Water as the Solvent of Life

Water's oxygen hogs shared electrons more than its two hydrogens do, so oxygen ends up slightly negative and the hydrogens slightly positive. That uneven charge, plus water's bent shape, makes it a polar molecule. When salt, NaCl, hits water, the positive hydrogen ends of water molecules swarm the chloride ion and the negative oxygen ends swarm the sodium ion. Enough water molecules pulling from enough directions overpowers the ionic bond holding Na+ and Cl- together, and the ions separate, each wrapped in its own shell of water molecules called a hydration shell, oxygen-in for cations, hydrogen-in for anions.

Oil does not get this treatment, and it is not because oil pushes water away. Oil is nonpolar, so it has no charges for water to grab onto. Surrounding an oil molecule would force water molecules to break their own hydrogen bonds with neighboring water and get nothing back in return. Water molecules would rather stay hydrogen-bonded to each other, so they squeeze oil out and cluster tightly around it, which looks like repulsion but is really water protecting its own bonds. This is the hydrophobic effect: not a force pushing oil away, but water pulling itself together.

This same logic builds the cell membrane. Phospholipids have a polar, hydrophilic head and two nonpolar, hydrophobic tails, making them amphipathic. Dropped into water, their heads face outward to enjoy hydrogen bonding with water while their tails bury themselves away from water, together with other tails. The result is a bilayer that forms on its own, no enzymes or energy required, purely because water's polarity makes hiding the tails the only arrangement water will allow.