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Thermodynamics/ Heat,volume and temperature

  1. Nov 4, 2016 #1
    1. The problem statement, all variables and given/known data

    The formation of ice from water is accompanied by:

    A. absorption of energy as heat

    B. temperature increase

    C. decrease in volume

    D. an evolution of heat

    E. temperature decrease

    For the first one I think it's correct but I don't know the explanation

    For second one it is wrong since ice is cold

    For the third one it is wrong since ice has more volume than water for same quantity

    For the fourth one I'm confused what is heat? It is colder but doesn't mean heat evolute?

    For fifth one I think it is correct since ice is cold
     
  2. jcsd
  3. Nov 4, 2016 #2

    Simon Bridge

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    Then why do you think it is correct?
    ... can't water be cold too?
    Well done.
    ... the first step to wisdom is to say "I do not understand."
    Can't water be cold too?

    So your main confusion is about the nature of heat ... btw: what does "evolute" mean in this context?
    You should also revisit your ideas about state changes in matter - ie. at what temperature does water turn into ice (at std pressure)?

    Note: what are you supposed to do with the options?
    Are you supposed to call true/false and with reasoning, or are you supposed to find all the true statments, or are you supposed to find the only true statement, or what? The actual problem statement can provide clues here.
     
  4. Nov 4, 2016 #3
    I need only to circle the correct answer (only one answer)


    I think to be cold it must gain some heat energy
     
  5. Nov 4, 2016 #4

    Simon Bridge

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    Can't cold things lose heat energy to become even more cold?
    "Hot" and "cold" are relative terms ... what is the temperature that ice forms?
     
  6. Nov 4, 2016 #5
    The correct answer is A
    how did they reach that
     
  7. Nov 4, 2016 #6

    Simon Bridge

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    They did it by understanding the concepts involved ... to help you to understand I am asking you questions.
    If you do not answer questions, I cannot help you.
     
  8. Nov 4, 2016 #7
    can't we give them energy to become colder?
    I think it is less than or equal 0?
     
  9. Nov 4, 2016 #8

    Simon Bridge

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    You wrote in the other thread that ##Q=mC(T_f-T_i)## ... so: using that equation, can you show me ##T_f<T_i## (so the material gets colder) by making Q>0 (adding heat)?
    Normally, when you add heat to something, ie on a stove, does the material get hotter or colder?

    This is the core problem:
    ... water, at 1atmos pressure, turns into ice at exactly 0degC.
    That is, water at 0degC changes into ice at 0degC by removing heat at the rate of 334kJ for each kg of ice formed.
    http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html

    In general, phase changes occur at constant temperature.
    The heat gained or lost during the phase change is called "latent heat" because it is not associated with a change in temperature.
     
  10. Nov 4, 2016 #9
    Okay thanks alot
     
  11. Nov 4, 2016 #10

    Simon Bridge

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    OK So water loses heat at constant temperature to become ice ... and that heat has to go somewhere.
    So how does that inform your answers?
     
  12. Nov 4, 2016 #11
    it should absorb heat to change from liquid to ice
    but isn't the decreasing in temprature also correct???
    I think they are close to each other since Q is directly proportional to delta T
     
  13. Nov 4, 2016 #12

    Simon Bridge

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    Do you usually associate freezing things with adding heat to them?
    ie. do you use a freezer to add heat or remove heat from things?
    do you use an oven to add heat to things or remove heat from things?
    Do you make ice from water in the freezer or in the oven?
    Do you turn ice into water in the freezer or in the oven?

    What temperature does the water start at as a liquid?
    What temperature does it end up at as a solid?
    What is the change in temperature?

    ... what are you answering here?
    Go through the options from A through E again, this time use what I told you:
    "Water at 0°C changes to ice at 0°C by losing heat energy to the surroundings."
     
  14. Nov 4, 2016 #13
    273.16K
    the change is equal to zero??
     
  15. Nov 4, 2016 #14
    Ah okay I got your point
    solid transfer to liquid without changing in temperature.
    so if solid goes to liquid during the same temperature The factor of changing it is the heat energy
    right?
     
  16. Nov 4, 2016 #15

    Simon Bridge

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    That's the one.
    If you put water in a freezer, the freezer is usually much colder than the water ... the water decreaces temperature to 0deg, then stays at that temperature until it is frozen into ice at 0deg, then the temperature of the ice can decrease further. (0°C is 276.16K)
    ... solid to liquid is called "melting" - it is the opposite of freezing... to melt something you have to add heat, to freeze something you take heat away.
     
  17. Nov 4, 2016 #16

    Simon Bridge

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    Go through the options from A through E again, this time use what I told you:
     
  18. Nov 4, 2016 #17
    The answer is E
    I have a small question
    Can a process be isobaric and isothermal at the same time?
     
  19. Nov 4, 2016 #18

    Simon Bridge

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    Now you are just having me on!
    E = "there is a temperature decrease". What leads you to believe that this is the correct answer: explain?

    ... try to understand basic concepts before getting to the more advanced ones.
    Yes, a process can involve having something at constant pressure and temperature - there are quite a lot of things that can happen.
    For instance, for an ideal gas, there could be no change in volume either in this process, or maybe gas is added or removed from the container as well as the volume changing.
     
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