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Why the significant water level drop?

  1. Aug 24, 2012 #1

    davenn

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    hi guys

    OK, this has annoyed me for some time. I like my cups of noodles. The pack of noodles and sachet of flavour powder (obviously dry) go into the cup with hot water poured on them. Now if I fill the cup to the brim with the hot water, it will start to lower in level over a period of time ... say 10 minutes its very noticeable ~ 3mm loss of water level.

    Now, I appreciate that the water is soaking into the noodles/flavouring and the that would cause a drop in fluid level, but against that the noodles etc expand as they soak up the hot water. Added to that no significant amount of water has left the cup as steam.

    So if all the water is still in the cup, why am I still seeing a significant drop in fluid level ??

    is the soaking expansion of the noodles not totally counteracting the amount of water absorbed ? or something else is going on ?

    EDIT --- just had a new thought --- as the water cools its likely to become more dense and take up less volume -- maybe ?


    cheers
    Dave
     
  2. jcsd
  3. Aug 24, 2012 #2

    Bobbywhy

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    davenn, I’m no chemist, but I think the volume of your water did indeed decrease when you added those materials, probably because of the mechanism described below. (I reserve the right to be completely mistaken!)

    “Volumes do not add when you mix things together because the intermolecular forces of a mixture are different than the ones in the pure substances. The way this is handled is by using a property of mixtures called the "partial molar volume."

    "There are a couple of reasons why volumes of solution are not additive. One important one is that some chemical reaction occurs. The second is especially important if one component is water. Liquid water has an open structure, that is, the water molecules even in the liquid form molecule sized structures of lower density than the mean. The addition of ionic and/or polar solutes can "collapse" this structure resulting in an increased density. To complicate matters, the solute may also have structure that may be smaller than or greater than the average density of the solid. This will result in an expansion (or contraction) when the solute is dissolved.”
    http://www.newton.dep.anl.gov/askasci/chem03/chem03838.htm

    And see:

    http://en.wikipedia.org/wiki/Partial_molar_property

    Cheers,
    Bobbywhy
     
  4. Aug 24, 2012 #3
    Wow, I don't think I've ever known a noodle-science question before, nice work!

    I like Bobbywhy's answer a lot. I have had a hypothesis about why some pots seem to have more sauce than others, despite following the making instructions religiously, but without adding more water later. I think sometimes there may a lot of powder at the bottom that the water takes a while to absord into.
     
  5. Aug 24, 2012 #4

    davenn

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    Hi Bobbywhy

    you havent totally convinced me ;)
    interesting about the queried sugar and water senario in that link ... big difference there of course is that the sugar is actually disolving into the water. whereas my noodles are not, they are staying whole.
    Like you, I'm not a chemist either.. and Partial molar property isnt something I completely wrapped my head around

    Is there any validity about my added comment about the water cooling and becoming more dense thus taking up less volume ? remember we are going from boiling water to a water temp we can drink after ~ 10 mins

    Haha rooted :) first time for every thing ....

    cheers
    Dave
     
  6. Aug 24, 2012 #5
    My first guess would be that air pockets in the dry noodles, veg and powder displace the water. After soaking they get liberated to the surface the water goes down to fill their space.

    Just a guess.
     
  7. Aug 24, 2012 #6

    davenn

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    OK interesting thought :)

    Will need to fill a cup carefully with water and see if there are any visible bubbles ( they may not be visible)

    Dave
     
  8. Aug 25, 2012 #7

    sophiecentaur

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    The 'noodle effect' also happens in my cafetière. The level drops annoyingly, soon after adding the water to the ground coffee, even if you stir it whist water is added. Life can be tough when you want a full mug.
     
  9. Aug 25, 2012 #8
    You could try measuring the particle and bulk density of your powder.
    http://en.wikipedia.org/wiki/Particle_density
    http://en.wikipedia.org/wiki/Bulk_density
    There is always a lot of air in dry powder.

    Let's assume the density changes from 0.96 to 0.99 and the water level is initially at 10cm. You'd get 100mm/0.99*0.96 = 97mm. So the water level would drop by 3mm.
     
    Last edited: Aug 25, 2012
  10. Aug 26, 2012 #9
    There's some air stuck between the noodles which gets replaced by water. The air escapes, so the volume goes down.
     
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