Polar molecules dissolve in water because water is a polar molecule itself, meaning it has a slight positive charge on one end and a slight negative charge on the other. This property is due to the bent shape of the water molecule and the unequal sharing of electrons between the oxygen and hydrogen atoms.
When a polar molecule is introduced to water, the positive end of the water molecule (the hydrogen side) is attracted to the negative region of the polar molecule, and the negative end of the water molecule (the oxygen side) is attracted to the positive region of the polar molecule. This attraction, known as dipole-dipole interaction, allows the polar molecule to become surrounded or solvated by water molecules.
The solvation process occurs because water molecules can effectively "pull apart" the individual molecules of the polar substance, dispersing them throughout the water. The polar water molecules surround the solute molecules, with their positive ends orienting toward the negatively charged portions of the solute and their negative ends toward the positively charged regions.
Non-polar molecules, on the other hand, do not readily dissolve in water. Non-polar molecules lack a significant separation of charge within their structure, so they do not form strong attractions with the polar water molecules through dipole-dipole interactions. Instead, the water molecules tend to interact more strongly with each other than with the non-polar molecules.
Non-polar substances are typically composed of molecules that have similar electronegativities and symmetrical structures, resulting in an equal distribution of electron density. As a result, water molecules do not surround or solvate non-polar molecules as effectively as they do with polar molecules. Instead, non-polar molecules tend to clump together, minimizing their contact with water.
The phrase "like dissolves like" summarizes the principle that polar substances tend to dissolve in polar solvents like water, while non-polar substances tend to dissolve in non-polar solvents. This concept is important in understanding the solubility of different substances in various solvents.