distillation. This happens because when azeotrope is boiled, vapor has same proportion of constituents as unboiled mixture.

Maximum boiling azeotropes:

These are obtained from solution that shows negative deviation from ideal solution. We know, vapor pressure is inversely proportional to boiling point. Since first type of mixture showing negative deviation have lowest vapor pressure, they form maximum boiling azeotropes. Here the mixtures will boil at the temperature above boiling temperature of components from which they are made and the mixture will boil undistilled. Read more