phase diagram, graph that shows the relation between the solid, liquid, and gaseous states of a substance (see states of matter) as a function of the temperature and pressure. The graph is divided into three regions, one for each of the physical states, and it specifies the range of temperatures at which the substance exists in each state for any value of the pressure. For example, a phase diagram for water shows that at a pressure of 1 atmosphere water is a solid up to a temperature of 0°C, a liquid from 0°C to 100°C, and a gas above 100°C. At a pressure of 0.5 atmospheres, the graph shows that although the melting point of ice remains 0°C, the boiling point of water is lowered to 82°C. Each substance has its own phase diagram, which must be determined experimentally. The border between two regions on the graph represents an equilibrium state, such as a melting point or boiling point, at which two states can coexist (see chemical equilibrium). The point at which all three regions meet is called the triple point; at these conditions of temperature and pressure, the solid, liquid, and gaseous states can coexist in equilibrium. The triple-point conditions for water are a temperature of .01°C and a pressure of .006 atmospheres.

phase diagram A graphical method for examining stability in biological systems (e.g. host-parasite relationships). The system variables (the numbers of host and parasite organisms) at different times are plotted against one another, and the time taken to change is shown by a line connecting the coordinate points for successive time values. The form of the resulting curve indicates the stability of the system. A line spiralling inward indicates damped oscillations favouring ultimate stability; an outward spiral suggests ultimate instability. By testing different values for the number of organisms it is possible to establish limits for stability. The technique has considerable practical relevance in agriculture.

phase diagram Graphical method for examining stability in biological systems, e.g. host-parasite relationships. The system variables (the numbers of host and parasite organisms) at different times are plotted against one another, and the time taken to change is shown by a line connecting the co-ordinate points for successive time values. The form of the resulting curve indicates the stability of the system. A line spiralling inward indicates damped oscillations favouring ultimate stability; an outward spiral suggests ultimate instability. By testing different values for the number of organisms it is possible to establish limits for stability. The technique has considerable practical relevance in agriculture.

phase diagram A graphical method for examining stability in biological systems (e.g. host-parasite relationships). The system variables (the numbers of host and parasite organisms) at different times are plotted against one another, and change over time is shown by a line connecting the coordinate points for successive time values. The form of the resulting curve indicates the stability of the system. A line spiralling inward indicates damped oscillations favouring ultimate stability; an outward spiral suggests ultimate instability. By testing different values for the number of organisms, it is possible to establish limits for stability. The technique has considerable practical relevance in agriculture.

phase diagram Graph to show fields of stability for phases in a heterogeneous system; variables such as temperature, pressure, and concentration are plotted against each other.