A new twist to show a buoyant Lavoisier

September, 2016

Lautem, Timor-Leste

From the far mountains of Timor-Leste:  a stunning new development in the already stunning Lavoisier conservation of mass demo!

Standard activity:

*Part one:  bottle with vinegar, a bit of soda wrapped in tissue, and a balloon all weighed on a digital scale, and weight noted.  Tissue with soda then stuck into the bottle, and bottle quickly capped with balloon.  Weight again noted.


*Part two:  same experiment, but now capped with bottle cap instead of balloon.  (Best to use a carbonated soda drink bottle because water bottles and their caps are not designed to withstand pressure.  If it leaks, your result is no good.)


It is also possible to do this without the soda and vinegar, instead using a carbonated drink, shaken to stimulate the fizz.  You get the same results with no reaction, just CO2 coming out of solution.

Spoiler alert!  But this is so fascinating, I know you’ll want to try it even after I spoil it.

In part one, the weight (mass) displayed by the electronic scale goes down, seeming to contradict Uncle Lavoisier.  (When one of our trainers first saw that happen he said, with a look of disappointment, “It looks like we can’t use this to prove conservation of mass!”) But in part two the great French chemist is vindicated as the mass stays the same.

The extended discussion, in our experience, is not about Lavoisier at all, but properties of gas, atmospheric weight and pressure, functioning of scales, and buoyant force.  All quite complex and abstract unless we do more pratika!

I have often suggested the gedanken experiment of doing the above activity under water.  Then water instead of air is displaced by the balloon above the scale: much more visualizable.  But still, a real experiment would be better.

We were thinking of how to actually do it, and whether our scale would be ruined if we put it at the bottom of a tank when we realized:  we don’t need no stinking scale!

New activity:

*Part one: cap a carbonated drink bottle with a balloon and put it at the bottom of a big bucket or tank of water.  (If it floats, fill it completely to the top with water or more soda and try it again.  Balloon must be completely empty and flat.)  Shake it and observe.

*Part two: same experiment, now capped with bottle cap instead of the balloon.

Spoiler alert:  the following photo shows the result of part one.


It is easy to do this experiment in this part of the world, because most homes and schools have water tanks in the bathroom.   This photo is the bathroom of our host school for this training: Central Basic School, Lautem.



Some teacher always wants us to write down the chemical equation. Is this true interest, or are they just testing us? Fortunately for me, Mestra Mimi had the equation for the baking soda and vinegar reaction ready to present to them, and everyone was happy.

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