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.
Abundance of Water
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.