A solution forms when a solute dissolves evenly in a solvent, such as sugar dissolving in water. Saturation describes how much solute a solvent can hold at a given temperature and pressure. This idea matters in chemistry, cooking, medicine, geology, and manufacturing because solubility controls whether substances stay dissolved or form crystals.
Understanding saturation helps predict when adding more solute will dissolve, remain undissolved, or trigger crystal growth.
An unsaturated solution can still dissolve more solute, while a saturated solution has reached its solubility limit. A supersaturated solution contains more dissolved solute than is normally stable at that temperature, often made by dissolving solute in hot solvent and cooling it carefully. Supersaturated solutions are unstable, so adding a seed crystal or disturbing the liquid can start crystallization.
During crystallization, dissolved particles leave the solution and arrange into an ordered solid structure.
Key Facts
- Solute + solvent = solution.
- Unsaturated solution: dissolved solute amount is less than the solubility limit.
- Saturated solution: dissolved solute amount equals the solubility limit, so extra solute remains undissolved.
- Supersaturated solution: dissolved solute amount is greater than the normal solubility limit and the solution is unstable.
- Solubility is often written as grams of solute per 100 g solvent at a specific temperature.
- Percent by mass = mass of solute / mass of solution × 100%.
Vocabulary
- Solute
- The substance that dissolves in a solvent to form a solution.
- Solvent
- The substance that does the dissolving and is usually present in the larger amount.
- Solubility limit
- The maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature and pressure.
- Crystallization
- The process in which dissolved particles come out of solution and form an ordered solid crystal.
- Seeding
- The addition of a small crystal or particle that gives dissolved solute a surface on which to begin crystallizing.
Common Mistakes to Avoid
- Calling any clear solution saturated is wrong because a clear solution may still be unsaturated and able to dissolve more solute.
- Ignoring temperature is wrong because solubility often changes greatly with temperature, especially for many solid solutes in water.
- Thinking supersaturated solutions are stable forever is wrong because they can crystallize quickly when seeded, scratched, shaken, or contaminated.
- Confusing dissolving with melting is wrong because dissolving separates solute particles into a solvent, while melting changes a solid into a liquid by heating.
Practice Questions
- 1 At 25°C, the solubility of potassium nitrate is 38 g per 100 g water. If 30 g of potassium nitrate is added to 100 g of water at 25°C and all of it dissolves, is the solution unsaturated, saturated, or supersaturated?
- 2 A saturated solution at 20°C contains 36 g of sodium chloride dissolved in 100 g of water. If 45 g of sodium chloride is added to 100 g of water at 20°C, how many grams remain undissolved after equilibrium is reached?
- 3 A hot solution is cooled slowly without forming crystals, then a tiny seed crystal is dropped in and many crystals grow. Explain why this shows the solution was supersaturated rather than simply saturated.