Water can dissolve charged or polar substances because of its polarity (oppositely charged ends). Molecules such as sugars, salts, and gases are relatively soluble in water and become available for living things. For example, fish take in oxygen from the water across their gills through diffusion, the movement of molecules from an area of higher concentration to an area of lower concentration. There is more oxygen in the water than inside the fish's blood, so oxygen diffuses into the blood. A solute is a substance that is dissolved into a solution. The solvent is a substance in which another substance dissolves. In the case of the fish, the oxygen is the solute and the water is the solvent.
Because so many things dissolve in water, it is referred to as the universal solvent. However, some substances do not dissolve well in water. A hydrophobic substance is one that has a weak or no affinity to water. Conversely, a hydrophilic substance is one that has a strong affinity toward water. Examples of hydrophobic substances include oils, fats, and other greasy substances. Hydrophilic substances include sugars, salts, vinegar, and many amino acids.When water molecules bond together, they form a barrier against hydrophobic molecules. There is an expression that says, "Oil and water do not mix." Oil is an example of a lipid, which is a nonpolar molecule, whose sides do not carry a charge. Lipid molecules are not attracted to the charged polar ends of water molecules. Even if the solution is mixed, the molecules of the lipids will likely never dissolve in the water.