Calcium water vapour

Deliquescence and efflorescence. A substance is said to deliquesce (Latin to become liquid) when it forms a solution or liquid phase upon standing in the air. The essential condition is that the vapour pressure of the saturated solution of the highest hydrate at the ordinary temperature should be less than the partial pressure of the aqueous vapour in the atmosphere. Water will be absorbed by the substance, which gradually liquefies to a saturated solution water vapour will continue to be absorbed by the latter until an unsaturated solution, having the same vapour pressure as the partial pressure of water vapour in the air, is formed. In order that the vapour pressure of the saturated solution may be sufficiently low, the substance must be extremely soluble in water, and it is only such substances (e.g., calcium chloride, zinc chloride and potassium hydroxide) that deliquesce. 

Fig. 4.29 Adsorption isotherms of water vapour on caldte, after being balt-milted for different periods (A, B, C) and on precipitated calcium carbonate (D). Period of milling (A) 1000h (B) ISOh (C) 22h outgassing temperature 2S°C. Isotherms A, B and C (but not D) all showed extensive low-pressure hysteresis, but for clarity the desorption branch is omitted. The amount adsorbed is referred to 1 m of BET-nitrogen area. ...

Two absorbents are required, one for water vapour, the other for carbon dioxide. The absorbents for water vapour which are generally employed are (a) anhydrous calcium chloride (14-20 mesh), (b) anhydrous calcium sulphate ( Drierite or Anhydrocel ), and (c) anhydrous magnesium perchlorate ( Anhydrone ). Both (b) and (c) are preferable to (a) (c) absorbs about 50 per cent of its weight of water, but is expensive. Anhydrous calcium chloride usually contains a little free lime, which will absorb carbon dioxide also it is essential to saturate the U-tube containing calcium chloride with dry carbon dioxide for several hours and then to displace the carbon dioxide by a current of pure dry air before use. 

Fig. 13. Plot of variations of activation energy ( /kJ mole"1) with water vapour pressure (PHjO/Torr) for dehydration of calcium sulphate. Data from Ball et al. [281,590, 591] who discuss the significance of these kinetic parameters. Dehydrations of CaS04 2 H2O, nucleation ( ), boundary (o) and diffusion (e) control Q-CaSC>4 5 H2O, diffusion control, below (X) and above (+) 415 K j3-CaS04 5 H20, diffusion control ( ).

The concentrates were subsequently analysed for arsenic using Varian-Techtron AAS atomic absorption spectrophotometer fitted with a Perkin-Elmer HGA 72 carbon furnace, linked to a zinc reductor column for the generation of arsine (Fig. 5.3). A continuous stream of argon was allowed to flow with the column connected into the inert gas line between the HGA 72 control unit and the inlet to the furnace. Calcium sulfate (10-20 mesh) was used as an adsorbent to prevent water vapour entering the carbon furnace. The carbon tube was of 10 mm id and had a single centrally located inlet hole. 

In order to concentrate a solution the solvent is distilled through a downward condenser . For this purpose various forms of coil condenser are more convenient than the Liebig pattern. For working under reflux such coil condensers are less suitable because of the layers of liquid which form in the coil between the vapour and the external atmosphere. A condenser designed by Dimroth has proved suitable for both types of work. In it the cooling water passes through the coil (Fig. 1). In order to prevent condensation of water vapour on the coil it is advisable to fix a calcium chloride tube into the upper opening of the condenser. 

The apparatus for the determination of active hydrogen is shown in Fig. 45. The Lunge nitrometer a, of which the levelling bulb is not shown in the sketch, is filled with saturated brine. By means of the short calcium chloride tube 6, interposed between the reaction vessel and the nitrometer, the access of water vapour to that vessel is prevented. For the determination weigh accurately, according to... 

Liquid absorbents. If the partial pressure of the water in the gas is greater than the equilibrium partial pressure at the surface of a liquid, condensation will take place as a result of contact between the gas and liquid. Thus, water vapour is frequently removed from a gas by bringing it into contact with concentrated sulphuric acid, phosphoric acid, or glycerol. Concentrated solutions of salts, such as calcium chloride, are also effective. The process may be carried out either in a packed column or in a spray chamber. Regeneration of the liquid is an essential part of the process, and this is usually effected by evaporation. 

The absolute humidity of air refers to the mass of water vapour actually present in unit vol., and it is determined by passing a known vol. of air over a suitable drying agent—calcium chloride, phosphorus pentoxide, etc. (1. 7, 2)—and... 

E. Baur and G. L. Voerman studied the action of hydrogen—vide supra. According to G. J. Fowler, in air or oxygen, there is a very slight action at 120° and oxidation is visible at 200°—brown ferric oxide is formed, but no nitrogen is oxidized water vapour, at 1000°. produces a slow evolution of ammonia. A. Rossel prepared a nitride, which is readily decomposed by water, by heating a mixture of finely powdered calcium carbide and powdered iron while exposed to air. G. J. Fowler... 

With some substances, not necessarily salt hydrates, the reverse of efflorescence occurs. For example, if anhydrous calcium chloride is left in the air, it absorbs water vapour and eventually forms a very concentrated solution. This process is called deliquescence, and substances which behave like this are said to be deliquescent. Solid sodium hydroxide will deliquesce. 

Water vapour is removed from the ammonia gas by passing the gas formed through a drying tower containing calcium oxide (Figure 11.16). 

Calcium oxide is used to dry ammonia gas in its laboratory preparation. Write a word and balanced chemical equation to show how calcium oxide can react with the water vapour to remove it from damp ammonia gas. 

Measurement of the adsorption of gases on crystals are less numerous than on most other adsorbents because of the difficulty of securing a reasonably large surface area per unit weight. A few experiments have been performed for monolayer adsorption of methane, carbon monoxide on mica and adsorption of water vapours on finely powdered quartz and anthronite (a calcium aluminium silicate). 

At red heat calcium carbide reacts with water-vapour, liberating hydrogen and carbon dioxide 5... 

Atmospheric acidity is responsible for tbe attack in the presence of humidity whereas the SO2 reaction continues to occur in the presence of liquid water as well as water vapours. The calcium sulfate that forms is less preserved when the structures are exposed to direct rainfall. In protected regions SO2 continues its attack almost continuously and the gypsum obtained from this reaction forms crusts on protected surfaces. As a consequence most ancient buildings in the industrialized countries have a black appearance in some areas and a clean aspect in others. The black appearance is due to gypsum crusts, which have incorporated soot in the process of crystallization whereas the areas directly showered with the... 

In completion it may be remarked that we have arrived in two different ways at the conclusion that the lowering of the melting point of hydrated calcium chloride by addition of water or calcium chloride is less than normal. Both arguments proceed essentially from the same fundamental assumption the first from the partial dissociation of the hydrate for equilibrium in the fused state the second on the existence of a pressure of water vapour for the fused hydrate, which is intimately bound up with the partial dissociation of the hydrate. Hydrates, therefore, that when melted have no vapour pressure of water must give the normal lowering of melting point on addition of cither component that is very approximately the case for bodies like sulphuric acid, and completely so for the carbohydrates. 

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