If we return to that hot windy day at the beach and consider the relative humidity in the air, the cooling effect of the wind would be most effective if the relative humidity was low. Vapor pressure is an equilibrium property. Let's begin by discussing the vapor pressure of a pure substance and how it varies with temperature.
#Steam at 100 but still ing how to
In order to understand how to take advantage of these processes in purifying organic materials, we first need to learn how pure compounds behave when they are vaporized or sublimed. Both vaporization and sublimation are processes that can be used to purify compounds. This is a consequence of the process of sublimation. Many of you may have noticed that even on a day in which the temperature stays below freezing, the volume of snow and ice will appear to decrease, particularly from dark pavements on the streets. They simply vaporize directly into the atmosphere. Solid substances are not characterized by a similar phenomena as boiling. If the external pressure is atmospheric pressure, the temperature at which a pure substance boils is called the normal boiling point. When the vapor pressure of a liquid substance reaches the external pressure, the substance is observed to boil. The larger the vapor pressure, the greater the tendency to escape. Vapor pressure is a measure of the tendency of a condensed substance to escape the condensed phase.
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The vapor pressure of a pure substance is the pressure exerted by the substance against the external pressure which is usually atmospheric pressure. All substances regardless of whether they are liquids or solids are characterized by a vapor pressure. The wind facilitates the evaporation process and you supply some of the heat that is required. This is why even on a hot day at the beach, if there is a strong breeze blowing, it may feel cool or cold after you come out of the water. For a liquid, this process is called vaporization and for a solid it is called sublimation.
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Let us begin by describing the process by which a substance is transformed from the condensed phase to the gas phase. Many of these are terms that you are familiar with but the exact definitions may not be known to you. However, before we begin a discussion of distillation, it would probably be beneficial to define the terms that describe the process and related properties. Distillation is an important commercial process that is used in the purification of a large variety of materials.