What Is Solubility in Chemistry?
Solubility in chemistry is the maximum amount of a solute that will dissolve in a given amount of solvent at a specified temperature, producing a saturated solution. It is a number, not a yes-or-no property. Table salt has a solubility of about 36 grams per 100 grams of water at 25 °C. Because that figure shifts with temperature, a solubility value means little unless the temperature is stated beside it. It explains why sugar vanishes in hot tea and why soda goes flat.
Key takeaways
- Solubility is a quantitative limit: the most solute that dissolves in a fixed amount of solvent at a stated temperature, usually written in grams per 100 grams of water.
- A solution is unsaturated below that limit. A saturated solution sits exactly at it, in equilibrium with any undissolved solid.
- “Like dissolves like”: polar and ionic solutes dissolve in polar solvents, nonpolar in nonpolar.
- Most solids become more soluble as temperature rises, but gases become less soluble. Gas solubility also climbs with pressure.
- Stirring and crushing change how fast a solid dissolves, never how much dissolves.
- A product that cannot stay dissolved separates out as a solid precipitate.
What is solubility in chemistry?
Solubility is the maximum amount of a substance that dissolves in a given quantity of another at a specified temperature. The solute is the material being dissolved, the solvent is what does the dissolving, and together they form a solution. Unless another solvent is named, chemists mean solubility in water.
The important word is maximum. Solubility does not describe whether something dissolves; it tells you how much. You will see it written three main ways:
- grams of solute per 100 grams of water, the usual form in school chemistry;
- grams per liter of solution;
- moles per liter, called the molar solubility.
Whichever unit is used, the temperature has to be stated. Potassium nitrate dissolves to roughly 32 grams per 100 grams of water at 20 °C, but to around 245 grams at 100 °C. An unlabeled figure for that salt tells you almost nothing.
Solubility is also not the same as concentration, which says how much solute a sample actually holds. Solubility is the ceiling that concentration cannot stably pass.
What are saturated, unsaturated, and supersaturated solutions?
These three words describe how close a solution sits to its limit.
- Unsaturated: less solute is dissolved than the solvent can hold, so more will disappear if you add it.
- Saturated: the solution holds exactly its limit, and extra solid just sits on the bottom.
- Supersaturated: the solution holds more than its limit, which is unstable.
A saturated solution resting on undissolved solid is not doing nothing. It is at dynamic equilibrium: particles keep leaving the solid surface while others rejoin it at the same rate. The amount dissolved stops changing even though the movement never stops.
You make a supersaturated solution by saturating a solvent while it is hot, then cooling it slowly without knocking it. The extra solute stays dissolved because it has no surface to crystallize on. Drop in a seed crystal and the excess solidifies within seconds, which is how reusable hand warmers work.
Why does “like dissolves like” work?
Whether one substance dissolves another depends mostly on polarity, which is how unevenly charge is spread across a molecule. Water molecules are polar: the oxygen end carries a slight negative charge and the hydrogen ends a slight positive one.
Those partial charges let water pull ions and polar molecules apart and surround them. When salt dissolves, water molecules tug on the sodium and chloride ions until they leave the crystal lattice.
Nonpolar substances such as oil, wax, and iodine have no charged regions to offer, so water molecules stick to each other instead and squeeze them out. Put the same substances in a nonpolar solvent like hexane and they dissolve easily.
Treat this as a guideline rather than a law. Chalk is ionic yet barely dissolves, because the pull holding its lattice together beats anything water can offer.
How does temperature change solubility?
For most solids, heating the solvent raises the solubility. Potassium nitrate climbs from roughly 13 grams per 100 grams of water at 0 °C to about 245 grams at 100 °C, which is why hot solutions are the standard way to grow crystals.
The rule is not universal. Table salt hardly responds, moving from close to 36 grams at 0 °C to only about 39 grams at boiling point, and calcium hydroxide runs backwards: it becomes measurably less soluble as the water warms.
Gases are the exception worth memorizing, because they run the opposite way. Warm water holds less dissolved gas than cold water. Dissolving a gas releases energy, and the dissolved molecules are only loosely held by the liquid around them, so heating gives them enough kinetic energy to escape into the air above.
