Cohesion, Adhesion, and Surface Tension

Water is bent, with oxygen hogging electrons and leaving two slightly negative lone pairs near the oxygen and two slightly positive spots near the hydrogens. That lopsided charge lets each water molecule hydrogen bond to several neighbors at once. Cohesion is this attraction between water molecules themselves. Adhesion is the same kind of attraction, but aimed at a different surface, like glass or the inside wall of a plant's plumbing.

Surface tension shows up because molecules at the very top of a puddle have neighbors beside them and below them, but nothing pulling from above. Hydrogen bonds yank those surface molecules inward and sideways, pulling the surface into the tightest shape it can hold, like a crowd packed shoulder to shoulder. There is no film, no membrane, nothing physically separate sitting on top. It is just a layer of molecules under more inward pull than the molecules beneath them. That tension is strong enough to support light, spread-out weight, which is how a water strider's legs dent the surface without breaking through.

Trees exploit the exact same bonds to move water without any pump. Inside narrow xylem tubes, adhesion sticks water to the tube walls while cohesion keeps each water molecule hydrogen bonded to the next, forming one unbroken thread from root to leaf. At the top, water evaporates out of leaf pores, and as each molecule leaves, it tugs the molecule behind it upward through cohesion, and that pull transmits all the way down the column to the roots. This is why unit one keeps returning to hydrogen bonds: cohesion, adhesion, surface tension, and transpiration are really one property of water showing up in four different situations.