Insoluble Compound
Definition and meaning of Insoluble Compound in chemistry.
An insoluble compound is a substance that does not dissolve to any significant extent in a given solvent, typically water, instead remaining as a separate solid phase (a precipitate) rather than forming a true solution.
In more detail
Solubility is actually a matter of degree, not an absolute property: chemists usually call a compound "insoluble" if less than about 0.1 g dissolves per 100 mL of solvent at room temperature. For ionic compounds, insolubility results when lattice energy (the energy holding the ionic crystal together) exceeds the hydration energy released when ions are surrounded by solvent molecules. Chemists predict insolubility using solubility rules and quantify it with the solubility product constant, Ksp: a very small Ksp indicates an insoluble, or sparingly soluble, salt.
Key facts
| Common Example | BaSO4 (barium sulfate) |
|---|---|
| Typical Threshold | < 0.1 g dissolved per 100 mL solvent |
| Governed By | Solubility product constant (Ksp) |
| Field | Inorganic Chemistry |
Barium sulfate (BaSO4) is insoluble in water. This low solubility is medically useful: patients swallow a barium sulfate suspension ("barium meal") before an X-ray because it coats the digestive tract without being absorbed into the bloodstream.
Frequently asked questions
Is any compound completely insoluble?
No. Every compound dissolves to some measurable extent, even if extremely small. "Insoluble" means the solubility is negligible for practical purposes, not that it is exactly zero.
How can you predict whether a compound is insoluble?
Solubility rules for common ionic compounds provide quick predictions, while comparing Ksp values gives a quantitative measure, a smaller Ksp corresponds to lower solubility.