Recrystallization is the process of obtaining pure crystals from a compound containing impurities in a solvent. Hot gravity filtration is a process commonly used to remove these impurities from a solution prior to recrystallization.
Hot filtration is a type of filtration where the filtration equipment and the sample are heated during the process. Hot filtration is needed for recrystallization when impurities exist in solution. Recrystallization requires a hot solution because the solution needs to be supersaturated for crystals to form on cooling. Hot solutions can hold more solute in suspension compared to a cold solution because the solubility of most solids increases with a rise in temperature. This means that a saturated solution will contain more dissolved solute if prepared at a higher temperature than at a cold temperature. When the hot solution then cools, it will be supersaturated – it will hold more dissolved solute than its cold equivalent would.
The impurity may have a different solubility than the compound in certain solvents. The aim is to choose a solvent that dissolves the compound when heated, but that doesn’t dissolve the impurity at high temperatures. The impurity is then filtered out during the hot gravity filtration process.