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Thread: Need help confirming or not confirm arguement

  1. #1 Need help confirming or not confirm arguement 
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    Im in 10th grade and today in Chemistry we did this one lab which was pretty grade school. We took a 100ML beaker and put a cube of ice in it. The total mass of the beaker before the cube melted was 84.98 grams and the weight after the water melted was 84.97 grams so the weight of the whole thing had decreased by .01. The other kids in the class had also gotten the same result the mass of the entire beaker and the water had decreased. NOW my argument was to why it decreased was that the ice before was sitting in one spot so the weight of the ice cube was more concentrated in one spot, but as the water melted it distributed its weight along the bottom of the container making the weight seem less but the mass stayed the same. The other kids in the class argued that just because the water melted it should not change the weight of anything. BUT everybody's experiment showed that the water weighed less that the ice. All I wanted to know is if i was right so then when i go back to class i dont look like a fool.


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    Quote Originally Posted by FifthGoblin View Post
    Im in 10th grade and today in Chemistry we did this one lab which was pretty grade school. We took a 100ML beaker and put a cube of ice in it. The total mass of the beaker before the cube melted was 84.98 grams and the weight after the water melted was 84.97 grams so the weight of the whole thing had decreased by .01. The other kids in the class had also gotten the same result the mass of the entire beaker and the water had decreased. NOW my argument was to why it decreased was that the ice before was sitting in one spot so the weight of the ice cube was more concentrated in one spot, but as the water melted it distributed its weight along the bottom of the container making the weight seem less but the mass stayed the same. The other kids in the class argued that just because the water melted it should not change the weight of anything. BUT everybody's experiment showed that the water weighed less that the ice. All I wanted to know is if i was right so then when i go back to class i dont look like a fool.
    Regardless of where in the beaker the ice was sitting, and how the water distributed itself in the beaker, you were weighing the total weight of the beaker and water. The .01 difference in weight was probably due to evaporation of the water as it melted.


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    ▼▼ dn ʎɐʍ sıɥʇ ▼▼ RedPanda's Avatar
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    It could have been down to poorly made scales being effected by the distribution of weight.
    Or it could also be because of evaporation.

    Ice is less dense than water, but an ice-cube will weigh the same as the water it forms when it melts (as long as none of it evaporates).

    How did you melt the ice?

    Maybe you could do the experiment with a sealed container and see if the weight drops again.
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    We melted the ice by putting our hands under the container and warming the beaker up btw there was a lid on it but it wasn't very secure... so it changed because of evaporation? But how could the water go from a solid to liquid to gas so quickly?
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    Quote Originally Posted by PhDemon View Post
    The mass should remain the same.

    Was the outside of the beaker clean and dry? Was the beaker and ice/water on the balance the whole time? If not a plausible explanation could be that the beaker was wet when you did the first measurement and when you removed it and placed it back on for the second measurement a drop of water had fallen off the beaker.
    Also did you carefully clean and re-set the TARE on the balance and make sure it was exactly 0.0000 g before every measurement? If the balance was not clean and dry there could have been dust etc. on the pan which was brushed off between measurements.

    There could be any number of other reasons for very small discrepancies (other than those above) although you would expect to see both positive and negative artifacts if it was an issue with the balance.

    I suggest you do a test to see if the mass change is real and significant. Do the experiment a number of times, leaving the beaker on the balance as the ice melts and carefully cleaning and zeroing the balance before each experiment. Calculate the %mass change (not absolute mass change as the weight of each ice cube will be different) on melting. Calculate the average %mass change over all experiments and the standard deviation. From this you can calculate an error bar on the % mass change. You can do all this simply in Excel using the MEAN, STDEV and CONFIDENCE functions (your teacher should be able to help you with this if needed). I'm guessing the error bar will comfortably encompass zero. I.e. there is no significant mass change.

    Try this (it is a good exercise to learn how science works) and post your results.

    Yea I had a feeling that my argument was not holding up and by the way your probably right the little changes every time we measured it were probably the cause of the mass decrease. Also the previous groups had cleaned out the container without drying so that was probably another discrepancie. SO i was wrong! but that's okay because you learn by failing!
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    ▼▼ dn ʎɐʍ sıɥʇ ▼▼ RedPanda's Avatar
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    Quote Originally Posted by FifthGoblin
    We melted the ice by putting our hands under the container and warming the beaker up btw there was a lid on it but it wasn't very secure...
    You could try covering the top of the beaker with cling-film/shrink-wrap.

    Quote Originally Posted by FifthGoblin
    But how could the water go from a solid to liquid to gas so quickly?
    The moment the ice started melting (which was probably before you even put it in the beaker) there was water forming.
    That water instantly started evaporating. And the more your hands warmed the water, the faster it evaporated.

    Unusual fact #31: Even the ice-cubes in your freezer evaporate.

    Quote Originally Posted by FifthGoblin View Post
    SO i was wrong! but that's okay because you learn by failing!
    I would say that both you and the other students are correct.

    Evaporation is definitely a factor.

    But there are 'mechanical' aspects to take into account as well.
    For example (as PHDemon pointed out) it is worth weighing the same beaker several times, resetting the scales each time.
    Or (as PHDemon also pointed out) "Was the outside of the beaker clean and dry? Was the beaker and ice/water on the balance the whole time?"

    And looking for possible issues that could change your measurements is part of doing good science.
    Even if it turns out that the scales aren't affected by the position of the ice, it was worth thinking of.
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    As simple as this experiment seems, when you examine it closely, it's an excellent lesson in the care and rigor which should be applied to the scientific method.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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