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Section 1: Introduction
As chemistry evolved into a scientific discipline in the 17th and 18th centuries, scientists became increasingly skilled at analyzing and theorizing about the properties of matter in its various forms—solid, liquid, and gas—and understanding how a substance such as water could exist in multiple states. These states of matter are called "phases", and the transition from one form to another is called a "phase change."
Figure 2-1. A Phase Change: Condensation
On a cold day, water vapor in warm indoor air condenses back to liquid when it touches a cold windowpane.
© Wikimedia Commons. Creative Commons License 3.0. Author: Danielclauzier, 2008.
Phase changes are physical, not chemical—the substance's form is altered, but its chemical composition remains constant. Water is always H2O, whether it is liquid, frozen, or vapor. In contrast, reactions are chemical changes: Bonds are formed or broken, and the final products are different substances from the inputs used to produce the reaction. With an understanding of phase changes we can answer many questions about everyday phenomena. For example, why do cooks need to alter recipes for baking at high altitudes? Why do some liquids give off fumes when they are left in open containers at room temperature? And why does water vapor condense on windowpanes on cold days? (Figure 2-1)
Chemical phases are characterized by two features: the distance between individual particles of the material and the level of energy (temperature) in the system. This means that to understand phases and phase changes, chemists had to develop accurate tools and scales for measuring temperature. They also had to understand and quantify the role of pressure, which is especially important in gas chemistry. (See Section 5 of this unit.)
Modern study of gases did not begin until the 1600s, after chemists had been working with solids and liquids for centuries. This sequence is not surprising. Many gases were invisible, which made them difficult to measure. Moreover, according to the widely supported idea proposed centuries earlier by Aristotle that all matter was made up of four elements (fire, earth, air, and water), air was a unique and indivisible element. Scientists who viewed the world this way thought that the various gases present in air were different kinds of air, not unique substances with their own properties. Once this idea was disproved, however, pneumatic (gas) chemistry progressed rapidly.
By the mid-19th century, scientists had formulated a series of laws that explained how temperature and pressure combined to affect the behavior of gases under different conditions. With this knowledge, they were ready to quantify and analyze phase changes in various substances. The first version of a phase diagram, showing the full range of states for a chemical, was developed in 1897. Today, phase diagrams are standard tools in fields such as physical chemistry, geology, and materials science. (Phase diagrams will be discussed again in Section 7.)