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I was going through a bunch of interesting science 'facts' and one entry went this way:

Name an element whose mass decreases when it is dissolved in water and increases if it is burnt.

I tried guessing, but that didn't help. So I had a look at the answer: it was Sulfur

There are two questions that struck my mind after I saw the answer:

  1. What does mass decrease/increase as stated here mean?(Does it mean that mass of sulfur along with other elements combined with it changes/or anything ambiguous such as change in actual mass is taking place) )

  2. Is sulfur really the correct answer to that question? If yes, then why does sulfur show this anomalous property? Are there elements that display a similar property?

Any suggestion is welcome.

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    $\begingroup$ On combustion $\ce{SO2}$ is produced which has a greater mass than sulphur. $\endgroup$ – user14857 Oct 28 '16 at 11:19
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    $\begingroup$ Shouldn't this question be true for more elements then??or sulphur is exceptional one. $\endgroup$ – I am Back Oct 28 '16 at 11:21
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    $\begingroup$ Now this is one huge piece of bovine excrement. Sulfur does not dissolve in water, which means it does not change at all when you try to dissolve it in water, which in particular means its mass does not decrease. (It may dissolve in strongly alkaline solutions upon heating, but that's another story.) As for burning, sulfur forms gaseous products that fly away, which may be naively interpreted as "vanishing" or "decreasing mass", but hardly vice versa. $\endgroup$ – Ivan Neretin Oct 28 '16 at 11:47
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    $\begingroup$ Yes this 'interesting science facts' article appears to be a pile of do-do! An atom i.e. element, cannot change its mass during a reaction, other than to gain or loose one or more electrons if ions are formed. But as the electron mass is $\approx 1/1836$ that of the proton the effect is tiny. $\endgroup$ – porphyrin Oct 28 '16 at 12:04
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    $\begingroup$ Except for tricks, matter does not change mass in the circumstances mentioned. It reminds me of: "Matter cannot be created or destroyed, nor can it be returned without a receipt" $\endgroup$ – Paul Ogilvie Oct 28 '16 at 21:35
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Upon reading the answers on Quora (thanks S007 for pointing that out) I realized this trick question is a lousy play upon two somewhat peculiar features of sulfur:

  1. When submerged in water (not "dissolved", mind you, for sulfur does not dissolve), its apparent weight becomes less, thanks to Archimedes and the buoyancy force. This is indeed true of any element and compound, but sulfur has density of about 2, so its apparent weight decreases about twofold.
  2. When burnt, sulfur forms a compound with oxygen which (containing all that sulfur plus oxygen) naturally weighs more than sulfur alone. Again, this is true of any element or compound that can be burnt, which are numerous. If anything, sulfur is "special" in that its atomic weight is 32, and the molecular weight of $\ce{SO2}$ is 64, so the increase is exactly twofold.

The way it stands now, though, the question is quite meaningless. Mass of any element never changes (barring nuclear reactions), that's the law of conservation. If you count compounds, then the mass would increase in any reaction (presuming we started from the pure element) and stay unchanged otherwise. If you count all compounds except gases, then the mass would sometimes increase and sometimes decrease, so there might be an answer, but then sulfur fails both conditions: it does not dissolve in water at all, and it all transforms to gases (that is, "vanishes") when burnt.

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  • $\begingroup$ The "decrease" in weight isn't Archimedes' buoyancy force but surface tension. If it was really buoyancy then large particles would float in water too. $\endgroup$ – MaxW Oct 28 '16 at 17:04
  • $\begingroup$ I think I addressed this properly: I said "when submerged", which pretty much rules out the surface effects (strong as they are). $\endgroup$ – Ivan Neretin Oct 28 '16 at 17:18
  • $\begingroup$ Wouldn't the mass of the element decrease in most reactions (or at least thermodynamically favourable ones)? Sure, the difference between S bound in an allotrope and S bound in SO2 isn't large, but it's not nonexistent, when you factor in the energy it sheds in forming the more stable compound; right? $\endgroup$ – Williham Totland Oct 28 '16 at 18:18
  • $\begingroup$ You mean, because of $E=mc^2$? Yes, it would; I ignored that. $\endgroup$ – Ivan Neretin Oct 28 '16 at 18:23
  • $\begingroup$ @IvanNeretin Einstein must be spinning in his grave. ;) It's still a stupid question relying on intentional ambiguity, of course (obligatory xkcd: xkcd.com/169), but it still presents a good opportunity to remind readers of such things, I should think. $\endgroup$ – Williham Totland Oct 28 '16 at 18:27
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  • The decrease in weight is actually due to buoyancy force and not due to any chemical reaction as such. Though hydrogen sulphide gas may be formed but it will be in negligible quantities. The relative density of sulphur is about 2.

  • On combustion, $\ce{SO2}$ is produced which has a greater mass than sulphur.

P.S: Did you know that "magic" is science in disguise ?


