It affects phase changes by reinforcing the attractive forces that hold particles together. Pressure is a measurement of force pushing on an area. Water is constantly cycling between solid, liquid, and gas phases on Earth and in the atmosphere. The same process that produces clouds in the atmosphere is also what makes water condense on a cold surface, such as a windowpane, the outside of a cold glass, or eyeglass lenses when the wearer moves from outdoors into a warm room.
When water vapor in the air cools, it condenses back into droplets and falls as rain (or freezes and falls as snow). Water constantly cycles between land, oceans, and the atmosphere, changing between solid, liquid, and gas forms (Figure 2-3). To see phase changes happening around us every day, we can simply look outdoors at Earth’s water cycle. Materials also can change from a solid directly into a gas, a process called “sublimation,” or from a gas to a solid, which is called deposition. When materials cool, they lose kinetic energy: Gases condense into liquids, and liquids freeze into solids (Figure 2-3). If enough energy is added to a substance, the particles will overcome their attractive forces and the material will change to a less ordered state: A solid will melt into a liquid, or a liquid will evaporate into a gas. Heating materials will increase their kinetic energy. Particles of gases have much greater kinetic energy (energy of motion) than those of liquids, which in turn have more energy than solids. The particles of any substances are constantly in motion, but their energy levels vary. In a gas, the particles may occupy the entire volume of the container, so that their shape and volume are both defined by the container. In a liquid, the particles are close together but may move with respect to one another, giving the substance a definite volume but a fluid shape. In a solid, the particles are packed in a rigid configuration, giving the substance a definite shape and size. This schematic diagram shows the differences in physical properties and particle arrangement between a substance in the solid, liquid, and gas phases. (Phase diagrams will be discussed again in Section 7.) Today, phase diagrams are standard tools in fields such as physical chemistry, geology, and materials science.
The first version of a phase diagram, showing the full range of states for a chemical, was developed in 1897. With this knowledge, they were ready to quantify and analyze phase changes in various substances.
Once this idea was disproved, however, pneumatic (gas) chemistry progressed rapidly.īy 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. Scientists who viewed the world this way thought that the various gases present in the air were different kinds of air, not unique substances with their own properties. 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), the air was a unique and indivisible element.
Many gases were invisible, which made them difficult to measure. The modern study of gases did not begin until the 1600s after chemists had been working with solids and liquids for centuries. They also had to understand and quantify the role of pressure, which is especially important in gas chemistry. This means that to understand phases and phase changes, chemists had to develop accurate tools and scales for measuring temperature. 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)Ĭhemical phases are characterized by two features: the distance between individual particles of the material and the level of energy (temperature) in the system. With an understanding of phase changes, we can answer many questions about everyday phenomena. 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. Water is always H 2O, whether it is liquid, frozen, or vapor. Phase changes are physical, not chemical-the substance’s form is altered, but its chemical composition remains constant. On a cold day, water vapor in warm indoor air condenses back to liquid when it touches a cold windowpane.