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Thread: Proving Law of Conservation of Mass Doesn't Work

  1. #1 Proving Law of Conservation of Mass Doesn't Work 
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    I have often tried to use the baking soda and vinegar lab as a way for my students to prove the law of conservation of mass/matter. I have never been successful in having the before and after mass be the same. It has alway been less by at least a gram. This is despite sealing with tape, double bagging it etc. The only explanation I have read is that the buoyancy of air surrounding the inflated balloon or bag has more affect on the products due to the increase in surface area. Can anyone give me a definitive answer and a resource to support this conclusion.
    Thanks for your help. A frustrated middle school teacher!


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  3. #2  
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    If you try to measure the mass by measuring the weight, there will be an error due to buoyancy of the materials being weighed.

    For example, as you know, helium has mass, but a helium balloon will rise in the air. If you weighed it, you would get a negative weight.

    Buoyancy depends on volume, so if the volume of gasses is greater after the reaction, the products of the reaction will weigh less.
    Buoyancy - Wikipedia, the free encyclopedia

    According to Archimedes principle
    Buoyancy = weight of displaced fluid
    The density of air is about 1.2 kilograms per cubic meter and a cubic meter is 1000 liters, so the buoyant force is equal to about 1.2 grams per liter of air displaced.

    Density of air - Wikipedia, the free encyclopedia


    Last edited by Harold14370; February 3rd, 2013 at 05:52 AM.
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  4. #3 Thanks for the reply 
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    Thanks Harold for the reply
    So what you are saying is that there will always be a loss of mass when trying to prove LCM when a gas is produced. I assume if you did it in a vacuum it would work??

    Is there any lab that would be better to use, especially for middle schoolers as they are quite literal and what they remember from the vinegar and baking soda lab would be that the law is incorrect because of this observed change?
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    No, there isn't a loss of mass. There is an error in measuring the mass. Mass is not the same thing as weight, and you should be teaching your students the difference.

    I think if you had the students estimate the volume of gas produced, then make the correction for effect of buoyancy on weight, they should understand how mass is conserved.
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  6. #5  
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    I definitely agree with teaching them how to compensate for state change. That will come in very handy later on.
    "Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us." -Calvin
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  7. #6  
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    I like that approach.

    It's also an introduction to the concept that a lot of scientific time is devoted to designing and testing equipment. Not just for the process in question but in accurately observing and measuring the result. When is it necessary to be absolutely perfect and how much time, effort, expense is required. Where is the cutoff point for not quite perfect.
    "Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill
    "nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke
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    This experiment can also be done by using a plastic bottle, which doesn't expand when pressurized with gas. That should simplify matters.
    Lab: Conservation of Mass
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    i still have to find an answer to this question. currently i'm at this position where in femto photography,by ramesh raskar from mit have developed a camera that can take pictures at one trillionth of a second. femto photography - Bing Videos in this video you can see the glow in the tomato when light is bouncing inside it. i wonder what difference does that light make. and how much? is there a way to measure that?
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