Almost two thirds of our planet is covered by water, and water is the most abundant substance in our bodies. Because water is so common, we tend to take its unique chemical and physical properties for granted. We will see repeatedly throughout this text, however, that water possesses many unusual properties essential to support life on Earth.
One of the most important properties of water is its ability to dissolve a wide variety of substances. The water in nature, therefore, whether it is the purest drinking water from the tap or water from a clear mountain stream, invariably contains a variety of dissolved substances. Solutions in which water is the dissolving medium are called aqueous solutions.
Many of the chemical reactions that take place within us and around us involve substances dissolved in water. Nutrients dissolved in blood are carried to our cells, where they enter into reactions that help keep us alive. Automobile parts rust when they come into frequent contact with aqueous solutions that contain various dissolved substances. Spectacular limestone caves (Figure 4.1) are formed by the dissolving action of underground water containing carbon dioxide, CO2(aq):
CaCO3(s) + H2O(l) + CO2(aq) → Ca(HCO3)2(aq)
We saw in Chapter 3 a few simple types of chemical reactions and how they are described. In this chapter we continue to examine chemical reactions by focusing on aqueous solutions. A great deal of important chemistry occurs in aqueous solutions, and we need to learn the vocabulary and concepts used to describe and understand this chemistry. In addition, we will extend the concepts of stoichiometry that we learned in Chapter 3 by considering how solution concentrations can be expressed and used.