What Is a Saturated Solution? Solubility Limits
Keep stirring sugar into a glass of iced tea and at some point it stops disappearing. A little pile settles at the bottom and simply refuses to go. The water hasn't got tired — it's full.
The short answer: a saturated solution contains the maximum amount of solute that will dissolve in a given solvent at a given temperature. Add more and it just sits there undissolved, because dissolving and re-crystallising are happening at exactly the same rate.
The three kinds of solution
| Type | How much solute | What you see | Stable? |
|---|---|---|---|
| Unsaturated | Less than the maximum | More solute still dissolves when added | Yes |
| Saturated | Exactly the maximum | Extra solute sits undissolved at the bottom | Yes |
| Supersaturated | More than the maximum | Clear, but crystallises at the slightest disturbance | No — metastable |
What "saturated" really means
The undissolved crystals at the bottom of a saturated solution aren't doing nothing. At the microscopic level, two processes are running at once:
- Dissolving — solvent particles pull solute particles off the crystal.
- Crystallising — dissolved particles bump back into the crystal and rejoin it.
In a saturated solution these two happen at equal rates. It's a dynamic equilibrium: the amount dissolved stops changing, but the traffic never stops. Individual ions are leaving and returning constantly — the books just balance.
That's the proper definition, and it's a much better answer than "no more will dissolve."
How to make a solution saturated
Two routes:
- Add solute until no more dissolves. Keep adding and stirring; when a permanent residue sits at the bottom, you're at the limit.
- Cool a hot solution. Most solids are more soluble hot. Cool a hot concentrated solution and it will reach saturation on the way down — then start dropping crystals out.
That second route is how recrystallisation purifies compounds in the lab, and how rock candy is made at home.
Supersaturated solutions: the unstable one
Here's the strange case. If you cool a hot saturated solution very gently, without dust or scratches or knocks, the extra solute sometimes fails to crystallise. You're left with more dissolved than should be possible at that temperature — a supersaturated solution.
It's balanced on a knife edge. Drop in a single seed crystal, scratch the glass, or just give it a knock, and the whole thing crystallises in seconds, often visibly racing across the container.
Reusable hand warmers are the everyday version: a sealed pouch of supersaturated sodium acetate solution. Click the metal disc inside and you trigger crystallisation — which releases heat, because crystallising is exothermic. Boil the pouch to redissolve the crystals and it's ready again.
Temperature changes everything
Saturation is always quoted at a temperature, because the ceiling moves:
- Most solids: solubility rises with temperature. Water at 100 °C holds far more sugar than water at 20 °C.
- Table salt is a notable exception in degree: NaCl solubility barely budges — about 36 g per 100 g of water at 25 °C and only around 39 g at 100 °C. Heating your pasta water doesn't help it take much more salt.
- Gases: solubility falls with temperature. Warm water holds less dissolved oxygen and less CO₂, which is why a warm fizzy drink goes flat and why rivers in a heatwave stress fish.
So a solution that's saturated at 20 °C becomes unsaturated the moment you warm it — no solute added, no solute removed, but the ceiling just rose above it.
Worked examples
Decide before you read on.
- You add 30 g of NaCl to 100 g of water at 25 °C and it all dissolves. Unsaturated — the limit is about 36 g.
- You add 45 g of NaCl to 100 g of water at 25 °C; about 9 g stays at the bottom. Saturated, with excess solute.
- A clear solution suddenly crystallises solid when you drop one crystal in. It was supersaturated.
- A saturated solution at 20 °C is heated to 60 °C. What is it now? Unsaturated, because solubility rose.
- A saturated hot solution is cooled slowly and crystals appear. Solubility fell, so the excess came out of solution — this is crystallisation.
Common mistakes to avoid
- Confusing saturated with concentrated. "Concentrated" just means a lot of solute. Saturated means at the limit. A saturated chalk solution is extremely dilute — the limit is simply very low.
- Thinking nothing is happening in a saturated solution. Dissolving and crystallising continue at equal rates. It's a dynamic equilibrium, not a stopped process.
- Forgetting to state the temperature. "Saturated" is meaningless without one, because the maximum changes as soon as the temperature does.
FAQ
What is a saturated solution?
A solution holding the maximum amount of solute that will dissolve in that solvent at that temperature. Any extra solute stays undissolved, and dissolving and crystallising occur at equal rates.
What is the difference between saturated and supersaturated?
A saturated solution holds exactly the maximum amount of solute and is stable. A supersaturated solution holds more than the maximum, made by cooling a hot solution carefully, and crystallises suddenly if disturbed.
How can you tell if a solution is saturated?
Add a small amount of extra solute. If it dissolves, the solution was unsaturated; if it sits undissolved, the solution was already saturated.
Does heating a saturated solution make it unsaturated?
Usually yes, for solids — heating raises the solubility limit, so the same amount of dissolved solute is now below the new maximum. For dissolved gases the reverse happens and they come out of solution.
The takeaway
Saturation is a ceiling, not a wall — and the ceiling moves with temperature. Below it the solution is unsaturated and will take more; at it the solution is saturated and sits in dynamic equilibrium with undissolved solute; above it, briefly and precariously, the solution is supersaturated and waiting for an excuse to crystallise. Always quote the temperature, and always say at the limit, not full.
Background → [What Is Solubility?] and [Solute vs Solvent] (sibling posts). Related → [What Is a Phase Change?] (sibling) — crystallisation is the same equilibrium idea in a different costume. For concentration calculations → What Is the Mole? Avogadro's Number Made Simple.
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