Answer:
Polarity.
Step-by-step explanation:
Solubility is defined as the upper limit of solute that can be dissolved in a given amount of solvent at equilibrium. In such an equilibrium, Le Chatelier's principle can be used to explain most of the main factors that affect solubility. Le Châtelier's law dictates that the effect of stress upon a system in chemical equilibrium can be predicted in that the system tends to shift in such a way as to alleviate that stress.
The relationship between the solute and solvent is essential in determining solubility. Strong solute-solvent attractions equate to greater solubility, while weak solute-solvent attractions equate to lesser solubility. In turn, polar solutes tend to dissolve best in polar solvents, while non-polar solutes tend to dissolve best in non-polar solvents. In the case of a polar solute and non-polar solvent (or vice versa), it tends to be insoluble or only soluble to a minuscule degree. A general rule to remember is, "Like dissolves like."