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Homework Help: AP Chemistry -two interesting questions - PLEASE HELP!

  1. Mar 26, 2008 #1
    AP Chemistry -two interesting questions -- PLEASE HELP!

    1. The problem statement, all variables and given/known data
    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.

    2. Relevant equations
    3. The attempt at a solution
    i really have no idea....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
    IDEA....would the first one be b/c of freezing point depression? the solute and solvent have different FP's, and when the solute is in solution it prevents freezing but when the can is opened the solute is released and the solvent freezes....?
    i was trying to talk my way through it and this is what i came up with:
    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 that make any sense? could i word it better?
  4. Mar 26, 2008 #3
    When you add solutes to a liquid, the freezing point of the resultant solution decreases, which is why you still hear that liquid sound. What happens to the solute the instant you open the can? What happens to the water the instant you open the can now that you know what has happened to the solute?

    As for the second question. What components are in the flask when you heat the water and then put a stopper over it when it starts to boil? When you put cold water on the flask, what happens to the vapour inside the flask? How does this relate to the pressure outside the flask? What will the water do to offset this pressure difference?
  5. Mar 26, 2008 #4
    The instant you open the can the solute escapes and the pressure decreases...umm where does water factor in with the first question? and could you please look over my answer (2nd post) and see if its okay....

    when the water is heated there's water in liquid and gaseous states. When cold water is added KE decreases so more liquid water would form, and vapor would decrease. The pressure outside is constant but the pressure inside is less so...the water will boil to create more gas?? is that right??
  6. Mar 26, 2008 #5
    Your answer for 1 is close. Where does the water come in in the first question? Well, think about it. Water freezes at 0C. Water with carbon dioxide in it (a solute) will freeze at < 0C. What happens when you open the can? The carbon dioxide is gone, you're right, and the water is still there, but the temperature inside the can hasn't changed instantly once you open the lid. So what will the water do?

    Your answer for 2 is spot on.
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