Use the noun solubility to refer to how easily something dissolves in water or another liquid. Salt has high solubility, easily dissolving in water, while oil has low solubility in water, and sand is not at all soluble in water.
Solubility is also a synonym for solvability, or how easy or difficult a particular problem is to solve. The solubility of your math homework is pretty much a sure thing. The solubility of some of the planet's more thorny issues — racism, climate change, the perfect ratio of chocolate to milk in hot cocoa — is less certain.