Why is it so hard to shake the last drops of water out of a glass? asks a reader.Pour a glass of water into the sink, and the water streams effortlessly out, emptying the glass. Oops, wait: Some of the water isn't cooperating. Tiny drops cling to the sides and bottom, stubbornly refusing to accompany their water molecule friends down the drain.
Have some water droplets figured out how to outsmart gravity? Actually, it's a kind of electrical attraction.
Water molecules in liquid form slide past each other, rather like people moving in a crowd. That's what makes liquids shape-shifters, able to flow into nooks and crannies of a container. By contrast, in solid ice, water molecules sit frozen in a crystal lattice, vibrating in place.
But while molecules move more freely in liquid form, they are still electrically attracted to each other. Why? Each H2O molecule is shaped rather like a triangle, with one oxygen atom at the top and two hydrogen atoms at the base. Since the oxygen atom is bigger, it has more (positively charged) protons in its nucleus. And since opposites attract, it draws (negative) electrons from the hydrogen atoms toward itself. Result: The oxygen end of a water molecule has a slight negative charge; the hydrogen end is weakly positive.
So even in a flowy liquid, water molecules form bonds: The hydrogen end of each molecule is attracted to the oxygen end of other H2O molecules. This "hydrogen bonding" holds water molecules together in a loose mass.
But at the liquid's surface, water molecules aren't all that attracted to gas molecules in the air above. Their attraction is "downward," to other water molecules filling the glass, and to fellow H2O molecules on the surface. This makes the surface, a patchwork of highly bonded molecules, resemble the thin skin of a balloon. (Such "surface tension" is why some lightweight insects can actually walk on water.)
Surface tension is also what pulls small parcels of water into rounded drops. Turn a glass of water upside down, and most of it pours right out. But smaller groupings of water molecules form drops on the glass' surface. What holds them in place? Water molecules, it turns out, are very attracted to molecules in glass.
Bigger drops fall out with a shake, gravity tugging them earthward. The smallest, lightest drops cling on, their own surface molecules and the glass' locked in an electrical embrace. In fact, water is so fond of glass that rather than converging into drops, it can spread out, wetting the glass. Try to shake it out, and it may race around the rim of the glass, holding on tight.