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It isn't salt or sugar.

Was in a shaker at a restaurant - caused burning/bleeding sensation after consumption. Made up of small white granules that seem moist - all sticking together. The restaurant did not know what it was.

Tested properties:

Dissolves in water and vinegar.

When dissolved in water there was little effect. In vinegar, it produced substantial heat.

When mixed with lemon juice, the powder turned orange, heated up enough to crack the plastic container it was in, and gave off smoke.

No other information - based on these observations can any suggestions be made as to what it is?

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You kept some of the unknwon as evidence, didn't you? And a witness, too? Because if you get harmed, document it, and seek both medical as well legal counsel.

Turning to the chemistry: Likely, while dissolving X in water, you felt some heat, too? Was the colorless solution obtained, when diluted further with additional water and then applied on skin a bit "soap like", felt slippery?

Vinegar is acidic, because of the acetic acid present. Hence, mixing / dissolving the unknown proceeds with evolution of heat is indicative for the unknown acting as base that is neutralized. Such reactions generate heat. Similar token with lemon juice, equally reacting acidic, because of the presence of citric acid (even a more acidic than acetic acid): heat evolution possibly because of the neutralization of X (as a base) with it.

An additional indication that X is something basic were the action on diluted neutral juice of red cabbage. If basic, the addition of X would yield a blue/greenish stain, in contrast to acids turning it red.

Speculation: it could be remaining NaOH.

  • Because sodium hydroxide actually (NaOH) is a base, that dissolves with ease in water, it reacts with the emanation of heat while dissolved in acids. (Even if I am surprised the lemon juice experiment heated up enough to shatter the container.) Even while preparing a dilute solution of NaOH in water, say 1 M as commonly used in the chem lab, the heat generated is sensible across the glass wall of the container used.

  • Because sodium hydroxide actually is used as a cleaning agent, to remove oily residues, to remove fat and grease (in other instances, to prepare lye soap) from pans (provided they are not made of aluminum).

  • Because NaOH is hygroscopic, in other words, the little white grains of it quickly look wet and stick to each other when exposed to humid air.

  • Because in pure form, sodium hydroxide is corrosive; and aqueous solutions of it are hazardous, too (pointing to your bleeding sensation).

Thankfully, such accidents in restaurants occure not often; yet there are examples ref1, ref2. Bars ref3 use caustic soda to rinse their pipes of beer & other beverages, too, connecting the counter with the stock, often out of sight, in the basement.

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    $\begingroup$ Your answer makes sense, but how do you explain the orange colour when the powder was mixed with lemon juice? $\endgroup$ – getafix Sep 18 '16 at 11:42
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    $\begingroup$ (+1) My first thought was NaOH too. The only slight doubt I have is that the dissolution of NaOH is noticeably exothermic, as anybody who has tried to prepare a stock solution can verify. :) Of course, if you just dissolve a little bit of solid in a lot of water, you wouldn't be able to tell. $\endgroup$ – orthocresol Sep 18 '16 at 11:51
  • $\begingroup$ Is there anything about NaOH (or perhaps inside this particular mixture) that, when mixed with lemon juice, could lead to an orange colour? $\endgroup$ – Wharf Rat Sep 18 '16 at 12:09
  • $\begingroup$ @getafix and Elkal Anwa The stated observation of orange coloration is indeed a bit puzzling. Aqueous solutions of NaOH + (acetic or citric acid alone) should remain colorless. A lemon is more complex, a mixture. Currently, none of the said reagents are with me to probe it. $\endgroup$ – Buttonwood Sep 18 '16 at 17:01
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    $\begingroup$ @Buttonwood: I happened to have some lemon juice and NaOH lying around, so I tested it, and there is indeed a noticeable color change from yellow to orange. Adding clear vinegar reverses it, so it would seem that something in the juice is reversibly changing color with pH. $\endgroup$ – Ilmari Karonen Sep 18 '16 at 20:56

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