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How can i keep an ce cube from melting?

  1. Nov 5, 2009 #1
    what method can be used to keep a regular 2cmx2cmx2cm ice-cube from freezing. i need to find a way to keep an ice cube solid for 10 hours after taking it out of the freezer without using electricity or by adding more ice etc.
    the question comes from my chemistry lecturer so i would expect it to have something to do with chemistry, maybe raising ice's melting point.
    room temperature is about 24C
     
  2. jcsd
  3. Nov 5, 2009 #2
    Hi,
    maybe with salt or liquid nitrogen or liquid oxygen.
    I`m not sure but salt should increase the melting point and liquid gases are as far as I know very, very cold.
    I hope I could help you.
     
  4. Nov 5, 2009 #3

    Mapes

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    A phase change is a chemical reaction. What can you do to prevent a reaction from being spontaneous?

    It's not very helpful to have a list of things you can't do that ends with "etc." What specifically is excluded?
     
  5. Nov 5, 2009 #4

    chemisttree

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    Hmmmm. Maybe you meant to write 'melting' instead of 'freezing'. Not much can be done to keep it from melting after removing it from the freezer unless you do something that prevents it from absorbing any heat from the room whatsoever. It isn't a chemical reaction, merely a physical change so don't think along the lines of mixing something with the ice cube to change it's melting point.
     
  6. Nov 5, 2009 #5
    Maybe you can put the Cube under high pressure to prevent the molecules from moving any faster?
     
  7. Nov 5, 2009 #6

    Mapes

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    But it's pretty useful to treat it as a chemical reaction, since (1) the two phases behave different chemically and physically; (2) there's an associated enthalpy change and entropy change; (3) the process can occur forwards or in reverse and possibly be spontaneous in one direction; and (4) we have to match atomic amounts on both sides. So close enough? :smile:
     
  8. Nov 6, 2009 #7

    Borek

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    You mean water in ice reacts differently from water in liquid phase? That's novelty to me :wink:

    --
     
  9. Nov 6, 2009 #8
    Physics probably gives you the simplest answer.You have to prevent heat from entering the ice and there are three mechanisms by which it does this conduction, convection and radiation.Look these up and work it out.I assume your lecturer wants you to make something and test it. The designs I have in mind can be put together easily and cheaply using stuff you will find in most kitchens.Some melting is inevitable.
     
  10. Nov 7, 2009 #9

    Lok

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    Considering that ice is not at 0'C when you take it out of the freezer but rather a pleasant -4'C, this might work fine. Use a small polystyrene box taken out of the freezer to and cover it with lots of blankets. ( with blankets one can keep a hot water bottle for 2 days and it will still be warm, and the temp difference is greater than the ice and room temp)
     
  11. Nov 7, 2009 #10

    Moonbear

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    It looks like the constraints of your question are that you cannot alter temperature. What are the other components of a phase diagram that determine if something remains solid, liquid or gas? At least one of those can still be manipulated with an ice cube to maintain it in a solid state. You'll need to figure out which to solve your problem.
     
  12. Nov 7, 2009 #11
    pressure?? as someone said here, i could put the ice into a vacuum, otherwise what is the pressure that will raise ice's melting temp to around 24 degrees?? do you know where i can find a phase diagram for water online??
     
  13. Nov 7, 2009 #12
    Try googling triple point
     
  14. Nov 8, 2009 #13

    chemisttree

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    I'm sorry but no, it is not close enough. Great pains are taken in the teaching of chemistry both at the HS and college level to distinguish between these two concepts. In fact, there are usually questions in intro courses that ask the student to distinguish between physical and chemical changes... a perennial favorite is the melting of water (correct answer - physical change).
     
  15. Nov 8, 2009 #14

    Mapes

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    At the college level, really? http://www.av8n.com/physics/chemical-physical.htm" [Broken] that a distinction between chemical and physical changes is specious. I would definitely be interested in hearing a dissenting opinion, though.

    Would the phase change between C(graphite) and C(diamond) be chemical or physical?

    My original point is only that it's useful to treat a phase change as a chemical reaction, since we can use accumulated knowledge about how to force a reaction to go one way or another. For example, if the two phases have different entropies, we can change the temperature. If they have different densities, we can change the pressure. If they have different polarizations, we can change the electric field. If they have different surface tensions, we can change the area. And so on. As I see it, and as Denker argues, it only helps to make a connection with chemical reactions. It doesn't hurt.
     
