mixture of 80%glucose at 40 8 C, part (about 30%) of the glucose will be
crystalline, and that asa-monohydrate. When increasing the temperature
rapidly to 60 8 C, the solubility of the monohydrate would probably still be
surpassed, as indicated by the extrapolated broken line. However, the
solubility of the anhydrous form is lower. What will happen is that the
hydrate dissolves and that some glucose crystallizes in the anhydrous form
(provided that nucleation occurs within a reasonable time), until all of the
hydrate has disappeared. Cooling again will lead to the opposite: the
anhydrous form dissolves and monohydrate crystallizes. Similar changes
can occur near 90 8 C, where changes from thea-totheb-anomer, and vice
versa, are possible.
Figure 15.17a gives further details forsucrose. It is useful to express
the concentration in kg (or moles of) solute per kg water, especially if more
solutes are present. By plotting log concentration against temperature,
approximately straight lines are often obtained. The curves divide the
FIGURE15.16 Solubility, expressed as mass fractioncS, of some sugars in water
as a function of temperature T. a and brefer to different anomers, H to
monohydrate, and DH to dihydrate.