Thursday, April 30, 2015

Bleg: reactions that show conservation of mass?

I was talking with a middle school teacher that is looking for a reaction that is more interesting than "vinegar plus baking soda" that demonstrates conservation of mass in an interesting fashion. So, we're looking for:
  • A synthetic reaction that demonstrates conservation of mass (yes, they all do.)
  • This will be done in a school, so the ingredients need to be readily available (purchased at a supermarket, not at a lab supply house.)
  • The ingredients need to be relatively non-toxic.
  • Color changes, physical property changes would be better.
  • Can be a demo, doesn't need to have students doing all of them.
Any ideas? Thanks in advance. 

18 comments:

  1. Copper cycle lab

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  2. Not a chemical reaction, but I remember a middle school science teacher putting a few chunks of dry ice in a balloon, tying it shut, and weighing it. After the dry ice sublimed and the balloon was full of gas, he weighed the balloon again and showed us that the mass didn't change.

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  3. Maybe a baking soda and vinegar reaction with a balloon on the coke bottle. Sit the whole thing on a top-loading balance. Observe lack of weight change, then pop the balloon and observe the weight lost from the gas escaping.

    I remember a teacher doing this back in the 50's when I was in elementary school.

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    1. Yeah, I think that's the current idea, but they were hoping for something more interesting.

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    2. Well, they could repeat it with sodium and water, but it might be difficult to keep the balloon from melting! :-)

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  4. Hydrogen peroxide to water plus oxygen, catalyzed by iodine maybe? About a gram of peroxide makes about half a gram of oxygen, so scale could be reasonable (more reasonable with lab grade 30% than grocery store 3%). Could do the same balloon routine as baking soda vinegar.

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    1. Or catalyzed enzymatically (use a small piece of liver)

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    2. I like active dry yeast as a decomposition catalyst there. If you start with 12% peroxide, available as "40 Volume developer" at a beauty shop, it actually gets boiling hot. You can add soap for foam and food coloring for extra awesomeness, makes a great 'volcano' mixture. Note that 12% is strong enough to leave a painful white mark (that will last a couple hours) if it touches your skin, so wear gloves.

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  5. Make a basic aqueous solution solution containing an indicator. Add dry ice to the solution and quickly cap with a balloon. You will see a color change as carbonic acid is formed and reacts with the indicator. Of course you also get to see the phase change as CO2 sublimes. This will take some fine tuning to perfect the amounts, but just make sure you don't add so much dry ice that the flask gets cold and starts condensing moisture from the air! See below:

    https://www.youtube.com/watch?v=Hf2bCA1i-Oc

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  6. Personally, I always enjoyed the Iron oxide + aluminum can = Aluminum oxide + solid iron reaction. Of course, that may be more high school chemistry. Requires Thermite and magnesium.

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  7. I am the man to answer this question, as I demo this at a local CC as follows:

    First I blab about Antoine Lavosier, and then I do two Demos, in this order, A and B

    A.) Place 1 M KI solution in a test tube. Place 1M solution Pb(NO3)2 in flask. Carefully add test tube to flask, stopper, dont mix. State: "nothing enters, nothing leaves, a closed system." Weigh it, weight again. Flip, mix, get yellow PbI2 ppt. Weigh again, same mass. Here it is: https://www.youtube.com/watch?v=J5hM1DxaPLw

    B.) Next, pull out steel wool. Weight it. Ignite with 9 V battery. Say out loud: "what do you think? Increase mass, decrease mass, or stay the same". Weigh again. Point out the L of CM must hold, so what happened?

    Once you do them demo, you understand why the phlogistion theory was so persuasive.

    Your welcome.

    I'm trying now to think or Org Chem demos that wont kill my students...

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    1. Oops. Didnt see the limitations of purchasing.

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  8. Reaction of Magnesium and Sand is visually appealing and could be weighed...

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    1. When done in a test tube it will melt it and drop the bottom. Maybe a bit "hot" for a classroom. Same with Thermite....

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  9. I believe what I did in high school was to heat calcium carbonate in a crucible over a Bunsen burner. Get accurate mass, heat for a while, get another accurate mass. But where did the rest of it go?! Into the atmosphere, (boys and) girls.

    I'm not sure whether this counts as "interesting", though.

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    Replies
    1. (This would have been middle school in the US system, of course.)

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  10. In my school our teacher took a matchstick head, put it in a test tube, sealed the test tube by melting it shut, weighed it. Then applied bunsen burner to test tube to set off matchstick, weigh again. Works well when combined with prior experiment the students can do themselves: weigh match stick, burn match stick, weigh again (without sealed atmosphere).
    I remember most of us were mainly impressed by the molten glass, not the experiment itself...

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  11. They should make some cheese in class. http://www.epicurious.com/recipes/food/views/fresh-homemade-ricotta-234282 I do this one in my chem of cooking (and at home). Easy way at it: 1 tbsp vinegar for every cup of milk. Heat. Form curds

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