Physics · Ch 7 — Thermal Properties of Matter
Phase Diagram
Phase Diagram
A pressure-temperature (P-T) diagram, called a phase diagram, is a convenient way to see at a glance which phase (solid, liquid or vapour) a substance is in for any combination of pressure and temperature (Fig. 7.10). A "phase" here means a homogeneous form of the material -- note that even within the solid state a substance can have more than one phase, e.g. graphite and diamond are both solid carbon, but with different atomic arrangements, hence different phases.
Three curves partition a phase diagram into three regions. The VAPOURISATION curve (l-v) marks where liquid and vapour coexist in equilibrium -- essentially a graph of boiling point against pressure; for water it correctly passes through (100 °C, 1 atm). The FUSION curve (l-s) marks where solid and liquid coexist -- a graph of freezing point against pressure; for water (Fig. 7.10(a)) this curve slopes very slightly UPWARD TO THE LEFT (a small NEGATIVE slope), a peculiarity limited to substances, like water, that EXPAND on freezing. Most substances behave the opposite way: for carbon dioxide (Fig. 7.10(b)), which contracts on freezing like most materials, the fusion curve instead slopes upward to the RIGHT (a normal, positive slope); CO2's melting point is about -56 °C, but only at the comparatively high pressure of roughly 5.1 atmospheres. The SUBLIMATION curve (s-v) marks where solid and vapour coexist; water sublimates below about atmosphere, whereas solid CO2 ("dry ice") sublimates even at ordinary atmospheric pressure, at temperatures as low as -78 °C -- which is exactly why dry ice is never seen as a liquid at everyday pressures. …
What this figure shows. A pressure (vertical axis) versus temperature (horizontal axis) diagram for water, divided by three curves into three labelled regions -- SOLID (upper-left), LIQUID (upper-middle/right) and VAPOUR (lower-right). The fusion curve (labelled l-s), separating solid from liquid, is drawn sloping slightly UPWARD TO THE LEFT (a small negative slope) -- unusual, and specific to substances like water that expand on freezing. The vaporisation curve (labelled l-v), separating liquid from vapour, slopes upward to the right and passes through the point (100 °C, 1 atm), matching water's normal boiling point. The sublimation curve (labelled s-v), separating solid from vapour, lies below and to the left of the other two curves, at low pressures. All three curves meet at a single point, the TR …
What this figure shows. A pressure (vertical axis) versus temperature (horizontal axis) diagram for carbon dioxide, with the same three regions (SOLID, LIQUID, VAPOUR) and three boundary curves as Fig 7.10(a), but with the fusion curve (l-s) sloping upward to the RIGHT (a normal, positive slope, unlike water) since CO2 contracts, not expands, on freezing. The triple point of CO2 (about -56.6 °C at roughly 5.1x10^5 Pa, i.e. about 5.1 atmospheres) is marked where all three curves meet; the diagram also indicates that at ordinary atmospheric pressure (1 atm), which lies BELOW the triple-point pressure, solid CO2 (dry ice) sublimes directly to vapour without ever passing through a liquid region, consistent …