A supersaturated solution is one of the most interesting and unstable types of mixtures discussed in IB Chemistry Topic 1 (Stoichiometry) and Topic 7 (Equilibrium). Supersaturation helps explain crystallization, solubility curves, and the dynamic behavior of solutions. Because supersaturated solutions contain more dissolved solute than they normally should at a given temperature, they demonstrate how temperature, solubility, and equilibrium interact in the real world.
What Is a Supersaturated Solution?
A supersaturated solution is a solution that contains more dissolved solute than it can normally hold at a specific temperature and pressure.
This makes it:
- Unstable
- Temporary
- Prone to crystallization
Supersaturated solutions exist only under special conditions and form when a saturated solution is heated, more solute is added, and the mixture is cooled without crystallizing.
How Supersaturated Solutions Form
Supersaturation requires three steps:
1. Heat a solvent
Heating increases solubility, allowing more solute to dissolve.
2. Add excess solute until the solution becomes saturated at the high temperature
At this stage, all added solute dissolves.
3. Cool the solution carefully
As temperature drops, solubility decreases.
However, if the cooling is slow and no crystals form, the solution temporarily holds more solute than it normally could.
The result is a supersaturated solution.
Why Supersaturated Solutions Are Unstable
A supersaturated solution is in a metastable state.
This means it wants to return to a lower-energy, stable condition.
The extra dissolved solute is not comfortable staying dissolved, so the solution will readily crystallize if:
- A seed crystal is added
- The container is scratched
- The solution is stirred
- A shock or vibration occurs
- Dust or impurities enter
This sudden release of excess solute can be dramatic and fast.
Examples of Supersaturated Solutions
1. Sodium acetate “hot ice” demonstration
Heating a concentrated solution of sodium acetate and cooling it forms a supersaturated mixture.
Touching it triggers rapid crystallization, releasing heat.
2. Rock candy formation
A sticky solution of sugar cools until crystals grow on strings or sticks.
3. Carbonated beverages
Under pressure, CO₂ dissolves.
Opening the bottle reduces pressure and creates a supersaturated state, causing bubbling and fizzing.
4. Sodium thiosulfate
Often used in labs for crystallization demonstrations.
These examples show how supersaturation is both practical and observable.
Supersaturation and Solubility Curves
Solubility curves show how much solute dissolves at various temperatures.
A supersaturated solution lies above the solubility line.
IB students must understand:
- Below the curve → unsaturated
- On the curve → saturated
- Above the curve → supersaturated
Supersaturated solutions cannot remain above the curve indefinitely; the excess solute will eventually crystallize.
Crystallization and Supersaturation
Supersaturation is essential for crystallization techniques used in:
- Purifying substances
- Producing crystals for analysis
- Industrial processes like candy production
In the lab, crystallization is triggered by:
- Adding a seed crystal
- Scratching the container
- Cooling further
Once crystallization begins, the solution moves back toward its saturation point.
Why Supersaturated Solutions Matter in IB Chemistry
They help explain:
- Dynamic equilibrium in solutions
- Temperature effects on solubility
- Intermolecular forces in dissolution
- Enthalpy changes during crystallization
- Lab techniques for purification
Supersaturation also reinforces ideas about stability, energy, and molecular interactions.
Common IB Misunderstandings
“A supersaturated solution is more stable than a saturated one.”
False—supersaturated solutions are highly unstable.
“Supersaturation happens at any temperature.”
No—it requires heating and controlled cooling.
“Supersaturated means a chemical reaction occurred.”
It is a physical process involving solubility, not reactivity.
“All substances can form supersaturated solutions.”
Only substances with strong temperature-dependent solubility behavior can.
FAQs
What happens if you add a seed crystal?
It triggers rapid crystallization as excess solute precipitates.
Is supersaturation reversible?
Yes—reheating the solution dissolves the crystals again.
Why must the solution cool undisturbed?
Movement can trigger crystallization and destroy supersaturation.
Conclusion
A supersaturated solution contains more dissolved solute than it should at a given temperature. Formed by heating, saturating, and cooling carefully, it is a metastable and temporary state that demonstrates the dynamic relationship between temperature and solubility. Supersaturated solutions are important in crystallization, laboratory techniques, and understanding phase stability in IB Chemistry.
