Triple point pressure

If the triple point pressure of a solid is below one atmosphere, it will melt if the heating is conducted rapidly so that the vajiour pressure can exceed that at the triple point. If camphor is heated in a closed space, the vapour pressure increases and when the value of 360 mm. is reached, the solid will melt further heating results in an increase in the vapour pressure and the camphor will boil when the vapour pressure is 760 mm. 

For substances with a moderate triple point pressure e.g., benzoic acid, 6 mm., m.p. 122° naphthalene, 7 mm., m.p. 80° ), the simple process described above for camphor will not give a satisfactory yield of a sublimed product. Thus, for example, if naphthalene is heated it will melt at T (80°), and will boil when the vapour pressure is 760 mm. (218°) ... 

At a pressure below the triple point pressure, the solid can change directly to a gas (sublimation) and the gas can change directly to a solid, as in the formation of carbon dioxide snow from the released gas. 

Iodine sublimes more readily than ice because its triple-point pressure, 90 mm Hg, is much higher. Sublimation occurs on heating (Figure 9.6) below the triple-point temperature, 114°C. If the triple point is exceeded, the solid melts. Solid carbon dioxide (dry ice) has a triple-point pressure above 1 atm (5.2 atm at — 57°C). Liquid carbon dioxide cannot exist at 1 atm pressure regardless of temperature. Solid C02 always passes directly to vapor if allowed to warm up in an open cantainer. 

The critical point is unique for (vapor + liquid) equilibrium. That is, no equivalent point has been found for (vapor + solid) or (liquid + solid) equilibria. There is no reason to suspect that any amount of pressure would eventually cause a solid and liquid (or a solid and gas) to have the same //m, Sm, and t/m. with an infinite o and at that point. mC02 was chosen for Figure 8.1 because of the very high vapor pressure at the (vapor + liquid + solid) triple point. In fact, it probably has the highest triple point pressure of any known substance. As a result, one can show on an undistorted graph both the triple point and the critical point. For most substances, the triple point is at so low a pressure that it becomes buried in the temperature axis on a graph with a pressure axis scaled to include the critical point. 

The liquid state exists only below the critical point pressure and above the triple point pressure. When a vapor below the triple point pressure is cooled down, we encounter a discontinuous and abrupt phase change to solid but, above the critical point pressure, a cooled vapor turns into the supercritical state where the properties of the fluid... 

When we look at the critical states and triple points of other gases, we find the situation shown in table 4.34. The liquid phase exists only when the pressure is between the critical and the triple-point pressures. If we cool down hydrogen, helium or water at room temperature and pressure, we will get liquids before we get solids. But if we cool down CO2 from room temperature and pressure, we get dry ice rather than liquid carbonic to obtain liquid carbon dioxide we have to raise the pressure to at least 5.1 atm to exceed the triple-point pressure. The melting point is not as sensitive to the pressure as the boiling point, which is stated usually for a room pressure of 1 atm, which prevails at sea level on Earth and not in Colorado or the Himalayas. 

Stability Limit 1, With the exception of helium and certain apparent exceptions discussed below. Fig. I gives a universal phase diagram liir all pure compounds The triple point of one P and one T is the single point at which all three phases, crystal, liquid, and gas. are in equilibrium. The triple point pressure is normally below atmospheric. Those substances, c.g.. CO . / - 3H85 mm. 7, = -5ft.fi C. for which it lies above, sublime without melting ai atmospheric pressure. 

Ice and water vapor are in equilibrium only below the triple-point pressure, Pt = 6.0 X 10-3 atm, which needs to be converted to millimeters of mercury. 

The adiabatic flash pressure Pf, maintained slightly below the triple-point pressure, causes liquid to spontaneously vaporize and solidify. The ratio of solid to vapor is determined by the heats of fusion and vaporization for carbon dioxide about 1.7 moles of solid are formed for each mole vaporized. The solid, more dense than the liquid, falls through a liquid head and forms a loosely packed crystal bed at the bottom. The liquid head is about 10-12 feet, and increases the hydrostatic pressure on the solid to melter pressure Pm. The crystal bed depth is about two... 

At the triple point pressure, liquid water cannot exist without a change in temperature. Instead, heated ice skips the liquid stage and changes directly from a solid to a gas in the sublimation phase... 

Water, of course, makes transitions from solid ice to liquid water (at 0°C) to water vapor (at 100°C) as it is heated at 1.0 atm pressure. Because at the pressure of the triple-point, the transformation is directly from solid to gas, the triple point pressure must be below 1.0 atm. Because Amelt V for water is negative, by Eq. (38), the solid-liquid equilibrium line has negative slope, and the triple point must be above 0°C. Considering the steepness of the line, the difference is very small and the measured triple point of water is 0.0098°C and 611 Pa ( 4.6 torr). 

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