Physical Constants of Liquids The Boiling Point and Density

Physical Constants of Liquids The Boiling Point and Density 

1 The Boiling Point As a liquid is heated, the vapor pressure of the liquid increases to the point at which 

As a rule of thumb, the boiling point of many liquids drops about 0.5°C for a 10-mm decrease in pressure when in the vicinity of 760 mm Hg. At lower pressures, a 10°C drop in boiling point is observed for each halving of the pressure. For example, if the observed boiling point of a liquid is 150°C at 10 mm pressure, then the boiling point would be about 140°C at 5 mm Hg. 

The vapor pressure-temperature curve for a typical liquid. 

Pressure-temperature alignment nomograph. How to use the nomograph Assume a reported boiling point of 100°C (column A) at 1 mm. To determine the boiling point at 18 mm, connect 100°C (column A) to 1 mm (column C) with a transparent plastic rule and observe where this line intersects column B (about 280°C). 

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Physical Constants of Liquids The Boiling Point and Density, Part A. "Boiling Points and Thermometer Correction"... 

Determination of the physical constants and the establishment of the purity of the compound. For a solid, the melting point is of great importance if recrystallisation does not alter it, the compound may be regarded as pure. For a liquid, the boiling point is first determined if most of it distils over a narrow range (say, 1-2°), it is reasonably pure. (Constant boiling point mixtures, compare Section 1,4, are, however known.) The refractive index and the density, from which the molecular refractivity may be calculated, are also valuable constants for liquids. 

If any doubt exists and if the specific gravity of the pure liquid is accurately known, it is best to determine that of the distillate if both the boiling point and the specific gravity of the distillate agree with those of the pure substance, we may conclude with confidence that the distillate is really pure. Instead of the specific gravity, the refractive index, the vapour density, the melting point (if the liquid can be easily solidified), or some other physical constant may be determined for comparison, or a chemical analysis of the liquid may be made. 

All substances also exhibit physical properties that can be observed in the absence of e in composition. Color, density, hardness, melting point, boiling point, and electrical and thermal conductivities are ph)rsical properties. Some ph)rsical properties of a substance depend on the conditions, such as temperature and pressure, under which they are measured. For instance, water is a solid (ice) at low temperatures but is a liquid at higher temperatures. At still higher temperatures, it is a gas (steam). As water is converted from one state to another, its composition is constant. Its chemical properties change very litde. On the other hand, the physical properties of ice, liquid water, and steam are very different (Figure 1-8). 

Although less pronounced than the effect on gases, pressure can have a considerable effect on the physical chemistry of liquids. Pressure can influence many of the physical attributes of the mobile phase such as melting point or boiling point, density, and viscosity. In addition, the attributes of a compound in solution can also be somewhat influenced by pressure, for example, diffusion coefficient. The distribution of a solute between solid stationary phase and liquid mobile phase can also be affected by pressure, for example, equilibrium constant and phase ratio. 

Besides having provisions for simulating most of the principal types of process equipment, the more extensive of these programs have routines that generate physical properties of hundreds of chemical species, either from built-in data tables or by using estimation formulas. Included in the stored data are boiling and melting points, liquid densities, critical constants, heat capacities, latent heats, solubilities, and vapor pressures. 

To an organic chemist, the more important physical properties of a liquid are the density, refractive index, and boiling point. All three are useful for the characterization of liquids, and the phenomenon of boiling is the basis of the most important method of separating and purifying liquids. Other properties such as the viscosity, surface tension, and dielectric constant are of lesser importance and will not be considered here. 

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... 

Organic compounds have a number of physical properties that allow their precise characterization. These include the classical physical constants boiling point, density, and refractive index for liquids and melting point for solids. The rapid development of modem chemical instrumentation, however, has also made easily accessible many of the spectral properties of these materials. Specfroscopy, in particular, provides information that is extremely powerful for establishing the structure of unknown molecular systems and for rapidly identifying known materials. 

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