    Last edited by a moderator: May 4, 2017
  16. Nov 8, 2009 #15
    1) Freeze the ice at a low temperature-- say -40C.
    2) Place it in a container of airogel that has been in the freezer as well.
    3) Surrounded the aerogel with aluminum foil to keep the light out.

    http://www.boingboing.net/200602061740.jpg [Broken]
    Aerogel
     
    Last edited by a moderator: May 4, 2017
  17. Nov 8, 2009 #16

    chemisttree

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    Ha ha!:biggrin: I just got back from Denker's site. He does make some good points but unfortunately he is definitely not mainstream! I loved his complete annotation of the characters in Harry Potter! He's wrong... And he's allowed to be (wrong) on his web page but NOT HERE!

    I would say that it is chemical. The chemical bonding is radically different in the two compounds. They have different physical and chemical properties.

    Unless you are being asked on a test to distinguish between them.
     
    Last edited by a moderator: May 4, 2017
  18. Nov 8, 2009 #17

    Mapes

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    Where does he err?
     
  19. Nov 8, 2009 #18

    chemisttree

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    In many places. Notably here...
    There is lots of this stuff throughout his 'analysis'. In this case he makes up a rule about science (pretty reasonable one at that) and then chides teachers for not following the scientific method in the teaching of basic concepts. I cannot imagine a more obtuse analysis than that which he gives. After a few of these gems he goes on to recite a litany of carefully chosen examples (cutting paper or nylon as a chemical change!) that supposedly refute the fact that there are meaningful differences between the concepts.

    Utterly useless discussion like this is no help to anyone.
    Macromolecules can be broken down in their melt simply by stirring them. This is also true. From that do you expect me to teach students that merely stirring something is a chemical change? Nonsense! (edit: I'm not a teacher unless you consider some of what we do here as Homework Helpers and Science Advisors, 'teaching')

    Could you sit through a class where such basic material is accompanied by endless examples (...one should always consider all the data, or at least a fair sampling of the data.) that contradict the lesson? You think this is useful?

    I could continue a reasonable critique of Denker's ideas but I fear the nauseum in the ad nauseum....
     
    Last edited: Nov 8, 2009
  20. Nov 8, 2009 #19

    Mapes

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    These are counterexamples to conceivable distinctions between chemical and physical changes (e.g., the physical mechanism of cutting certain materials results in the rupture of many covalent bonds, surely a chemical phenomenon). I invited you to point out a logical error. He is (and I am) mystified about the precise definition of a physical change as distinguished from a chemical change. You said in your post #13 that the phase change between ice and liquid water is a physical change, and in your post #16 that the phase change between graphite and diamond is a chemical change. How are you distinguishing the two? How would you describe the reactions ice(VII) -> ice (VIII), or C(liquid) -> C(vapor), or Fe(bcc) -> Fe(fcc)?

    Well, it's altering its chemical potential. But why make a distinction in the first place? It they're taught that it's a "physical change," not a "chemical change," what can they do that they couldn't do before? What's the logical argument against Denker's position, apart from an appeal to incredulity?
     
  21. Nov 9, 2009 #20

    chemisttree

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    No, you aren't mystified. Merely argumentative. The distinction between physical change and chemical change is meaningless to you. Great! This is not the Forum to discuss your own definitions of well-defined terms.

    Simple. A physical change is a change that doesn't involve a change in the substance's chemical identity. A chemical change is one where the chemical identity changes. Ice is H2O and so is water. Hydrogen bonding in ice... hydrogen bonding in water. Diamond is a lattice of carbon with SP3 hybridization.... graphite has SP2 hybridization (those are chemically distinct differences). Stirring water doesn't change its chemistry even though bonds are being broken and reformed all the time. The nature of water is that it has loosely bound hydrogen atoms that can be exchanged between molecules of adjacent water.

    Using the illogic of Mr. Denker as a guide you would have to conclude that a piece of paper sitting on a table is an example of a chemical reaction because at some level and at some nearly imperceptible rate the paper is combining with oxygen and will eventually be converted entirely to CO2 and water. He would also have to conclude that crumpling the paper is an example of a chemical change. While technically there is some chemical reaction occuring as a result of the crumpling process, the rest of the world views that as a physical change. All of the salient properties of the paper remain if not ALL the original molecules of cellulose.

    Some other useless examples of a Denker chemical change would be the stretching of a rubber band, the rolling of a marble across a hard surface, heating a cup of water, filling a glass with water, etc...
    So many examples that the meaning of chemical change becomes meaningless and can simply be replaced with the term 'change', any change. All of which is useless and confusing to students.
     
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