Hoar-frost point

Nitrogen is used to blanket the reaction and the raw materials to stop the absorption of moisture and to prevent oxidation of the product. The dew point of a gas is the temperature at which the water vapor present in the gas saturates the gas and begins to condense. Dew points of below 0°C are sometimes called the hoar-frost point. Industrial-grade nitrogen gas normally is dry enough for use. 

The vap. press, curve of solid iodine is indicated by PO, Fig. 16 that of liquid iodine by 00 and the effect of press, on the m.p. of iodine by ON. At the triple point 0 these curves meet. Fig. 18 shows a similar curve for water. The curve PO thus represents the sublimation curve or hoar-frost line OC. the boiling or vaporization curve, i.e. the effect of press, on the b.p. of the liquid. The same phenomenon occurs with water, iodine, etc., and the principle involved is the same as indicated in the law represented by Clapeyron-Clausius equations with respect to the lowering of the m.p. by an increase of press. Consequently, if the vap. press, of iodine be less than that of th,e triple point, the solid does not melt, but rather sublimes directly without melting at the triple point at 114-15° (89 8 mm.) and A. von Richter at 116 1° (90 mm.). According to R. W. Wood, if the condensation of iodine vapour occurs above —60°, a black granular deposit is formed, but below that temp, a deep red film is produced. 

The specific volumes and the latent heats are generally quite different from the three changes of state, and therefore the slopes of the three curves at the triple point are also different. The difference in the slopes of the tangents of the solid-vapour (hoar frost line), and the liquid-vapour (steam line) curves of water (Fig. 39) is... 

Hence calculate the difference in the slope of the hoar frost and steam lines for water at the triple point. Latent heat of water = 80 L12 = 80 x 42,700 T = 273, v3 - v2 = 209,400 c.c. Substitute these values on the right-hand side of the last equation. Ansr. 0-059. 

Condensation of water vapor occurs when the dew point is undershot onto cooler surfaces resulting in a subsequent diffusion process in the direction of the surface (that is dry deposition, similar to surface removal of other molecules than H2O, but these are ad- or absorbed onto the surface - only water can condense into liquid in the earth s climate system). When the temperature is above 0°C, liquid drops (not film) are formed on the surface - most likely on condensation or crystallization centers - and we call it dew. If r < 0 °C, water vapor is transferred into ice on the surface frost) in different sizes and forms, mostly depending on temperature and wind on trees, filigreed ice needles and structures are formed hoar frost). 

---