Alexander Blakie, of Aparima College, asks :-

Why is water wet?

Keith Dawber, a physicist at Otago University, responded.

Because of the chemical structure and shape of water molecules.

Our common observation is to do with the so-called "angle of contact" between liquids and solids. Clean water makes very close to zero angle of contact with some solids such as glass. A very small droplet of pure water placed on a horizontal plate of very clean glass will be pulled downward by gravity and will be spread out over the glass by the surface tension (the relative attraction between water molecules and atoms in the glass) until it is a monolayer, a layer only one water molecule thick. A thicker film of water on the glass plate breaks up into drops separated by the monolayer. In both cases the surface is wetted.

The presence of any grease, oil or other dirt makes a very considerable difference. The water molecules are attracted more to each other than to the grease molecules. Hence the angle of contact between water and grease is much greater than 90 degrees so that a droplet's surface tucks under itself, rather than spreading out when placed on a greasy horizontal glass plate. No monolayer forms and the plate effectively remains "dry." Similarly, when a drop of water falls onto the back of a duck, the wax in the duck's feathers causes the water to remain as a drop which rolls off the duck's back.

By adding soap or detergent to water, the angle of contact is altered and becomes close to zero for nearly all solids. Hence these products are used in order to wet, and thereby clean, dirty surfaces.