You can watch it happen: the bubbles lining a pan long before it boils are dissolved air escaping.
Pressure matters for gases too. Henry's law states that the mass of gas dissolved is proportional to the pressure of that gas above the liquid, which is why a sealed can of soda stays fizzy and goes flat once opened. Pressure barely affects solids and liquids.
How soluble are common substances in water?
The figures below are approximate values near room temperature, in grams of solute per 100 grams of water. Published numbers vary a little with the measurement method, so treat them as a guide to scale.
| Substance | Formula | Approximate solubility (g per 100 g water) | Temperature | Note |
|---|---|---|---|---|
| Table sugar (sucrose) | C₁₂H₂₂O₁₁ | ~200 | 20 °C | One of the most soluble everyday solids |
| Table salt (sodium chloride) | NaCl | ~36 | 25 °C | Barely changes with temperature |
| Potassium nitrate | KNO₃ | ~32 | 20 °C | Rises steeply on heating |
| Baking soda | NaHCO₃ | ~10 | 20 °C | Moderately soluble |
| Calcium hydroxide | Ca(OH)₂ | ~0.17 | 20 °C | Less soluble when heated |
| Chalk (calcium carbonate) | CaCO₃ | ~0.001 | 25 °C | Treated as insoluble |
| Oxygen gas from air | O₂ | ~0.0009 (about 9 mg per liter) | 20 °C | Falls as the water warms |
What is the difference between dissolving rate and solubility?
This is the most common mistake in solubility questions. Stirring a mixture and grinding a solid into powder both make it dissolve faster, but neither raises the solubility by a fraction of a gram.
Stir 50 grams of salt into 100 grams of water at 25 °C and about 36 grams dissolve, leaving roughly 14 grams at the bottom. Stir harder or use finer grains and that leftover mass stays the same; you only reach the ceiling sooner.
- Change the rate: stirring, particle size, and temperature.
- Change the amount: the solute and solvent involved, temperature, and pressure for gases.
Temperature appears on both lists, and that overlap is exactly what confuses people. Heating tea speeds the sugar up and lifts the limit; stirring only does the first job.
What do “soluble,” “insoluble,” and “precipitate” mean?
Chemists use a rough working scale for these labels. Anything dissolving above about 1 gram per 100 grams of water counts as soluble, below about 0.1 gram it is insoluble, and in between it is slightly soluble.
Strictly speaking, nothing is completely insoluble. Silver chloride is the classic insoluble salt, yet about 0.0002 grams of it still dissolves in 100 grams of water at 25 °C.
Precipitates come straight out of these limits. Mix two solutions and, if any pairing of ions forms a compound that cannot stay dissolved, that compound drops out as a solid:
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Solubility rules let you predict this before you mix anything. All nitrates are soluble, so both nitrates stay in solution, while chlorides are soluble except with silver, lead, and mercury(I). That makes silver chloride the solid that appears.
Frequently asked questions
What is solubility in simple terms?
Solubility is how much of one substance will dissolve in another before no more can fit. Add solute past that point and the extra just sits there undissolved. Because the limit shifts with temperature, every solubility figure needs a temperature attached.
Does stirring increase solubility?
No. Stirring brings fresh solvent to the surface of the solid, so dissolving finishes sooner, but the maximum amount that can dissolve is unchanged. To move that maximum you have to change the temperature, the solvent, or, for a gas, the pressure.
Why are gases less soluble in hot water?
Dissolving a gas releases energy, and dissolved gas molecules are held loosely by the surrounding liquid. Heating gives them enough kinetic energy to escape back into the air. It is why warm river water carries less oxygen for fish.
What is the difference between solubility and concentration?
Solubility is a fixed maximum for one solute and solvent pair at one temperature. Concentration is how much solute a solution actually holds. A solution can have any concentration up to the solubility, and only an unstable supersaturated solution exceeds it.
Looking for clear definitions of solute, solvent, and saturated solution? Search any word in the A to Z Chemistry Dictionary.