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Homework Help: 2 interesting think questions

  1. Mar 26, 2008 #1
    these are really intestesting but i have no clue what the answers are...

    1. You are having some friends over, so you venture to the store to purchase some soda and snacks. Unfortunately the only pop available is at room temp. In order to cool the cans of soda quickly you place the cans in the freezer for approximately one hour to cool them. Upon hearing liquid in the can swish back and forth you confirm that the soda had not frozen. However, when you open the can to drink, the soda freezes! Propose an explanation for why the soda froze when the can was opened.

    2. You fill a flask about a tenth full with water and heat it over a flame. You boil the water rapidly for a few minutes and then quickly place a stopper over the top of the flask. you remove the flask from the flame, and after a few seconds the water in the flask stops boiling. You then place the sealed flask under cold tap water, and behold -- the water starts boiling again! Make the tap water even colder and the water in the flask boils faster! Explain in detail what's going on.

    i really have no idea what the answers are....i think the first one has something to do with pressure and the second with pressure/temp (as temp increases so does volume) but beyond that i have no idea...if you could even guide me or give a hint in the right direction i would really appreciate it
  2. jcsd
  3. Mar 26, 2008 #2
    okay i tried to talk my way through the first one and came up with this:
    When the cans were placed in the freezer, the temperature decreased drastically, and with it the kinetic energy of the soda molecules. As the temperature decreases, the solubility of the gas increases, and more of the gas suspended above the soda begins to dissolve into the soda. Hence the vapor pressure on the soda is decreased. Also, when the gas was dissolved, the freezing point of the solution was depressed. When the can is opened, the soda can is depressurized, and the dissolved gas begins to escape from solution. Hence the freezing point of the solvent is no longer depressed by the solute, and the soda freezes.

    does it seem right?
  4. Mar 27, 2008 #3
    That's the same conclusion I reached.
  5. Mar 27, 2008 #4

    1.) Very famous diagram you probably have seen or will see in the future for the triple point diagram for H20.

    http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/pics/941.jpg [Broken]

    Notice how water, under the right pressures, is actually a liquid at 0 degrees C. Ever gone ice skating? What happens to the "ice" underneath your skate? If you look closely you will see that it actually turns into liquid water. This is because you are putting an enormous amount of pressure on the ice from all the weight of your body concentrated on such a small area from the blade of the skate. Lift your foot up and you will see that the water turns right back into solid ice. Same concept here. The soda in the can may be at 0 degrees, but it has yet to turn into a solid because it is under pressure. Open up the can, the pressure is lowered, so the liquid immediately freezes.

    2.) You are creating a vacuum. When you boil the open flask you are pushing out all of the air in the flask with the water vapor. After you seal it all that is in the flask is water and water vapor. Boiling happens when there is enough energy in the liquid to overcome the vapor pressure force over it. After you put the cap on the flask the water is still hot, thus a lot of the water in the flask is still in its vapor phase. This exerts a force over the liquid, so the water will not boil. Cool the water down and the water vapor in the flask will go back into its liquid phase. Since you created a vacuum in the flask there is nothing in the flask exerting a force over the liquid. This means that the water will boil easily if you cool it down since there is nothing over it exerting a force.
    Last edited by a moderator: May 3, 2017
  6. Apr 11, 2008 #5
    wow ,very interesting !!
    i'll try them both
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