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UNITS OF VAPOR PRESSURE The most common unit for vapor pressure is the = a force of 1 Newton per square meter = 10 Dyn /cm&2 = 0.01 Mbar = 0.0075 mmHg = 0.00000969 Atm = 0.00014 Psi . EQUILIBRIUM VAPOR PRESSURE OF SOLIDS Equilibrium vapor pressure can be defined as the pressure reached when a condensed phase is in equilibrium with its own vapor. In the case of an equilibrium solid, such as a Crystal , this can be defined as the pressure when the rate of Sublimation of a solid matches the rate of deposition of its vapor phase. For most solids this pressure is very low, but some notable exceptions are Naphthalene , Dry Ice and ice. Ice will still continue to disappear even though the ambient temperature is below the freezing point of water. All solid materials have a vapor pressure. However, due to their often extremely low values, measurement can be rather difficult. Typical techniques include the use of Thermogravimetry and Gas Transpiration . RELATION BETWEEN SOLID AND LIQUID VAPOR PRESSURES It may be noted that the vapor pressure of a substance in liquid form is usually different from the vapor pressure of the same substance in solid form. If the temperature is such that the vapor pressure of the liquid is higher than that of the solid, liquid will vaporize but vapor will condense to a solid, i.e. the liquid is '''freezing'''. If the temperature is such that the vapor pressure of the liquid is '''lower''' than that of the solid, solid will vaporize but vapor will condense to a liquid, i.e. the solid is ''' Melting '''. At the temperature that equalizes the two vapor pressures, an equilibrium exists between solid and liquid Phases . This temperature is referred to as the Melting Point . WATER VAPOR PRESSURE Water starts to boil when its vapor pressure reaches the vapor pressure of the surrounding atmosphere. At higher elevations the atmospheric pressure is lower and water will boil at a lower temperature. The boiling temperature of Water for pressures around 100 KPa can be approximated by where the temperature is in degrees Celsius and the pressure ''p'' is in Pascal s. One gets the vapor pressure by solving this equation for ''p''. PARTIAL PRESSURES Raoult's Law approximately governs the vapor pressure of mixtures of liquids. EXAMPLES OF CHEMICALS WITH A HIGH VAPOR PRESSURE
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