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Every day I take 10g of hydrolysed collagen dissolved in about 200ml of water.

First I put the cup of water on my digital scale (which measures in 0.1g increments) then I add exactly 10g of collagen and stir it with a spoon until it dissolves.

As the collagen dissolves, the total weight always gradually decreases by about 1.5g - 2g in total. If the total weight of the cup, spoon and water is 300g, when I add the collagen, it becomes 310g, then as it dissolves over the next 15 seconds it decreases to about 308g. All I've done is stir it.

I can't see how this is due to evaporation since I can leave the pure water on the scale for 5 minutes beforehand and the weight doesn't change; but as soon as I stir the collagen in it always decreases.

What's happening?

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    $\begingroup$ It is likely that the scale does not keep the tare. What happens if you stir the water only? Or does your container at start and after you have prepared your solution and emptied it weight the same? Check theses. $\endgroup$ – Alchimista Oct 14 '17 at 9:43
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    $\begingroup$ The change in density might make up for a few micrograms (buoyancy in air). Do you use a large spoon, that you first lay on the cup, but afterward leave in the cup, so it creates a torque to one side? $\endgroup$ – Karl Oct 14 '17 at 12:02
  • $\begingroup$ @Karl: With our scales, torque was never an issue. The maintenance guy explicitly showed me after gauging how it was irrelevant where you put your object (for our lab scale at least). You could even place very heavy and very light object right on the edge of the scaling plate. $\endgroup$ – basseur Oct 14 '17 at 12:32
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    $\begingroup$ This is quite not like scale should be used - that's the problem. No adding anything to container on scale, no mixing on scale, otherwise you see what happens. $\endgroup$ – Mithoron Oct 14 '17 at 15:28
  • $\begingroup$ True, this only leads to contamination and even corrosion of the scale. Seen it in other labs in our institute. $\endgroup$ – basseur Oct 14 '17 at 16:45
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Because collagen does not react with water and you say, no (measurable) evaporation of water is evident, there are only two options as to what is happening:

  1. Matter is converted into energy: If virtually no evaporation or spillage took place, and your scale is working properly, this is what must have happened. Using the formula of mass-energy equivalence $E=mc^2$ (E = energy, m = mass, c = speed of light in a vacuum) we can calculate the amount of energy that should result from a "loss" of 2 grams: $$ E = 0.002~kg\cdot(2.998\cdot10^8~m/s)^2 $$ $$ E = 0.002~kg\cdot8.988\cdot10^{16}~m^2/s^2 $$ $$ E = 1.798\cdot10^{14} (kg~m^2)/s^2 = 1.798\cdot10^{14}~J$$ so... that would be a LOT of energy.

  2. Your scale is not working as intended.

I'd go with option 2.

Now on a serious note: We do have a scale (precision of 0.0001 g) in our lab that occasionally has the same problems. Sometimes I would measure an empty reaction flask and it would be fine but some other time I could watch the measured weight drop by the second, "losing" up to 100 mg. The scale was checked by a professional but he could not find anything wrong with it.

My advice is: try another scale, see if the problem persists.

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    $\begingroup$ Option 3 OP isn't using his scale properly. $\endgroup$ – Mithoron Oct 14 '17 at 15:26
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    $\begingroup$ I tried stirring some plain water and the weight decreased by about one gram over 20 seconds, so the machine is the reason. $\endgroup$ – EmmaV Oct 14 '17 at 19:24
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    $\begingroup$ The apparent weight displayed on the will depend how much of the spoon is below the water surface. You should be able to demonstrate that effect easily in a controlled, repeatable way. (This is just Archimedes' principle, and buoyancy). $\endgroup$ – alephzero Oct 14 '17 at 20:00
  • $\begingroup$ "weighing accuracy of 0.0001 g" - perhaps you mean "precision", especially since you seem to indicate it is nowhere near as accurate as that? $\endgroup$ – Aaron Hall Oct 14 '17 at 21:18
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    $\begingroup$ @alephzero Sorry, that is just plain nonsense. $\endgroup$ – Karl Oct 16 '17 at 17:20

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