What Is Solubility?
Solubility is the maximum mass of a solute that can dissolve in 100 g (or 100 cm³) of solvent at a particular temperature. It is usually measured in grams per 100 g of water (g/100 g).
When no more solute can dissolve, the solution is saturated. Adding more solute to a saturated solution just results in it settling at the bottom undissolved.
Solubility is often written as, for example: "The solubility of potassium nitrate at 20 °C is 31.6 g per 100 g of water." This means a maximum of 31.6 g of KNO₃ can dissolve in 100 g of water at that temperature. Any extra KNO₃ added would remain as a solid.
Factors That Affect Solubility
1. Temperature
Solids dissolving in water: Increasing temperature usually increases solubility. Sugar and most ionic salts dissolve more easily in hot water than in cold water. This is because the hotter water molecules move faster and can pull solute particles apart more effectively.
Gases dissolving in water: Increasing temperature decreases solubility of gases. Oxygen, carbon dioxide, and nitrogen are all less soluble in warm water than in cold water.
2. The Nature of the Solute and Solvent
Whether a substance dissolves depends on the chemical similarity between solute and solvent ("like dissolves like"). Salt dissolves in water (both polar/ionic). Grease dissolves in petrol (both non-polar).
3. Stirring and Particle Size
Stirring and crushing a solid into smaller pieces do not change the maximum solubility, but they increase the rate of dissolving (how quickly the saturation point is reached) by increasing surface area and mixing.
Solubility Curves
A solubility curve is a graph of solubility (y-axis) against temperature (x-axis). For most solids, it slopes upward — solubility increases with temperature. You can use the graph to:
- Find the solubility at any temperature.
- Predict how much solute crystallises out when a saturated solution is cooled.
Reading a solubility curve
If potassium nitrate has a solubility of 32 g/100 g at 20 °C and 85 g/100 g at 50 °C, what happens if a saturated solution made at 50 °C is cooled to 20 °C?
At 50 °C, 85 g was dissolved. At 20 °C, only 32 g can remain dissolved. So 85 − 32 = 53 g of potassium nitrate crystals form (crystallise out).
This is how large, pure crystals of salts are grown in laboratories and industry: a saturated solution is made at high temperature, then cooled slowly. The excess solute crystallises out as the solubility decreases. The slower the cooling, the larger and purer the crystals formed.
Real world: Fizzy drinks go flat faster when warm
Carbonated drinks (cola, sparkling water) contain dissolved CO₂ gas at high pressure. When you open a bottle and the pressure drops, CO₂ starts escaping. Leave the drink at room temperature and it goes flat in hours; put it in the fridge and it stays fizzy for longer. This is because gas solubility decreases with temperature — cold water holds more dissolved CO₂. The same principle explains why warm, polluted rivers have less dissolved oxygen and struggle to support fish life.
Real world: Supersaturated sodium acetate in hand warmers
Reusable hand warmers contain a supersaturated solution of sodium acetate — more solute dissolved than would normally be possible at room temperature. When you flex the metal disc inside, it triggers crystallisation. The sodium acetate rapidly crystallises, releasing heat (it's an exothermic process). To reuse the warmer, you boil it in water, which re-dissolves the crystals back into a supersaturated solution. This is solubility chemistry producing warmth on demand.
Real world: Salting icy roads uses solubility and freezing point depression
Road salt (mostly sodium chloride or calcium chloride) is spread on icy roads in winter. The salt dissolves in the thin film of water on ice. Adding a solute (salt) to water lowers its freezing point below 0 °C — so the icy road melts even when the temperature is below freezing. Calcium chloride works at even lower temperatures than sodium chloride. This application of solubility and freezing point depression prevents millions of road accidents each winter.
Where do you see this in real life? Fizzy drinks going flat faster when warm (gas solubility decreases with temperature), hand warmers using supersaturated sodium acetate, road salt melting ice in winter (dissolved salt lowers freezing point), hot water dissolving more sugar for jam-making — temperature and solubility are connected in countless everyday situations.
Practice Problems
Hot water can dissolve more sugar than cold water. Does this mean stirring cold water will dissolve more sugar than the maximum for that temperature?
Show answer
Why do fish struggle to survive in warm, polluted rivers compared to cold, clean rivers?