Chapter 12 Phase Diagrams Chemistry 2012/2013

Matter can exist in four phases (or states), solid, liquid, gas, and plasma plus a few other extreme phases, like critical fluids and degenerate gases. Generally, as a solid is heated (or as pressure decreases), it will change to a liquid form, and will eventually become a gas. For example, ice (frozen water) melts into liquid water when it is heated. As the water boils, the water evaporates and becomes water vapor. Sometimes, a solid will go directly from solid to gas - this is call subliming. An example of sublimation is dry ice, the solid (frozen) form of carbon dioxide, CO 2, which turns into gaseous carbon dioxide at standard temperature and pressure - there is no liquid phase of CO 2 at standard temperature and pressure. Phases of Matter

Solid: A solid is matter in which the molecules are very close together and cannot move around. Examples of solids include rocks, wood, and ice (frozen water). Liquid: A liquid is matter in which the molecules are close together and move around slowly. Examples of liquids include drinking water, mercury at room temperature, and lava (molten rock). Gas: A gas is matter in which the molecules are widely separated, move around freely, and move at high speeds. Examples of solids include the gases we breathe (nitrogen, oxygen, and others), the helium in balloons, and steam (water vapor). Phases of Matter

Plasma: A plasma is a gas that is composed of free-floating ions (atoms stripped of some electrons - positively charged) and free electrons (negatively charged). A plasma conducts electrical currents. There are many different types of plasmas. There is plasma in stars (including our Sun), and the solar wind in our Solar System is made of plasma. Supercritical Fluid: A supercritical (or critical) fluid is a liquid/gas under extreme pressure. These supercritical fluids have unique characteristics, the density of a liquid and the mobility of a gas. Supercritical fluids exist deep inside some planets; for example, there is supercritical water deep inside the Earth. Degenerate Gas: A degenerate gas is one that is super-compressed and very dense. The molecules of this degenerate gas are virtually touching one another and the gas acts much like a solid. Unlike gases under normal conditions, the temperature in a degenerate gas does not depend on the pressure. These gases follow quantum mechanical laws. Phases of Matter

Phase Diagrams Temperature vs. Pressure or Temperature vs. Composition Map of stable phases at equilibrium Information on phase transformations From a phase diagram you can identify –Stable phases –Composition of Phases –Amounts of Phases

Pressure-temperature diagrams

Phase Diagram of H 2 O A critical point is where the properties of two phases become indistinguishable from each other

Phase Diagram of H 2 O

Phase Diagrams Lines of equilibrium or phase boundaries refer to the lines that identify where phase transitions occur. A triple point is, in a pressure-temperature phase diagram, the unique intersection of the lines of equilibrium between three states of matter, usually solid, liquid, and gas. For a phase diagram with temperature on the vertical axis, a solidus is a line below which the substance is stable in the solid state. A liquidus is a line above which the substance is stable in a liquid state. There may be a gap between the solidus and liquidus; within the gap, the substance is replaced by a mixture of solid crystals and liquid.