How can you make crystals?

• A. Heat to the point of crystallisation; Leave to cool gradually; Filter, dry and collect the crystals.
• B. Allow the solution to cool quickly and then filter and dry.
• C. Boil off all solvent completely and collect solids.
• D. Add more solute until crystals appear spontaneously.

Answer: (A)

Crystallisation happens when a saturated solution is allowed to cool slowly. By heating, you dissolve the solid in a solvent to make a saturated solution; as the solution cools, its ability to hold the solute decreases and crystals begin to form. Cooling gradually lets crystals grow larger and purer because most impurities stay dissolved in the remaining liquid. After crystals form, filtering separates them from the liquid (the mother liquor), and drying removes the solvent so you’re left with solid crystals ready to collect.

If you cooled quickly, you’d get many tiny crystals with less-defined shapes and possibly more impurities trapped in them. Boiling off all the solvent removes the medium needed for controlled crystal growth and can leave a messy solid rather than clean crystals. Adding more solute until crystals appear spontaneously isn’t a controlled way to form well-formed crystals and can yield an impure or uneven product.