Which statement best defines a polar molecule?

A polar molecule is best defined as one that exhibits a separation of electric charge, leading to the presence of distinct positive and negative poles. This occurs because of an uneven distribution of electron density within the molecule, often due to differences in electronegativity between the atoms involved. In polar molecules, one atom attracts electrons more strongly than another, creating regions with partial positive and partial negative charges.

This characteristic of polarity is fundamental because it influences the molecule's interaction with other substances, particularly its solubility in polar solvents like water. For example, the polar nature of water allows it to dissolve many ionic and polar compounds effectively. Thus, the correct definition emphasizes the key feature of polar molecules—their positive and negative charge separation due to varying electron density, thereby explaining their behavior in various chemical contexts.

The other statements do not accurately represent the concept of polarity; for instance, a molecule with no electric charge or an even distribution of electron density does not possess polarity, while the inability to dissolve in water does not inherently define a polar molecule.