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Water or Steam?

  1. Jan 30, 2010 #1
    1. The problem statement, all variables and given/known data
    Will 1ml of 100 degrees C water or 1ml of 100 degrees C steam be more dangerous to your skin?

    2. Relevant equations
    not too sure

    3. The attempt at a solution
    I keep hearing that steam would be more lethal than the water but I would like some math to back it up. What math can give me an answer?
     
  2. jcsd
  3. Jan 30, 2010 #2

    rock.freak667

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    Well I would just check to see which one would have more heat energy.
     
  4. Jan 30, 2010 #3
    have heard of the term latent heat?
     
  5. Jan 30, 2010 #4
    I have heard of latent heat. Its units are energy per mass. I'm guessing that since steam is a gas it would have more heat energy but just out of curiosity how much more heat energy compared to the water?
     
  6. Jan 30, 2010 #5

    Pengwuino

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    Well, just a second, think about how much water there actually is in 1ml of steam vs. 1 ml of liquid water?
     
  7. Jan 30, 2010 #6
    Well there would be .001kg of water in 1 ml and a hell of a lot less steam in 1ml
     
  8. Jan 30, 2010 #7
    See this is were I get confused. I see that there is more water per volume compared to the steam but I would also think that the steam has higher heat energy because it is a gas.
     
  9. Jan 30, 2010 #8

    Pengwuino

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    Yes, so while there may be more energy per mass in steam, the amount of actual mass in 1ml is far greater with the liquid water
     
  10. Jan 30, 2010 #9
    Actually, it should make perfect sense then to say that heat energy in an object is equal to mass times latent heat

    E=mL

    Is this correct?
     
  11. Jan 30, 2010 #10

    Pengwuino

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    Not exactly because that is only the heat released/absorbed during a phase change, not the energy of the actual substance.
     
  12. Jan 30, 2010 #11
    ok but then how would you know which one will cause the greater burn? Am i just suppose to say that because the water has more mass it will burn more?
     
  13. Jan 31, 2010 #12

    ehild

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    You need to estimate the heat your skin gets in both cases.
    Assuming that the temperature of your skin is 36 °C, the change of temperatue is -64 °C If it is water, Qw=c*mw*64. It it is steam, your skin gets the latent heat L when the steam condenses and then some more when the condensed water cools down. Qs=L*ms+c*ms*64.

    If you look after the thermal properties of water, you would find that

    the latent heat of evaporation/condenzation is L= 2,270 kJ/kg
    c is the specific heat capacity of water c= 4.187 kJ/(kgK)
    density of water : 1000 kg/m^3

    You can estimate the mass of steam from the universal gas law: pV=m/M*RT P~105 Pa, V=106 m3, M =0.018 kg, T=373 K, R=8.31 J/(K mol)

    ehild
     
  14. Feb 2, 2010 #13
    So wait. What I really like about what you said about specific heat. It seems to describe the amount of energy needed to change the temperature of a certain mass by 1 kelvin. If you know the mass and multiply it by its specific heat, you will get the amount of energy needed to change the temperature of that mass by 1 kelvin. Wiki says the specific heat for steam vapor is 2.080 kJ/(kg*K) (I know I didn't do the calculation but I want to make sure my point gets across).

    Since we know that the mass of steam is going to be much less than water, multiplying these mass of steam by its specific heat will turn it into a small KJ/K ratio compared to the water. Since the water's ratio will be bigger, it means that it will have to transfer more heat energy to you in order to cool off which means that the water will cause a greater burn than steam. Is this correct?
     
  15. Feb 2, 2010 #14

    ehild

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    I think that 1 ml water will cause higher damage than 1 ml steam, but that steam will condense into water before further cooling down, and releases the latent heat, which is much higher than the heat released because of cooling. Anyway, this is still much less than the heat coming from the water.

    ehild
     
  16. Feb 2, 2010 #15
    That makes perfect sense actually because even once it changes state, it is only a fraction of the mass of the 1ml of water. Ok I think I have a definite answer now. Thanks :smile:!
     
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