Evaporation and Vapor Pressure ```Name: Elliot Status: N/A Age: 14 Location: N/A Country: N/A Date: 2000-2001 ``` Question: Evaporation is different in different liquids why is this? Also why does evaporation cause a cooling effect? Replies: The evaporation of a liquid depends upon its vapor pressure -- the higher the vapor pressure at a given temperature the faster the evaporation -- other condition being equal. The higher/lower the boiling point the less/more readily will a liquid evaporate. The amount of energy required to convert one mole of liquid to one mole of vapor, the more energy is required for evaporation. You can find some interesting trends in these behaviors by perusing a handbook such CRC/s Handbook of Chemistry & Physics. Water is a unique liquid with regard to its vaporization properties. It must absorb ~10 k.cal/mol of heat at 25 C to convert one mole = 18 gm = 18 ml of water to vapor and of course must release the opposite amount of heat for the opposite process. This is a HUGH energy lever that has a large impact on weather and climate!! Estimate how much energy is released in a thundershower of 1 inch of rain over 1 mile squared!!! Vince Calder Hi Elliot Evaporation in different liquids is different because of differences in weights of the molecules of the liquid and (generally more importantly) differences in attraction of molecules for each other within the liquid. For example, water (H2O, molecular weight 18) has a much much higher boiling point than ethane (C2H6, molecular weight 30) even though it has a lower weight because of hydrogen bonding between water molecules. But when you look a series of chemicals with similar composition (for example the alkanes- methane, ethane, propane, butane, pentane, hexane), as you get higher weights you generally get higher boiling points. Evaporation takes energy from whatever source and converts it into breaking the attractions between molecules- as the now gaseous molecules float off, they carry this energy with them. Don In a material at a certain temperature, each molecule has a different "temperature". What we measure is an average value. In evaporation, some of the fastest-moving (i.e. hottest) molecules on the surface manage to break free of the liquid. Because only the hottest molecules, the ones with the most energy, actually evaporate, only the cooler molecules are left behind in the liquid. This is why evaporation is a cooling process. The molecules of different liquids hold on to each other with different amounts of force. For liquids that hold tight, a great deal of energy is required for a molecule to break free. Such a liquid will take a long time to evaporate. Liquids that do not hold on very tightly will evaporate quickly. Temperature also affects evaporation rate. A hot material has more energy per molecule. It is easier for a molecule to get the extra energy required to evaporate. At lower temperatures, evaporation is more difficult. Kenneth Mellendorf Evaporation is the process of molecules leaving a condensed liquid state and going into a gaseous state. The liquid state arises because molecules attract each other, making the condensed state lower in energy than the gaseous state. To form the condensed state from the gaseous state, the molecules need to give off energy in the form of heat. To form the gaseous state from the condensed state, the reverse must ccur, that is, the system must absorb heat. That's why evaporation has a cooling effect. The reason that different liquids evaporate differently is that the attractions between molecules are different for different substances. Richard E. Barrans Jr., Ph.D. Assistant Director PG Research Foundation, Darien, Illinois Click here to return to the Physics Archives

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