An ideal solvent for recrystallization is one that dissolves the compound of interest but not any impurities. It is unreasonable to expect a 100% yield from recrystallization because some product will remain dissolved in the mother liquor or be lost during filtration. Using too much solvent can make crystals difficult to form and collect. Impurities remain in the mother liquor after crystals have formed and been separated by vacuum filtration. If crystals do not form after cooling, the solution can be seeded with existing crystals or the solvent adjusted to better promote crystallization.