Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

Monthly Update sign up
Mailing List signup

Unit 2: The Behavior of Atoms—Phases of Matter and the Properties of Gases

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."

A Phase Change: Condensation

Figure 2-1. A Phase Change: Condensation

© Wikimedia Commons. Creative Commons License 3.0. Author: Danielclauzier, 2008.

View More

A Phase Change: Condensation

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.

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.)



A low-density phase of matter that has no shape or volume, and will fill the entire volume of a container.


A phase of matter that has a distinct volume but no shape.

Phase change

The conversion of a substance from one state to another. Phase changes include melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas), condensation (gas to liquid), sublimation (solid to gas), and deposition (gas to solid).


A phase of matter that has a distinct volume and shape.


© Annenberg Foundation 2017. All rights reserved. Legal Policy