Basic Properties of Water

Water is the most abundant and ubiquitous liquid on earth and has played a central role in scientific thought for millennia. Nearly every aspect of life is influenced or controlled by water. No wonder scientists have huge interest in properties of water, among which the hydrophobicity is the one we shall delve into.

We begin by reviewing a few basic facts about water. Water is a small and compact molecule. As the first approximation it can be regarded as a sphere with a radius of 1.4 Å. The H-O-H angle is $104^{\circ}27'$, and the OH bond length is $0.9584$ Å. Water has a permanent dipole moment that comes from partial charge transfer from hydrogen to oxygen. Two hydrogen atoms and two lone pairs of electrons are arranged in nearly tetrahedral symmetry around oxygen. It is able to form up to four hydrogen bonds with other water molecules. The formation of hydrogen bonds results in tetrahedral structure in liquid water and ice, which is responsible for its many peculiar properties such as relatively high boiling and melting temperatures.

Unusually high dielectric constant (81.7) in water makes it an excellent solvent for ions comparing with other liquids. Water also has high surface tension (about $70$ dyn/cm) compared with the alkanes (about $30$ dyn/cm). The high interfacial tension of the water/oil system leads to low solubility for apolar solutes in aqueous solution.

For most materials, the density of the solid phase is higher than that of its liquid form. This is due to the closer congeries in solids. In contrast, liquid water has higher density than ice. Water molecules in ice form regular tetrahedral structures and are well-ordered by hydrogen bonds with each other. As ice melts, the regular structure is broken, creating larger space in its liquid phase. Whence liquid water is denser.