Water, boiling point dissociation constant

Pure sulphuric acid is a colourless, viscous and rather heavy liquid (density 1.84 g cm ). On heating, it decomposes near its boiling point, forming sulphur trioxide and a constant boiling (603 K) mixture of water and sulphuric acid containing 98% of the latter. This is concentrated sulphuric acid, which is usually used. Further heating gives complete dissociation into water and sulphur trioxide. 

Absolute 100 per cent, sulphuric acid when heated begins to boil at 290° C.,4 but the temperature rapidly rises to a constant boiling-point of 338° C., when 98-4 per cent, acid distils. This behaviour is due to dissociation 5 of the pure acid, which at temperatures above 30° C. is made evident by its fumes, and which can be demonstrated by partial separation of the dissociation products by making use of their unequal rates of diffusion.6 The vapour of sulphuric acid is largely dissociated into sulphur trioxide and water acid above 98-4 per cent, concentration loses trioxide more rapidly than water, whilst acid below this strength parts preferentially with water, the tendency in both cases being for the remaining liquid to attain the concentration of 98-4 per cent., which is that of the acid mixture of minimum vapour pressure. 

Other investigations include the refraction constants,1 and the dissociation-pressure of the water of crystallization of the dodecahydrate at the transition-point2 and such properties of solutions as density,3 specific heat,4 boiling-point,5 vapour-pressure,6 molecular depression of the freezing-point,7 electric conductivity,8 and hydrolytic dissociation.9... 

Autoignition temperature 955°C Boiling point 141.1°C Dissociation constant pK = 4.874 Flash point 52-58°C (open cup) Melting point —21.5°C Partition coefficients Octanol water Refractive index = 1.3848 = 0.33. 

Chemical identity Physical-chemical data Melting point Boiling point Vapor pressure Water solubility Dissociation constant... 

ScHOOEL has already pointed out the influence of heat on the color of an indicator. He found that boiling a solution of an alkali sensitive indicator shifted the color to the basic side, while the color of acid sensitive indicators was displaced towards the acid side. He explained this on the basis of an increase in the dissociation constant of water. This interpretation is illustrated by the following discussion. 

Were the dissociation constant of dimethyl yellow to remain unchanged by heating, then at the boiling point of water this indicator should undergo transformation at two pH units less... 

Water is unusual in all its physical and chemical properties (Table 2.23). Its boiling point (abnormally high), its density changes (maximum density at 4 °C, not at freezing point), its heat capacity (highest of any liquid except ammonia), and the high dielectric constant as well as the measurable ionic dissociation equilibrium, for example, are not what one would expect by comparison of water with other similar substances (hydrides). All the physical and chemical properties of water make our climate system unique and have shaped the course of chemical evolution. Water is the medium in which the first cell arose, and the solvent in which most biochemical... 

ISIS Base Physical Properties Data Base includes melting point, boiling point, water solubility, vapor pressure, dissociation constant, octanol/water partition coefficient, and Henry s law constant as well as chemical structures of approximately 130p0 substances. 

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