Triple point converting between

At high pressures, solid II can be converted (slowly) to solid III. Solid III has a body-centered cubic crystal structure. Line bd is the equilibrium line between solid II and solid III, while line be is the melting line for solid III.P A triple point is present between solid II, solid III, and liquid at point b. Two other triple points are present in this system, but they are at too low a pressure to show on the phase diagram. One involves solid II, liquid, and vapor while the other has solid I, solid II, and vapor in equilibrium. 

Other features of interest in the phase diagram of 4He include triple points between various liquid and solid phases of the element. At point c in Figure 13.11, liquid I, liquid II and a body-centered cubic (bcc) solid phase are in equilibrium. The bcc solid exists over a narrow range of pressure and temperature. It converts by way of a first-order transition to a hexagonal close packed (hep) solid, or to liquid I or liquid II. At point d, liquid I and the two solids (bcc and hep) are in equilibrium liquid II and the two solids are in equilibrium at point e. 

The variance is the number of independent variables or degrees of freedom determining the state of the quantity. For a single substance,/< 3. Where there are three phases, the variance is zero and we speak of the substance at its triple point, a specific temperature and pressure at which the substance exhibits three phases. The usual situation is where the phases in question are solid, liquid and gas, which in the case of water occurs at 273.16 K (0.01 °C) and 611.72 Pa pressure (0.0060373 atm) and is used to fix a temperature above 0 K with which to calibrate the absolute (Kelvin) scale of temperature. Water also exhibits a number of distinct solid phases between which other triple points occur. Arbitrary small changes in temperature or pressure will convert the entire quantity of water at a triple point to one of the phases. The pressure of the ordinary triple point for water is the minimum pressure at which water can be liquid, so that in the low pressures of outer space, heating ice converts it directly to vapour. Here we see how the theoretical criterion of being a single substance points to sorts of properties that can serve as characteristic features of the substance. 

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