The setting I am working with has several individuals with shapeshifting powers. To deal with the inevitable “shapeshifters seem to violate conservation of mass while shapeshifting” issue, the rules of the setting are such that shapeshifters can seemingly gain excess mass from nowhere (they cannot but it appears that way from the characters’ limited perspective), but they cannot get rid of it in the same way. Instead they get rid of excess mass when they shift by having the excess mass slough off, break down supernaturally fast into simple gaseous compounds of their component elements, and disperse into the atmosphere.

The shapeshifting process is relatively hard magic, interacting with regular physics in a consistent, documentable manner. Having the excess mass evaporate away in gaseous form (even if the reverse isn’t true) makes it seem to the reader like the magic is at least “doing business” with conventional physics, similar to how The Dresden Files solves the shapeshifting problem with ectoplasm or Animorphs does with Z-space.

The four most common elements of the human body (making up 96.2% of the body by mass, though not molarity), carbon, hydrogen, oxygen, and nitrogen, can all form common atmospheric gasses (methane, carbon dioxide, O2, N2, water vapor) and be lost that way. However, it seems as though several elements cannot be easily lost in gaseous form (calcium in excess bone matter is the primary offender) and would likely be left behind as solid residue or waste products neither turned into gas nor part of the shapeshifter’s new form.

Several elements would also have to be bound up in larger compounds to avoid being highly reactive or toxic (e.g., calcium as CaCO2), so it is not a simple manner of figuring out what percentage of the human body is not made of CHON by mass. Calcium phosphate/calcium sulfate is one way I thought of maybe getting rid of calcium, phosphorus, and sulfur, but I am not sure if there are better/more efficient compounds available.

I am interested in figuring out how much solid waste matter would be left behind in this shapeshifting process, and what kind of compounds would it form assuming the end products are relatively stable and non-toxic. For example, the characters come across a white paste, subsequent analysis finds the paste to be high in calcium, potassium, chlorine, and magnesium, and based on this the characters are able to go “yep, we’re in shapeshifter country”. The best way I thought to address this question is to figure out how much waste matter would be left over if as much of the human body as possible was transmuted into simple gaseous compounds of its respective materials and then rescale it for however much excess mass is being lost in the shapeshifters.

Additional Parameters:

  • As little mass as possible is left behind - Ideally no mass left behind if at all possible, I can work with that (and actually would prefer it), but I am trying to minimize the amount of excess waste matter that is left behind in a solid form. I am trying to minimize the amount of solid waste matter produced in the process to reduce the chances of shapeshifter residue being highly noticeable. The shapeshifters try to hide it through their own actions but I am trying to avoid results that are so obvious that it is impossible to ignore. Avoiding extremely exotic compounds is also encouraged for the same reasons.
  • Assume the composition of the shapeshifters' body is identical to the human body and they have the same physiological needs as humans. So “their shifting produces compounds that would be toxic to humans but their non-human physiology tolerates it” doesn’t work.
  • More or less room temperature conditions, or at least conditions encounterable in everyday life - This is mostly because I don’t want the shapeshifters to get cooked by their own bodies as they shift. Plus some compounds may be gaseous at states of 400+ °C but they will condense quickly as they lose heat. Water vapor gets a bit of a pass because steam is something that is not inherently lethal and excess water condensation isn’t really noticeable, other compounds that fit the same parameters are okay. Ways of doing it without producing significant amounts of hot water vapor that could scald someone are encouraged.
  • No toxic and/or highly combustible products - Some components of the human body can be broken down into toxic or highly reactive substances. Like potassium metal (of which approximately 120g can be produced from the average human body), which reacts violently and exothermically when it comes in contact with water, or hydrogen cyanide, which is…hydrogen cyanide. I’d like to avoid these compounds for the simple virtue of not having the shapeshifters burst into flames or poison themselves when they try to shift. Hydrogen gas might get a pass because while highly flammable it won’t burn without a spark, nevertheless I’d like to avoid it if possible.
  • Input and output energy of the reaction is explicitly compensated for by the magic - The former because if there wasn't some supernatural means of causing compounds to be altered there couldn't be any rapid shape-shifting in any given setting period, and the latter because the act of breaking down a lot of organic tissues into simpler compounds would release a lot of energy (given this is basically what digestion is on a shorter timescale) and again, it would be bad if shapeshifters burst into flames whenever they shifted.
  • New compounds can be formed as needed without regards for things like activation energy but atoms cannot be changed into different elements - So it is not possible to simply atomically transmute all the calcium atoms into hydrogen ones and call it a day.

EDIT: Significantly revised the question to make it clearer what I am asking, what the parameters are and why, and what rules are allowed to be bent within the fictional setting.

  • $\begingroup$ It takes five minutes to collect this information from wikipedia, and sum up the numbers. After you have decided if you actually want elements or not, because your last point leaves much to be guessed. $\endgroup$ – Karl Feb 4 at 20:09
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    $\begingroup$ Matter in a gaseous state is not 'lost'. Nothing about your question makes sense, scientifically. $\endgroup$ – Morris The Cat Feb 4 at 20:14
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    $\begingroup$ @MorrisTheCat 'Lost' as in 'removed from the object to dissipate into the atmosphere', not 'violating the conservation of matter'. In the same way that boiling a pot of water eventually leaves it empty because the water evaporates into water vapor. The atoms aren't 'lost' because they miraculously disappear, they're lost because they disperse into the air and are outside the pot. $\endgroup$ – user2352714 Feb 4 at 20:26
  • $\begingroup$ Sulphur is not a gas at room temperature. While it's true that it can form sulphur dioxide, in doing so it prevents the formation of an equivalent amount of carbon dioxide. So it's probably best to leave either carbon or sulphur out of the list. $\endgroup$ – WhatRoughBeast Feb 4 at 21:09
  • $\begingroup$ @WhatRoughBeast lots of other gas-phase carbon bearing oxygen-free wossnames, though. Methane, for one. Hydrogen cyanide for another, if it were warm enough. Even sulphur can form hydrogen sulphide, and there are gaseous thiols like methanethiol that have a carbon, too. $\endgroup$ – Starfish Prime Feb 4 at 21:24

Cremation does what you propose.


This estimates 3-5 pounds of ash after cremation. Volatiles leave and carbon leaves as CO2. The ash will mostly be calcium salts and oxides from the bone. Sodium and potassium salts will be present too since that is a major component of organic ash generally and we contain a fair bit of those. There will be traces of metals since those are in the body too; mostly iron and small amounts of the others.

  • $\begingroup$ In addition to trace amounts of heavy metals if any consumption of them had occured in the individuals life time. Humans cannot excrete heavy metals as waste products, so they remain in the system forever. $\endgroup$ – hszmv Feb 4 at 21:01
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    $\begingroup$ @hszmv That is not true. An acute lead intoxication is usually down to normal after ten years or so. Most other metals are also purged over time, unless fixed in bone tissue or similar. Of course a permanent damage might remain, or you might not survive that long. $\endgroup$ – Karl Feb 4 at 21:32
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    $\begingroup$ @hxzmv Also, lead toxicity kicks in around 50 ug / dL. The average adult has 5 L of blood volume, so that's ~2.5g of Pb. Even in very acute cases you wouldn't notice this extra mass vs. the variation within a population $\endgroup$ – Punintended Feb 6 at 17:21

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