Physical vapor pressure lowering

The presence of a solute affects some of the physical properties of a solution, but the identity of the solute makes little difference in the colligative properties. Vapor-pressure lowering, freezing-point depression, boiling-point elevation, and osmotic pressure are four such properties. 

Other colligative properties can similarly be shown to be related to the left-hand side of Eq. (2-62). Vapor pressure lowering is related, for example, through Raoult s law and Eq. (2-54). Reference should be made to standard introductory physical chemistry textbooks. 

The four colligative properties that are of importance are 1) the vapor pressure lowering 2) the elevation of boiling point 3) the freezing-point depression and 4) the osmotic pressure. An attempt is made below to describe qualitatively and quantitatively each colligative property of solutions, with an emphasis on their interrelationship and their application later in measurement and adjustment of the tonicity of solutions, with particular reference to parenteral formulations. Although theoretical derivations based on thermodynamics can be used to show how each of the colligative properties of solution arises and relate to each other, textbooks on physical chemistry for theoretical derivations are recommended. 

Solutes affect some of the physical properties of their solvents. Early researchers were puzzled to discover that the effects of a solute on a solvent depended only on how many solute particles were in solution, not on the specific solute dissolved. Physical properties of solutions that are affected by the number of particles but not the identity of dissolved solute particles are called colligative properties. The word colligative means depending on the collection. Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. 

Fiquid physically and/or chemically bound to a solid matrix so as to exert a vapor pressure lower than that of pure liquid at the same temperature. 

Bound water may exist in several conditions. Liquid water in fine capillaries exerts an abnormally low vapor pressure because of the highly concave curvature of the surface moisture in cell or fiber walls may suffer a vapor-pressure lowering because of solids dissolved in it water in natural organic substances is in physical and chemical combination, the nature and strength of which vary with the nature and moisture content of the sohd. Unbound water, on the other hand, exerts its full vapor pressure and is largely held in the voids of the solid. Large wet particles, such as coarse sand, contains only unbound water. 

The physical properties of a solution dHfer from those of the solvent These properties (vapor-pressure lowering, boirng-point elevation, freezing-point depression, and osmotic pressure) are caled cdlSga-tive because they depend on the number, not the chemical nature, of the dssolved particles. In salt solutions, rteractions among ions cause deviations from expected properties. 

Physical properties of solutions that depend on the number, but not the kind, of solute particles in a given amount of solvent are called colligative properties. There are four im- Co///gaf/vemeans"tied together." portant colligative properties of a solution that are directly proportional to the number of solute particles present. They are (1) vapor pressure lowering, (2) boiling point elevation,... 

In Chapter 14 (Solutions and Their Physical Properties), we have added a section to describe the standard thermodynamic properties of aqueous ions. We use the concepts of entropy and chemical potential in Chapter 13 to explain vapor pressure lowering and why gasoline and water don t mix. 

One of the most significant sources of change in isotope ratios is caused by the small mass differences between isotopes and their effects on the physical properties of elements and compounds. For example, ordinary water (mostly Ej O) has a lower density, lower boiling point, and higher vapor pressure than does heavy water (mostly H2 0). Other major changes can occur through exchange processes. Such physical and kinetic differences lead to natural local fractionation of isotopes. Artificial fractionation (enrichment or depletion) of uranium isotopes is the basis for construction of atomic bombs, nuclear power reactors, and depleted uranium weapons. 

Extensive hydrogen bonding takes place in phosphoric acid solutions. In concentrated (86% H PO solutions, as well as in the crystal stmctures of the anhydrous acid and the hemihydrate, the tetrahedral H PO groups are linked by hydrogen bonding. At lower (75% H PO concentrations, the tetrahedra are hydrogen-bonded to the water lattice. Physical properties of phosphoric acid solutions of various concentrations are Hsted in Table 2 the vapor pressure of aqueous H PO solutions at various temperatures is given in Table 3. 

The physical characteristics of /i /f-amyl alcohol diverge from the standard trends for the other alcohols it has a lower boiling point, higher melting point, higher vapor pressure, and low surface tension. Most notably, organic molecules are highly soluble in /i /f-amyl alcohol. 

Physical and chemical properties of isopropyl alcohol reflect its secondary hydroxyl functionaHty. For example, its boiling and flash poiats are lower than / -propyl alcohol [71-25-8], whereas its vapor pressure and freezing poiat are significantly higher. Isopropyl alcohol bods only 4°C higher than ethyl alcohol. 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

Across a control valve the fluid is accelerated to some maximum velocity. At this point the pressure reduces to its lowest value. If this pressure is lower than the liquid s vapor pressure, flashing will produce bubbles or cavities of vapor. The pressure will rise or recover downstream of the lowest pressure point. If the pressure rises to above the vapor pressure, the bubbles or cavities collapse. This causes noise, vibration, and physical damage. 

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