Organic Chemistry MCAT Practice Exam 2025 - Free MCAT Practice Questions and Study Guide

The solubility of a compound in water is largely determined by its ability to interact with water molecules, and this interaction is most significantly influenced by the presence of polar groups in the compound. Polar groups, such as hydroxyl (-OH), carboxyl (-COOH), and amino (-NH2) groups, can form hydrogen bonds with water, allowing for better solvation of the molecule. When a compound contains polar functional groups, it is more likely to be soluble in water because these groups facilitate interactions with the polar water molecules.

In contrast, while the size of the molecule can play a role, larger molecules tend to be less soluble if they do not possess adequate polar groups. The melting point can indicate some characteristics of a substance but does not directly relate to its solubility in water. Similarly, the molecular weight compared to water may provide context on how heavier molecules might interact with water, but it is the presence of polar groups that fundamentally drives the solubility behavior in aqueous environments. Therefore, the importance of polar groups in determining solubility stands out as the primary factor.