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Salt water steam pressure vs fresh water steam pressure

  1. Jan 29, 2012 #1
    1. The problem statement, all variables and given/known data

    My son picked a science experiment for his 5th grade project. It was supposed to be on evaporation. Basically a steam powered boat with candles heating up a cigar tube with a hole for the steam to escape and push the boat. However he got ambitious and changed the experiment with his teacher to see what would work better fresh water vs. a salt water solution. My son theorized that fresh water would push the boat better than salt water after researching about boiling points and elevated boiling points due to the addition of salt water. The conditions were the same as far as time frame, amount of water, and temp of water added to the tube. However his results show that on average the salt water powered boat traveled farther than the fresh water in the same given time. His observations were that the fresh water started pushing steam out sooner than the salt water, however the salt water powered steam was more consistent than the fresh water. Can you point me in a direction that supports these results. Do they sound valid or do we have a case of experimental error. I helped him and made sure that we kept all processes the same so we weren't influencing a result. Is the added solution with a higher boiling point releasing steam at a higher pressure due to more molecular bonds being broken?

    2. Relevant equations



    3. The attempt at a solution
    He is looking into vapor pressure and is trying to come to a conclusion. Vapor pressure is lowered with salt added to the water so it takes more heat to release vapor so would this create more pressure thus pushing the boat faster.
    Thanks
     
  2. jcsd
  3. Jan 31, 2012 #2

    Borek

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    Staff: Mentor

    For the salty water escaping vapor has a higher temperature - I would try to think how it can change the propulsion effectiveness. But I am just thinking aloud, I don't have any solid idea about what is happening.
     
  4. Jan 31, 2012 #3
    It's been a while for me but I am thinking that because we have a higher boiling point the steam would be at a higher temperature thus increasing the pressure in the tube. Therefore we should have an increase of kinetic energy. I'm just trying to make sure we have a valid conclusion.
     
  5. Jan 31, 2012 #4

    Borek

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    Staff: Mentor

    Steam will be hotter, but the pressure in the tube will be identical in both cases - that's because boiling means vapor pressure is equal to the surrounding pressure (which doesn't change).
     
  6. Jan 31, 2012 #5
    Yes but if you have a higher temp you should be releasing more energy you couldn't have the same result with water vs a solution. Somehow there should be a change more energy in = more energy out.
     
  7. Feb 1, 2012 #6

    Borek

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    Staff: Mentor

    Hotter steam contains more energy at the same pressure.

    At least per mass unit.

    I think I will move this thread to the engineering section.
     
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