Sunday, February 6, 2011

What makes water wet?

Water is pretty fabulous stuff. It makes life possible and over enough time acts as a universal solvent. On earth, water is everywhere from the glass on my desk to covering over 70% of the surface. Oceans contain approximately 1,360,000 cubic kilometers worth of water – a lot of water even before land based, sub-surface, frozen or any other place water hides is considered.

Our current understanding of water came relatively recently. We held onto the ancient Greek concept that water was one of the four basic elements (along with fire, earth and air) up until the Renaissance. Water is very stable, making it difficult to break into its parts, but it was done at a time when most people considered it a fundamental element. The French chemist Lavoisier managed to break water apart into its components (two hydrogen molecules and one oxygen on) on 28 February 1785. In time it was determined that covalent bonds hold water together, that is, by sharing electrons between atoms.

A water molecule resembles Mickey Mouse's head. Two ears on top, which are hydrogen atoms, and the head would be the oxygen atom. The top, where the hydrogen is, is somewhat positive and the bottom, where the oxygen is, is somewhat negative resulting in a polar molecule. Since positive and negative are attracted to each other a hydrogen atom from one water molecule is attracted to the oxygen atom of a neighboring water molecule.

How many water molecules must we have before it can be called a liquid? In the field of fluid mechanics, they have what is called a 'continuum hypothesis'. This hypothesis assumes that to be a fluid there must be over a million water molecules present within a reasonable volume (by reasonable, I mean some where between packing them in so tightly they become like a black hole or spreading them out so much it looks like a vacuum).

So, assuming we have enough water molecules to make a liquid, let's think about putting them all into a glass of water. In the middle of the glass, each water molecule attracts its neighboring molecules (remember the polar trait from above). This attraction occurs from all direction at once, resulting in a balance and no net force. Things don't work out so nicely at the surface. Here, the attraction doesn't balance because there is only attraction from the molecules to the sides and below as there are no water molecules above the surface. This causes surface tension. Molecules at the surface are pulled towards the center of the liquid, minimizing the surface area. A very small drop of water pulls itself into a sphere because a sphere has the smallest ratio of surface area to volume. Water in a glass will form a flat surface (ignoring the meniscus) as it is the minimum space it can take up.

Since wet is defined as 'consisting of, containing, covered with, or soaked with liquid (as water)'. Water acts to makes something else wet (as opposed to being wet itself) – so if I go walking outside on a rainy day, I end up wet from the rain.

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