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I am not a chemistry person at all, so my apologies if my question seems dumb.

I am working on the set for a short film and in one of the scenes we would like the main actor to look at a sample of water, drop something into it and watch a black precipitate form. Clouding the water.

I have looked online to see what reaction would yield the desired result, however, my chemistry knowledge is very basic.

Could somebody here help me out and let me know what I could use? Safely. I understand that we need to handle all chemicals with care.

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    $\begingroup$ Could you just have the actor drop in some dark food coloring? Do you actually need solids to be formed that can separated out? $\endgroup$
    – J. Ari
    Apr 7, 2020 at 11:51
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    $\begingroup$ Too bad you can’t have a green precipitate and green screen in the black in post-processing. ;-) $\endgroup$
    – Ed V
    Apr 7, 2020 at 17:00
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    $\begingroup$ @EdV Recalling the old, tube-based TV and their color knobs, I speculate one may record with a non-green precipiate in front of a green screen, and then «simply» alter the color channels in the PAL / SECAM color space. Because changing colors selectively via huemap on .png with ImageMagick works so fine (e.g. imagemagick.org/discourse-server/viewtopic.php?t=20280). $\endgroup$
    – Buttonwood
    Apr 7, 2020 at 17:43
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    $\begingroup$ @Buttonwood Thanks for that link and information! I was just joking about the green screening, but barium sulfate gives an excellent white precipitate, starting from colorless solutions, and it could be flipped to black fairly easily with software post-processing. $\endgroup$
    – Ed V
    Apr 7, 2020 at 17:49
  • $\begingroup$ @EdV Indeed, I understood the comment as friendly. Re colorless / white BaSO4, I recall a lecture by Hervé This (a French chemist active to popularize science) stating «...and as soon you use yellow light instead of day light, you see the albumen («egg white», in French) turning into egg yellow, without [further] cooking, or chemicals» (because egg yolk literally translates into «egg yellow» in French). $\endgroup$
    – Buttonwood
    Apr 7, 2020 at 18:02

5 Answers 5

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I can think of iron(II) sulfide($\ce{FeS}$). There are many ways to make iron(II) sulfide but one reaction procedure will suit your purpose. It uses iron(II) sulfate and lime sulfur(calcium polysulfide) to form a clear black precipitate of iron(II) sulfide with a clear supernatent. See this e-book excerpt.

You can use other sulfides as well. See this picture (full picture here):

enter image description here

From left to right: silver sulfide ($\ce{Ag2S}$, black), lead sulfide ($\ce{PbS}$, black), nickel sulfide ($\ce{NiS}$, black-brown), copper sulfide ($\ce{CuS}$, dark indigo). These are formed by reacting $\ce{Na2S}$ with corresponding metal ions. If you want to fully make your solution black, then your options would be to form silver sulfide and nickel sulfide. You could also make your solution black by forming elemental mercury solution but that would be too toxic for your purpose.

Other ways include adding iodide to excess amount of bismuth(III) nitrate in dilute nitric acid but be careful not to overdo as excess concentration of iodide will make solution orange-red(See here).

If you do not want any reaction to happen then just dump manganese(IV) dioxide in water. They are black and insoluble.

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    $\begingroup$ Many transition metal sulfides are brown/black so they're enticing, but most of their water-soluble salts start out already coloured, sometimes quite intensely, and they could impart that colour momentarily to the aqueous sulfide solution. Iron(II) sulfate has a relatively light colour, though. $\endgroup$
    – Nicolau Saker Neto
    Apr 7, 2020 at 11:54
  • $\begingroup$ The reaction with Iron sulfate could be a good choice but the solution will be greenish so I don't know if the user is okay that the water will have this tint because he said a clear liquid. I would not use mercury for these reactions, they should then disposed of it in special containers. $\endgroup$
    – G M
    Apr 7, 2020 at 13:35
  • $\begingroup$ @GM see updated answer. $\endgroup$
    – Nilay Ghosh
    Apr 7, 2020 at 14:28
  • $\begingroup$ +1 that's better, the only cons is that by experience iron sulphate will give some greenish nuance to the solution $\endgroup$
    – G M
    Apr 7, 2020 at 14:38
  • $\begingroup$ What about managanese? I think the sulfate is almost colorless and the sulfide is fairly dark. $\endgroup$
    – barbecue
    Apr 9, 2020 at 17:28
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I would suggest an iodine starch reaction as shown in this video. This reaction is safer compared to the use of other heavy metals that must always be disposed of properly.

