3.3A: Ideal Temperature Profile

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The crystallization procedure is possible as most solids tend to become more soluble in solvents as their temperature is increased (Figure 3.7 shows a yellow solid dissolving in ethanol as the temperature increases on a hotplate).

Figure 3.7: Time-lapse dissolving of benzil (yellow solid) in ethanol as the temperature increases on a hotplate. The boiling stick is used for bump prevention.

Caffeine is a white solid that follows this usual patter, as 1 gram of caffeine dissolves in \(46 \: \text{mL}\) of room temperature water, \(5.5 \: \text{mL}\) of \(80^\text{o} \text{C}\) water, and \(1.5 \: \text{mL}\) of boiling water.\(^2\) If you have ever had a difficult time cleaning solidified starch residue from a kitchen colander (perhaps after you have strained pasta water), you may have learned that the colander is more easily cleaned in hot water than cold. Starch has an increased solubility in hot water, and this trend in solubility is partly what makes a dishwasher so effective.

The solubility profile of compounds is an empirical determination. Some solids increase solubility regularly with temperature (Figure 3.8a), and others increase in an exponential fashion (Figure 3.8b). In unusual cases, a solid's solubility can decrease with temperature. For the best crystallization, the compound should be very soluble in the hot solvent and minimally soluble (or insoluble) in the cold solvent.

Figure 3.8: Graphs of solubility versus temperature data for glutaric acid in water (left) and diphenyl amine in hexane (right).\(^3\) In both sets, solubility increases with temperature. Note that glutaric acid could not be crystallized from water, as it is highly soluble even at cold temperatures.

\(^2\)From: The Merck Index, 12\(^\text{th}\) edition, Merck Research Laboratories, 1996.

\(^3\)Data from: A. Seidell, Solubilities of Inorganic and Organic Substances, D. Van Nostrand Company, 1907.