Fractional Crystallization

Separate two salts by exploiting different solubility curves

Difficulty: Medium | Time: 45 minutes | Visual Impact: Medium

Historical Context

Fractional crystallization is one of the oldest purification techniques in chemistry, used long before anyone understood why it worked. Medieval alchemists purified saltpeter (potassium nitrate) from crude natural deposits by dissolving it in hot water and cooling the solution - the potassium nitrate crystallized preferentially, leaving impurities in solution.

The technique became industrially critical in the 19th century. The Stassfurt potash deposits in Germany contained a complex mixture of potassium and sodium salts. German chemists worked out precise solubility curves for each salt at different temperatures, allowing the mixture to be separated into pure components by carefully controlled cycles of heating, cooling, and filtering. These methods supplied the fertilizer potassium that helped drive the agricultural revolution.

Marie Curie used fractional crystallization to concentrate and ultimately isolate radium from tons of uranium ore - performing thousands of crystallization cycles in her Paris laboratory, each one slightly enriching the radium fraction.

Materials

  • Potassium nitrate - 15g
  • Sodium chloride - 5g
  • Water - 50mL
  • Two beakers (150mL)
  • Hot plate or stove
  • Thermometer
  • Stirring rod
  • Filter paper and funnel (or coffee filter)
  • Ice bath (bowl of ice water)

Procedure

  1. Mix potassium nitrate and sodium chloride together in a beaker.
  2. Add 50mL of water and heat while stirring until everything dissolves (the solution will be near boiling).
  3. Carefully transfer the hot solution to the second clean beaker using the filter to remove any undissolved particles.
  4. Place the hot solution in the ice bath or simply allow to cool to room temperature.
  5. As the solution cools, crystals will form. Potassium nitrate has a steep solubility curve - it crashes out of solution as temperature drops, while sodium chloride (whose solubility changes little with temperature) stays dissolved.
  6. Filter the crystals once cooled to ~10-15°C. The crystals on the filter are mostly potassium nitrate; the filtrate contains most of the sodium chloride.
  7. Dry and weigh the crystals. Optionally, test a small amount with a flame test: potassium nitrate burns with a violet flash.

The Science

The key to this separation is the difference in how solubility changes with temperature:

Salt Solubility at 20°C Solubility at 80°C
KNO₃ 32 g/100mL 170 g/100mL
NaCl 36 g/100mL 38 g/100mL

At high temperature, both salts dissolve freely. As the solution cools, KNO₃ solubility drops sharply - it must crystallize out because the solution can no longer hold it all. NaCl solubility barely changes, so it stays dissolved.

This is fractional crystallization: the component with the steeper solubility-temperature curve crystallizes first and most completely. Multiple cycles (redissolve the crystals, cool again) progressively improve the purity.

Safety

Potassium nitrate is an oxidizer - keep away from combustible materials. Do not heat the dry salt with organic material present. In solution, it is low hazard. Sodium chloride is harmless. Handle hot glassware with care.

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