0.4 to around 0.15 as the liquid mixture boils to the gas phase. Temperature glides Different zeotropic mixtures have different temperature glides. For example, zeotropic mixture R152a/R245fa has a higher temperature glide than R21/R245fa. A larger gap between the boiling points creates a larger temperature glide between the boiling curve and dew curve at a given mass fraction. However, with any zeotropic mixture, the temperature glide decreases when the mass fraction of a component approaches 1 or 0 (i.e. when the mixture is almost separated into its pure components) because the boiling and dew curves get closer near these mass fractions. A larger difference in boiling points between the substances also affects the dew and bubble curves of the graph. A larger difference in boiling points creates a larger shift in mass fractions when the mixture boils at a given temperature. Zeotropic vs. azeotropic mixtures Azeotropic and zeotropic mixtures have different dew and bubble curves characteristics in a temperature-composition graph. Namely, azeotropic mixtures have dew and bubble curves that intersect, but zeotropic mixtures do not. In other words, zeotropic mixtures have no azeotropic points. An azeotropic mixture that is near its azeotropic point has negligible zeotropic behavior and is near-azeotropic rather than zeotropic. Zeotropic mixtures differ from azeotropic mixtures in that the vapor and liquid phases of an azeotropic mixture have the same fraction of constituents. This is due to the constant boiling point of the azeotropic mixture. Boiling When superheating a substance, nucleate pool boiling and convective flow boiling occur when the temperature of the surface used to heat a liquid is higher than the liquid's boiling point by the wall superheat. Nucleate pool boiling The characteristics of pool boiling are different for zeotropic mixtures than that of pure mixtures. For example, the minimum superheating needed to achieve this boiling is greater for zeotropic mixtures than for pure liquids because of the different proportions of individual substances in the liquid versus gas phases of the zeotropic mixture. Zeotropic mixtures and pure liquids also have different critical heat fluxes. In addition, the heat transfer coefficients of zeotropic mixtures are less than the ideal values predicted using the coefficients of pure liquids. This decrease in heat transfer is due to the fact that the heat transfer coefficients of zeotropic mixtures do not increase proportionately with the mass fractions of the mixture's components. Convective flow boiling Zeotropic mixtures have different characteristics in