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Rates of Evaporation

  1. Mar 18, 2012 #1
    Hi

    I was just wondering how rates of evaporation changes with initial temperature. I was also confused because does say water have to be at 100° C before it starts evaporating or can it start earlier?

    Thanks
     
  2. jcsd
  3. Mar 19, 2012 #2
    It's going to be evaporating even at room temperature. Increasing the temperature causes those water molecules to speed up, thus speeding up evaporation (or so how I understand it!)

    --Matthew
     
  4. Mar 19, 2012 #3
    100° C is the boiling point of water, not the evaporation point. Boiling and evaporation are different things. Evaporation is the loss of molecules at the surface of a liquid to the external gas, whereas boiling is the formation of gas bubbles through-out the entire liquid. Evaporation happens at all temperatures to some extent, but boiling happens when the vapor pressure equals the external pressure. By reducing the external pressure, you can get liquids to boil more readily without changing the temperature.
     
  5. Mar 20, 2012 #4
    This will be a long reply and may cover some material you already understand, so forgive me if I'm being too longwinded.

    Evaporation:

    Evaporation happens all the time, even at room temperature. A body of water is made of a large number of water molecules, all going a different speeds but having an average speed (and speed distribution) proportional to the temperature of the water. At the surface, the very fastest of these molecules have enough energy to break away from the molecular bonds of the other water molecules and fly off the surface, becoming water vapor. As the temperature increases, the average speed of the molecules increases, and more and more molecules have the required energy to fly off the surface. So the evaporation rate increases with temperature.

    Vapor Pressure:

    If you were to have water in a sealed contained, these molecules of water vapor would bounce around the empty space above the water. They would thus exert pressure on the walls of the container and on the surface of the water. Eventually some of them would come back down and collide with the surface of the water, possibly becoming trapped in the liquid once again. So in addition to the evaporation rate, there is a condensation rate. As the pressure due to the water vapor increases, more frequent collisions with the liquid surface occur, and the condensation rate increases. Eventually, the condensation rate will become equal to the evaporation rate. Once this happens, the water is evaporating and condensing at the same rate, and the amount of vapor in the empty space can longer increase or decrease. The whole system has reached equilibrium. The pressure exerted by the gas once it reaches this equilibrium is called the vapor pressure for that specific temperature. It is important to note that for a substance like water, there is a unique vapor pressure for every temperature.

    Boiling:

    Evaporation also tends to form bubbles inside the body of water (don't ask me how, bubble formation is actually a pretty complex subject). But these bubbles aren't stable unless the pressure of the vapor inside the bubble is equal to the pressure of the surrounding water. The pressure of the surrounding water is equal to the pressure due to the depth of the water (which is usually small) plus the pressure of the atmosphere that the water is open to (which is very large). So the vapor pressure must roughly equal the atmospheric pressure in order for bubbles to form and not immediately collapse. This is why water makes lots of noise as it gets close close to boiling. Evaporation is forming tiny bubbles that cannot survive very long and are almost immediately crushed by the pressure of the surrounding water. Only when the vapor pressure becomes equal to the atmospheric pressure do the bubbles survive and float to the top. This is boiling. Since the bubbles are no longer being crushed, the loud noise goes away.

    So in answer to your question, the evaporation rate increases with temperature, and so does the vapor pressure. The boiling point of a substance is when the vapor pressure equals the surrounding atmospheric pressure. So you can either heat water up to 100 degress Celsius in order to boil it at normal atmospheric pressure, or you can decrease the surrounding pressure until bubbles can form in the water.
     
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