What primarily determines a molecule's solubility in water?

The primary factor that determines a molecule's solubility in water is the polarity of the molecule. Water is a polar solvent, meaning it has a partial positive charge on one side (the hydrogen atoms) and a partial negative charge on the other side (the oxygen atom). Molecules that are also polar can interact favorably with water through dipole-dipole interactions and hydrogen bonding, which allows them to dissolve.

For instance, polar molecules like alcohol (which contain hydroxyl groups) and salts easily dissolve in water because of their ability to form strong attractions with the water molecules.

While the presence of hydroxyl groups can enhance solubility by introducing polarity, it is the overall polarity of the entire molecule that is the critical factor in determining how well a substance will dissolve in water. Molecular weight and size do play a role in solubility, but they are less significant compared to polarity. Larger nonpolar molecules, regardless of their molecular weight, typically do not dissolve well in water.

Thus, while other factors are important, it is mainly the polarity of the molecule that dictates how well a substance can be solubilized in a polar solvent like water.