Rhombic and monoclinic sulphur

Point 0 (Fig. 13) is the triple point for rhombic sulphur—monoclinic sulphur— vapour, and is therefore the point of intersection of the three univariant curves, Srh— V, Smoa—and S h—Smon At this triple point reversible transformation of rhombic and monoclinic sulphur can take place, these two forms of sulphur being enantiotropic. 

To obtain this modification the discovery of which is due to Gernez, molten sulphur, after being heated in a test-tube to above 150°, is cooled down to and maintained at a temperature of about 98° e.g, in a water bath). On gently rubbing the inner walls of the tube with a glass rod, the sulphur crystallises in the nacreous form. This form of sulphur is monotropic with respect to rhombic and monoclinic sulphur. At all temperatures up to its melting-point, io6-8, mother-of-pearl sulphur is metastable. 

Curve BE is known as transition curve and the two phases in equilibrium along this curve are rhombic and monoclinic sulphur. Both the phases are solid. The system is again univariant. 

Like sulphur, selenium exists in a number of allotropic forms. These include both crystalline, rhombic and monoclinic modifications... 

In water system, there are three phases, viz., ice (solid), water (liquid) and vapours (gas). Similarly, in sulphur system, there are four phases, viz., rhombic sulphur and monoclinic sulphur (solids), liquid sulphur (liquid) and vapour sulphur (gas). 

Sulphur exists in two crystalline forms, viz., rhombic and monoclinic. Normally, sulphur exists in rhombic form which is octahedral in shape. When sulphur is heated to 95.6°, the rhombic sulphur changes into monoclinic variety which is prismatic in shape. Above 95.6°, monoclinic sulphur exists as the stable phase. If monoclinic sulphur is cooled then at 95.6°, it changes into rhombic type. Therefore, it is clear that below 95.6°, rhombic sulphur exists as a stable phase, whereas above 95.6°, only monoclinic variety occurs. At 95.6°, these two crystalline forms are in equilibrium with one another. Hence, 95.6° is the transition temperature of sulphur. 

A single substance which can exist in two or more solid modifications has therefore several triple points, the maximum number of which can be computed by the law of combinations. Thus for sulphur, which can exist in two crystalline forms (rhombic and monoclinic), we have the following triple points ... 

Sulphur exists in two well-known enantiotropic crystalline forms— rhombic and monoclinic. At the ordinary temperature, rhombic sulphur can exist unchanged, whereas, on being heated to temperatures somewhat below the melting-point, it passes into the monoclinic variety. On the other hand, at temperatures above 96°, monoclinic sulphur can remain unchanged, whereas at the ordinary temperature it passes slowly into the rhombic form. 

Curve OC is the transition curve and represents the change of the transition point with pressure. Since the passage of the rhombic into monoclinic sulphur is accompanied by an increase of volume (At> = 0 01393 c.c. per gram), it follows from the theorem of Le Chatelier that the transition point is raised by increase of pressure. By means of this value of Az and the heat of transformation, 3 12 cal. cM... 

Melting Points of Sulphur.— When sulphur is heated to above its melting-point it fuses to an amber-coloured, mobile liquid, which passes, as the temperature is raised, into a dark brown viscous liquid. This peculiar behaviour of sulphur was attributed by A. Smith and his collaborators to the existence in molten sulphur of two modifications, Sx and S, in dynamic equilibrium. Of these two modifications, S is soluble in carbon disulphide, and is the normal molecular form corresponding with the crystalline rhombic or monoclinic sulphur. It is the form which is first produced when crystalline sulphur is melted. S/x, on the other hand, is insoluble in carbon disulphide, and is formed in increasing quantity as the temperature is raised. Molten sulphur, therefore, is not a pure liquid consisting of only one kind of molecules, but is a homogeneous mixture or solution of in Sa, the composition of which varies with the temperature. The attainment of equilibrium is accelerated by the presence of traces of ammonia, and is retarded by the presence of sulphur dioxide. 

As a result of the recent investigations of the pseudo-binary systems of the substance sulphur we obtain the diagram shown in Fig. 67. Here, the points A, D, and G represent the ideal freezing-points of monoclinic, rhombic, and nacreous sulphur respectively, or the temperatures at which these three crystalline forms are in equilibrium with pure molten Sa. The curve HEB represents the dynamic equilibrium curve for Sa, S i, and S,r in molten sulphur and the points B, E, and H, where this equilibrium curve cuts the freezing-point curves, represent the natural freezing-points of the three modifications of sulphur. 

Sulphur crystallises in at least two distinct forms, rhombic and monoclinic. Rhombic sulphur, SA, is stable at atmospheric pressure up to 95.5at which temperature transition to monoclinic sulphur, SB, can take place. Monoclinic sulphur is stable up to its natural melting point of 114.5 0. 

The equilibrium curves for the various modifications of sulphur are shown in Fig. 21. rd is the curve for rhombic sulphur-gas, md for monoclinic sulphur-gas, and/d for liquid-gas. The modification which has the lowest vapour pressure is the most stable modification, and always forms at the expense of those which have a higher vapour pressure. The four intersections of each... 

The possibility of determining this point experimentally is dependent on the rate at which rhombic sulphur changes into monoclinic sulphur above 100°. We know experimentally that the velocity of transition is so small that the melting point of rhombic sulphur can be determined without difficulty. The curves 3 4, 3 1, and 3 2 (obtained in part by extrapolation) show the variation of the transition temperature and of the two melting points with pressure. All three curves must inter-... 

Expenment has shown 1 that if we start with solid rhombic sulphur, on heating it rapidly it melts at a temperature 1140 C (point H) If, however, we keep rhombic sulphur heated at any temperature between 96° C and 1140 C it will be found to have become transformed into monoclinic sulphur, which will now melt at 120° C This rathei anomalous behaviour is easily explained when we study the system systematically, from the standpoint of the Phase Rule Thus Reicher showed that at a tempeiature 955° C, and under a certain pressure, namely, the pressure of the saturated sulphur vapour, the rhombic form passes into the monoclinic This is represented by the point A This is a triple point corresponding to the stable coexistence of the three phases, solid rhombic—solid monoclinic—vapour According to the... 

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