Sublimation without fusion

The phenomenon that FeFg(cr) sublimes without fusion at temperatures near 1000 C has been observed and reported by Poulenc (5). Based on this data, the value of T was estimated. The value of heat of sublimation, (298.15 K), is calculated using an... 

According to Lacombe it insoluble in all solvents and as it sublimes without fusion, he did not determine its melting or boiling point. Tanatar (1907 12) claimed to make it by mixing the calculated amounts of carbonate and acid and boiling in water, which is impossible. 

Arsenic III oxide (arsenic trioxide, arsenious oxide) [1327-53-3] M 197.8, three forms m 200°(amorphous glass), m 275°(sealed tube, octahedral, common form, sublimes > 125° without fusion but melts under pressure), m 312°, pKj 9.27, pK 13.54, pK 13.99 (for H3ASO3). Crystd in octahedral form from H2O or from dil HCl (1 2), washed, dried and sublimed (193°/760mm). Analytical reagent grade material is suitable for use as an analytical standard after it has been dried by heating at 105° for l-2h or has been left in a desiccator for several hours over cone H2SO4. POISONOUS (particulary the vapour, handle in a ventilated fume cupboard). 

Ammonium bromide volatilizes without fusion or decomposition. Ammonium bromide sublimes without giving any bromine but the vapour given off at the beginning of the sublimation reacts alkaline, and at the end of the operation, acid. The vapour density obtained by H. St. C. Deville and L. Troost is 1 67 at 440°, and 1 71 at 860°. The theoretical value for complete dissociation is 1 69. As in the case of ammonium chloride, the vapour in H. St. C. Deville and L. Troost s experiment was not completely sat. with ammonium bromide, and the system was not in equilibrium. A. Smith and R. P. Calvert find the dissociation pressure of ammonium bromide to be 760 0 mni. at 294 6°, and that the observed results can be represented by log 2056 541/T+9 54014 log T—20 98468. A. Smith and R. H. Lombard find the sat. vapour densities in grms. per c.c. the percentage degrees of dissociation the averaged dissociation constants, X and the latent heat of vaporization of ammonium bromide from... 

Sublimate, a solid material obtained by the condensation upon a cold surface of a vapour arising directly (without fusion) from a heated solid. 

Ckemicdl.—When heated, it is decomposed without fusion or sublimation. Its constitution is unknown. Heated in Cl it yields cyamiric acid and HCl. When Cl is passed for some time through H,0 holding uric acid in suspension, alloxan, parabanic and oxalic acids, and ammonium cyanate are formed. Similar decomposiUon is oduced by Br and L It is simply dissolved by HCl. It is dissolved by H,SO, from a hot solution ill which a deliquescent, crystalline compound, 4H SO is... 

Needles. M.p. 237-8° decomp. Sublimes without decomp. FeClj —> cherry-red col. Sols,. shew blue fiuor. (alk. sol. colourless). KOH fusion —>. 2 and 4-hydrox.yisophthalic acids. Ox. —> hydroxytrimesic acid. 

Many solid substances (camphor, iodine, naphthalene, etc.), are known which are appreciably volatile at ordinary temperatures. Others, such as the metals, are apparently quite fixed, but they probably possess a definite, although very small vapour-pressure, even at ordinary temperatures. Thus, if magnesium is heated to 550° for a few hours in a magnesia boat enclosed in a vacuous tube it sublimes in beautiful crystals on the cool part of the tube. The vaporisation of a solid without previous fusion is called sublimation the vapour-pressure (like the vapour-pressure of a liquid), is definite for each temperature, is independent of i the volume of the vapour space, and increases with rise of temperature. 

If a solid is heated under atmospheric pressure, its vapour-pressure increases with rise of temperature, and it may happen that the vapour-pressure of the solid becomes equal to atmospheric pressure before the melting-point is reached. In this case, the substance sublimes away without previous fusion. But if the melting-point is reached before the vapour-pressure reaches atmospheric pressure, the substance will melt before boiling away. 

Fusion and vaporization are the most familiar phase changes, but sublimation is also common. Sublimation is a phase change in which a solid converts directly to a vapor without passing through the liquid phase. Dry ice (solid CO2) sublimes at 195 K with A ii/subl — 25.2 kJ/mol. Mothballs contain naphthalene (Cio Hg,... 

In a 5-I. round-bottom flask (Pyrex) is placed a mixture of 500 g. of phthalic anhydride and 400 g. of 28 per cent ammonium hydroxide. The flask is fitted with an air condenser not less than 10 mm. in diameter and is then slowly heated with a free flame until the mixture is in a state of quiet fusion at a temperature of about 300°. It requires about one hour before all the water has gone and about one and a half to two hours before the temperature of the reaction mixture reaches 300° and the mixture is a homogeneous melt. It is advisable, during the heating, to shake the flask occasionally some material sublimes into the condenser and must be pushed down with a glass rod. The hot reaction mixture is now poured out into a crock, covered with a paper to prevent loss by sublimation, and allowed to cool. The product is practically pure without further treatment, and melts at 23 2-23 5 °. The yield is 470-480 g. (94-95 per cent of the theoretical amount). 

LATENT HEAT. Hem tunned by a substance or system without tin accompanying rise in temperature during a change ol state. As examples, the latent heat of fusion is the amount of heat necessary to concert a unit mass of a substance trnin the solid stale to the liquid stale at the same temperature, the pressure being that to allow coexistence of the two phases. A considerable pari of the latent heat arises from the entropy increase consequent on the greater disorder of the liquid state. The latent heat of sublimation is the amount of heat necessary to convert a unit mass of a substance from the solid state to the gaseous stale at (he same temperature, the pressure being that to allow coexistence of the two phases. 

The three states of water are so common because they all are stable under ordinary conditions. Carbon dioxide, on the other hand, is familiar as a gas and a solid (dry ice), but liquid CO2 occurs only at external pressures greater than 5 atm. At ordinary conditions, solid CO2 becomes a gas without first becoming a liquid. This process is called sublimation. Freeze-dried foods are prepared by sublimation. The opposite process, changing from a gas directly into a solid, is called deposition—you may have seen ice crystals form on a cold window from the deposition of water vapor. The heat of sublimation (Aff°ubi) is the enthalpy change when 1 mol of a substance sublimes. From Hess s law (Section 6.5), it equals the sum of the heats of fusion and vaporization ... 

The absorption or release of heat without any accompanying temperature change is characteristic of a change in the state of aggregation of a substance. The quantity of heat absorbed in the transformation of solid to liquid is the heat of fusion. The quantity of heat absorbed in the transformation of liquid to vapor is the heat of vaporization. The direct transformation of a solid to vapor is called sublimation. The quantity of heat absorbed is the heat of sublimation, which is equal to the sum of the heats of fusion and vaporization. 

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