Water on the Hood

November 27, 2005

Dan_waxed_hood copy

Provided and copyright by: Rob Sheridan
Summary authors & editors: Rob Sheridan

This photograph of water beading on a waxed automobile hood, illustrates one of water's more important and unique physical properties, cohesion. The H2O molecule forms a 105 degree angle, with oxygen at the corner, resulting in the formation of a polar molecule and "hydrogen bonding" between positive and negative poles of adjacent water molecules. Hydrogen bonds between water molecules are about 10% as strong as the covalent bonds linking hydrogen and oxygen, but they account for many of water's important physical properties.

Cohesion is the tendency for water molecules to attract each other. This results in an unusually high heat of vaporization (the energy required to cause liquid water to become a gas). Cohesion is the explanation for why water remains a liquid over such a broad range of temperature, and it explains why water vapor carries so much heat. Of the solar energy absorbed at the Equator and transported to the northern and southern hemispheres (equalizing heat across the planet), approximately 70% is moved by atmospheric water vapor and 30% by ocean currents.

Adhesion is the tendency for liquid water to cling to most surfaces, making them "wet." It's also a result of the polar nature of the water molecule. However, if the surface is "hydrophobic", or coated with an uncharged petroleum product such as automobile wax (shown above), cohesion is stronger than adhesion, and beads of water result.

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