# Chem lab in violation of gas laws

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So I have to explain how a water balloon, when placed over the top of a bottle with a piece of burning paper in it, gets sucked into the bottle. From what i understand, the combustion reaction within the bottle should expend the Oxygen, and thus decrease the volume of gas inside the container (Avogadro's law) However, if the volume of gas decreases inside the container, then the pressure should actually rise (Boyle's Law), giving an opposite effect of what I saw (the balloon being sucked into the bottle). Am i doing something totally wrong? I'm not so clear i should count the combustion reaction as changing the moles of gas inside the bottle, but i assumed correctly is it because I have to use the ideal gas law?

This is confusing the fuck out of me

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You're not decreasing the volume of the gas. That would be if you were crushing the bottle or something.

I think what you're affecting is the mass (moles), though chemistry is kind of fading from my memory...

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Avogadro's law states that mass (moles) and volume are directly related, a decrease in moles would yield a decrease in volume.

Plus, I don't think theres any law connecting moles and pressure, aside from the ideal gas law

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Avogadro's law states that mass (moles) and volume are directly related, a decrease in moles would yield a decrease in volume.

I'm pretty sure that's only if other things (pressure, temperature...) are held constant.

So, if the moles in the bottle decrease, different things could happen depending on what you're holding constant. Assuming the bottle maintains its shape (constant volume), the pressure in the bottle should go down.

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OH! Right

I forgot to mention the bottle was made of glass, so while moles is being changed, volume cannot change and thus pressure must change

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Look at your equation for the combustion of of whatever you're burning. (would be so kind as to provide it?)

Until then, I'll use a generic methane + oxygen equation as an example:

CH4 + 2O2(g) → CO2(g) + 2H2O(l)

Notice that the overall number of molecules of gas decreases from 2 to 1 as the reaction proceeds, resulting in the decrease in pressure, since according to the ideal gas law one unit of ANY gas will want to take up a given space totally regardless of how complex the molecule is.

You could argue that the heat due the the candle or whatever would expand the resultant gas but the effect of that would be insignificant compared to the change in the number of moles of gas.

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It was a live demonstration, so the combustion of Oxygen and paper (whats the chemical formula for that?) would be needed.

I understand it now though, the phenomenae is ONLY due to Avogadros law, my chemistry teacher misled me a bit saying during the lab that it was due to atmospheric pressure acting on the top of the balloon. However, I now see it was because the reduction in moles caused a reduction in volume. The reduction in volume couldn't of had occured with the glass bottle, so the volume decreased with the only elastic part of the closed space (the balloon) which then sucked it into the bottle. It has nothing to do with pressure

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well the pressure is kind of a side thing

the atmospheric pressure does push the thing in (due to pressure differences inside and outside the bottle), but that's not the best way to explain the concepts here

(whats the chemical formula for [wood]?

more complex than necessary here now that I think about it

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i don't get what's the issue. the oxygen inside the bottle is being used up, therefore reducing the pressure inside the bottle. the balloon gets sucked into the bottle because the pressure inside is less than the atmospheric pressure outside.

did i miss something?

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Argh, why do people say that the oxygen is "getting used up" to explain the decrease in total moles of gas. They make it sound as if the oxygen simply ceases to exist because of combustion.

what if there was a combustion reaction that increased the number of moles of gas while still "using up" the oxygen

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guess this is why i'm not a scientist.

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