Fractional Crystallization
Summary
By dissolving a mix of two salts in hot water and cooling it, you can make one salt crystallize out while the other stays dissolved — separating them by their different solubility1 curves. You’ll be able to explain why potassium nitrate “crashes out” on cooling while table salt doesn’t, and how repeated cycles push toward purity.
History
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.
Hazards & preparation
PPE: safety glasses; handle hot glassware with care.
- Potassium nitrate is an oxidizer — keep it away from combustible material and never heat the dry salt with anything organic present. In solution it’s low hazard.
- Near-boiling solution can scald — use a mitt and a stable surface.
Disposal: both salts are benign in solution — flush down the drain with water. See the Safety page.
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
- Mix the potassium nitrate and sodium chloride together in a beaker.
- Add 50mL water and heat while stirring until everything dissolves (near boiling).
- Carefully filter the hot solution into the second clean beaker to remove any undissolved particles.
- Place the hot solution in the ice bath, or let it cool to room temperature.
- As it cools, crystals form — potassium nitrate has a steep solubility curve and crashes out, while sodium chloride (whose solubility barely changes) stays dissolved.
- Filter the crystals once cooled to ~10-15°C. The crystals are mostly potassium nitrate; the filtrate holds most of the sodium chloride.
- Dry and weigh the crystals. Optionally flame-test a little: potassium gives a violet flash.
What you should see
As the hot, clear solution cools, needle-like potassium nitrate crystals suddenly bloom throughout it — often quite fast in the ice bath — while the liquid stays clear of salt. Filtered and dried, you’re left with a heap of KNO₃ crystals and a filtrate carrying nearly all the NaCl.
| Symptom | Likely cause | Fix |
|---|---|---|
| No crystals on cooling | Too much water, or not cooled enough | Evaporate some water and re-cool; use the ice bath |
| Crystals look cakey/impure | Some NaCl co-crystallized | Redissolve and cool again (a second cycle purifies) |
| Everything stays dissolved | Solution too dilute | Boil off water to concentrate before cooling |
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.
Questions to Explore
Why does KNO₃ solubility change so much with temperature when NaCl’s barely changes? Both are ionic salts. What’s different about them that gives such different solubility–temperature curves?
Hint / answer
For KNO₃, dissolving is strongly endothermic, so heat greatly helps it dissolve — solubility soars with temperature. For NaCl, dissolving barely absorbs or releases heat, so temperature has almost no leverage and its solubility stays nearly flat.
How pure can you get in a single step? The first crop is mostly KNO₃ but carries some NaCl. After three cool-and-redissolve cycles, how much NaCl remains, and is there a limit?
Hint / answer
Each cycle discards most of the remaining NaCl with the filtrate, so impurity drops roughly geometrically — after a few cycles it’s very low. The limit is set by how much liquid clings to the crystals (mother liquor) each time; you never reach perfectly zero, but you get arbitrarily close.
What happens to the NaCl? After filtering off the KNO₃, where is most of the sodium chloride, and how would you recover it pure?
Hint / answer
Most NaCl stays dissolved in the filtrate. Boil that filtrate down: since NaCl’s solubility hardly changes with temperature, it comes out mainly by evaporation rather than cooling — evaporate to dryness (or to the point KNO₃ hasn’t re-saturated) to recover it.
How did Marie Curie use this to isolate radium? She ran thousands of cycles on tons of ore. Given that radium and barium are chemically similar, what challenge did she face that a two-salt separation doesn’t?
Hint / answer
Radium and barium behave so similarly that their solubilities differ only slightly, so each cycle enriched the radium only a tiny bit — hence thousands of painstaking recrystallizations. Our KNO₃/NaCl pair separates in one or two cycles because their solubility curves differ enormously.
Where does this show up today? Fractional crystallization still purifies sugar, separates fertilizer salts, and concentrates minerals. What advantages does it have over distillation, chromatography, or filtration for these?
Hint / answer
It’s cheap, scalable, and gentle — no high heat to boil everything (distillation) or costly columns (chromatography), and it separates dissolved species that a filter can’t. For bulk solids like sugar and fertilizer salts, cooling a hot solution is by far the most economical route.
Going further
- Purify further. Redissolve your KNO₃ crop in the minimum hot water and cool again; weigh each generation and watch the purity climb.
- Recover the other salt. Boil the filtrate down to reclaim the sodium chloride by evaporation, contrasting the two separation modes.
- Next in the Crystals & Solutions track: store heat in a crystallizing salt with Glauber’s Salt.
Footnotes
Solubility — The maximum amount of a substance that will dissolve in a given amount of solvent at a given temperature.↩︎