Apparently the question seems to be quite popular. https://www.quora.com/Does-sulfur-really-weigh-two-kilograms-when-dry-one-kilogram-when-wet-and-three-kilograms-when-burnt

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    $\begingroup$ A.C. Clarke: "Any sufficiently advanced technology is indistinguishable from magic." (emphasis from mine) I don'tknow about science. ;-) $\endgroup$ – Karl Oct 28 '16 at 14:27
  • $\begingroup$ @Karl "Word Play" =) $\endgroup$ – user14857 Oct 28 '16 at 14:30
  • $\begingroup$ upvoted for including the title of the quora question, which is the only thing that makes any of this make sense. What's interesting is the 1:2:3 ratio, not just decrease/increase, so it makes sense that someone would try to point this out in a clumsy way, and that further distortion could lead to the nonsense that the OP saw. $\endgroup$ – Peter Cordes Oct 29 '16 at 1:14
  • $\begingroup$ If you want to say that buoyancy forces "decrease weight" then you're also saying that you're weightless when you sit on a chair, since the chair provides an upward force that "decreases your weight" to zero. $\endgroup$ – David Richerby Oct 29 '16 at 22:40
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This is, mostly, nonsense. Your question didn't speak to weight, it spoke of mass. It is generally agreed that elemental sulfur exists as an S8 ring. (cyclo-S8). (really we know it exists in a number of allotropes at STP) It is quite water insoluble, (with a solubility of 2½ ppm).

It is a basic assumption of chemistry (with the exception of radiochemistry) that mass is fixed (conserved) and does not change in any chemical process (including phase changes and dissolution). It is possible that when S dissolves in water, the S ring is broken up so that the molecular mass is decreased - I don't know. When it is burnt (oxidized with O2), the product's molecular mass is quite a bit less than the 256 of S8, meaning the "answer" is nonsense.

However. As any physicist knows E=mc². So an increase in an atom's ENERGY increases mass by E/c². Of course the energy change will be small in burning it, and c is a very large number, so that the tiny mass increase is not significant for a chemist.

Similarly, if S8 is dissolved in water cooler than it is, energy will be lost, but the same "insignificance" argument can be made. For a chemist (as I said, except for radioactive elements (or those in high energy environments like fission reactors, fusion experiments, or particle colliders (or when exposed to cosmic rays))) Mass is conserved, it does not change upon dissolution, burning, heating or cooling. The is no chemical element which changes mass upon dissolution or burning, although there are plenty of chemical compounds, including molecules of just one element, that may change molecular mass upon a phase change or dissolution, or that react to form products of more or less molecular mass.

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    $\begingroup$ Could you stop answering as unregistered? You already have an account. $\endgroup$ – Mithoron Oct 28 '16 at 19:50
  • $\begingroup$ When sulfur is burnt, energy is taken out of it, so its mass decreases (by a tiny amount, as you said, but it's worth getting the direction right). $\endgroup$ – Williham Totland Oct 28 '16 at 20:51
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Theoretically yes, if something loses energy it also loses mass (seriously no joke). Also if it gains energy it gains mass(By Einstein's mass energy equivalence) Of course the energies involved in chemical reactions are in no way affecting mass of the molecule atom in any measurable way.

This applies to all forms of energy, potential, chemical, doesn't matter.

Example: the planet Saturn would be pulling harder at my toenail then any change might due to the potential energy of being in a gravity well (Earth).

What is the mass gain loss? We use Einstein's equation: $$\Delta M=\frac{\Delta E}{c^2}$$

So you see the $c^{-2}=1.1126501 \times 10^{-17} $ part will slap the mass change to something almost nonexistent.
So yes,the mass will change,it depends on you how do you measure such a small change.

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    $\begingroup$ I don’t see how this answers the question. $\endgroup$ – Jan Oct 30 '16 at 22:17
  • $\begingroup$ You should write your answer to actually answer the question asked. I see no reference to sulphur in your answer at all. $\endgroup$ – Jan Dec 19 '16 at 22:00
  • $\begingroup$ Okay, let me state my criticism in a different way. Yes, you are technically correct about energy and mass and losing one means losing the other. To answer the question asked (‘(Why) Does sulphur’s mass decrease when dissolved and increase when burnt?’) requires relating what you posted in your answer to sulphur, the process of mixing sulphur and water, and the process of reacting sulphur with oxygen in a strongly exothermic way. That exactly is what your answer is failing to do and therefore it does not answer the question. $\endgroup$ – Jan Jan 25 '17 at 17:17
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    $\begingroup$ Words have meaning, heck if SOx is sulphur why not call everything a frog? $\endgroup$ – jacob bogers Jan 26 '17 at 3:59
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    $\begingroup$ Congratulations. You found the flaw in the question. That should be the answer. See the other answers. $\endgroup$ – Jan Jan 26 '17 at 20:18

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