What is the benefit of colder temperatures on recrystallization?
Colder temperatures promote more ordered recrystallization. It's not necessary, but it produces larger and nicer-looking crystals. It also makes them more pure, which is why it's encouraged to use an ice bath.
Recrystallization is when a solution is heated up in order to force the solvation of a precipitate at higher temperatures (since solubility increases at higher temperatures), then cooled down to induce crystallization (since solubility decreases at lower temperatures).
In general, since molecular motion is slower at lower temperatures:
- Crystallization is slower, allowing it to be more ordered.
- The impurities can be left behind while the pure compound is recrystallizing, instead of quickly crystallizing with the pure compound.
If you add ethanol for example, a crystalline ionic compound would form more quickly since organic alcohols are less polar than water (thus solvating more-ionic compounds more poorly, facilitating crystallization). So, the crystals would be less ordered, smaller, and actually less pure.