Water in three states
Water in three states: liquid, solid (ice), and (invisible) water vapor in the air. Clouds are accumulations of water droplets, condensed from vapor–saturated air.
Life on Earth began in water and evolved there for 3 billion years before spreading onto land. Modern life, even terrestrial (land‐dwelling) life, remains tied to water. All living organisms require water more than any other substance. Human beings, for example, can survive for quite a few weeks without food, but only a week or so without water. Molecules of water participate in many chemical reactions necessary to sustain life. Most cells are surrounded by water, and cells themselves are about 70–95% water. Three‐quarters of Earth\'s surface is submerged in water. Although most of this water is in liquid form, water is also present on Earth as ice and vapor. Water is the only common substance to exist in the natural environment in all three physical states of matter: solid, liquid, and gas.
The abundance of water is a major reason Earth is habitable. In this chapter, we will learn how the structure of a water molecule allows it to form weak chemical bonds with other molecules, including other water molecules. This ability leads to unique properties that support and maintain living on our planet.
Our objective in this chapter is to develop a conceptual understanding of how water contributes to the fitness of Earth for life. As mentioned earlier, water is the most abundant substance in living systems, making up 70% or more of the weight of most organisms. The first living organisms doubtless arose in an aqueous environment, and the course of evolution has been shaped by the properties of the aqueous medium in which life began. This chapter begins with descriptions of the physical and chemical properties of water, to which all aspects of cell structure and function are adapted. The attractive forces between water molecules and the slight tendency of water to ionize are of crucial importance to the structure and function of biomolecules.