Water and Its Properties

Objective: Explain unequal charge distribution in water.

Water has polar covalent O–H bonds because oxygen attracts shared electrons more strongly than hydrogen. Its bent geometry gives the molecule a partially negative oxygen side and partially positive hydrogen sides. Nearby water molecules can form hydrogen bonds between these partial charges. Water is overall electrically neutral; partial charges do not make one water molecule an ion. The collective effects of many hydrogen bonds contribute to several biologically important properties.

Objective: Relate hydrogen bonding to water movement and temperature stability.

Cohesion is attraction among water molecules; adhesion is attraction between water and another material. Together with narrow spaces, these effects contribute to capillary action, although water transport through plants also depends on evaporation and other processes. Water has a high specific heat relative to many common substances: considerable energy is needed to change its temperature. Evaporation removes energetic molecules and can cool a surface. None of these properties means water never changes temperature or always rises by itself.

Objective: Distinguish solvent, solute, and acidity without overgeneralizing.

A solution is a uniform mixture at the scale considered; a solute is dispersed in a solvent. Water dissolves many ionic and polar substances because its partial charges interact with particles, but it does not dissolve every material. pH is a logarithmic measure related to hydrogen-ion activity in an aqueous solution. Lower pH is more acidic and higher pH more basic; a one-unit change is a tenfold change in hydrogen-ion activity under the usual approximation. Biological systems often regulate pH because molecular structure and reaction rates can depend on it.