In this case, you have to dissolve first starch in water (for instant soaking bread in water and then filtering) or buying potato starch powder. The water will be still transparent but some starch will be dissolved in the solution.

You could try to add some ethanol to the solution this should improve the chance that the starch will precipitate.

The solution to be poured in should be made of iodine (I2) and an iodide for example potassium iodide (KI) (this solution can be found with the commercial name Lugol's solution). When you add this solution you will see appearing some dark spot the precipitate will not so evident if the solution is not saturated so you might want to try to maximize the amount of starch dissolved. enter image description here Furthermore is not actually black but a dark blue, however, I don't think you will notice the difference. Once finished, it can be disposed of in diluted water in the sink.

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    $\begingroup$ Chemically related (because the same compound is formed) is the iodine clock (e.g., youtube.com/watch?v=xLCDJ0m_qrk) with the added delay (movie speak: suspense) between mixing almost colorless liquids and the formation of the dark matter. $\endgroup$
    – Buttonwood
    Apr 7, 2020 at 20:24
  • $\begingroup$ Would this be a precipitate though? $\endgroup$
    – JMac
    Apr 8, 2020 at 17:19
  • $\begingroup$ @JMac No, both forming the iodine-starch complex equally will only cloud the colorless aqueous solution, and not form a solid precipitate. $\endgroup$
    – Buttonwood
    Apr 8, 2020 at 17:30
  • $\begingroup$ @Buttonwood the precipitate will not be very evident if the solution is not saturated, but you can try to add ethanol to facilitate the precipitation (I wrote it now in the answer) $\endgroup$
    – G M
    Apr 8, 2020 at 19:47
  • $\begingroup$ +1 for ingredients being (relatively) safe and widely available $\endgroup$
    – fraxinus
    Apr 9, 2020 at 15:19
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A classic lab experiment would be the formation of black lead(II) sulfide by mixing potassium sulfide with lead(II) acetate in an aqueous solution:

$$\ce{Pb(CH3COO)2(aq) + K2S(aq) -> PbS(s) + 2 CH3COOK(aq)}$$

Safety precautions must be taken due to neurotoxicity of soluble lead(II) salts. In place of $\ce{K2S}$ one can also use aqueous solution of ammonium sulfide or hydrogen sulfide.

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    $\begingroup$ Because of the considerable hazard of lead solutions, it may be prudent to avoid it altogether. A silver nitrate solution would give a similar effect, though ammonium sulfide may not work for this specific metal. I suspect there are better options still, but nothing comes to mind right now. $\endgroup$
    – Nicolau Saker Neto
    Apr 7, 2020 at 10:29
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    $\begingroup$ @NicolauSakerNeto Correct, silver would do too. Bismuth as well. I picked lead simply as the cheapest option assuming one would need a substantial amount of the precipitate to occur for the good footage. $\endgroup$
    – andselisk ♦
    Apr 7, 2020 at 10:53
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    $\begingroup$ Can someone tolerate the smell of hydrogen sulfide? TBH I can't ;) $\endgroup$
    – Nilay Ghosh
    Apr 7, 2020 at 11:00
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    $\begingroup$ @NilayGhosh Actors are paid to smell things that are even worse than this. Besides, I added $\ce{H2S}$ solution as the last option;) $\endgroup$
    – andselisk ♦
    Apr 7, 2020 at 11:14
  • $\begingroup$ I was planning to put condition infront of OP, "will tell only if you project me as the actor", but you ruined my plan ;) $\endgroup$
    – Zenix
    Apr 7, 2020 at 13:32
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I know fountain pen's are less often used nowadays, but you could drop some droplets of their ink into water and than take a macro photo / video:

enter image description here

(screenphoto from here)

There are plenty of companies in the field (e.g., Waterman, Caran d'Ache, etc.) offering ink. You might be surprised how little ink is necessary for this, e.g., just while rinsing the nib and the inner tank of your fountain pen. And you are not limited to black, or even a single color.

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    $\begingroup$ Yes, it would work, but mixing a clear or lightly colored liquid would be more surprising....that is the magic of chemistry, $\endgroup$
    – AJKOER
    Apr 7, 2020 at 17:41
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    $\begingroup$ @AJKOER Like pushing a white shirt into the yellowish aqueous solution of leuco-indigo which, just brought back into the air slowly becomes jeans-blue. Or the dark red by mixing the colorless solutions of $\ce{FeCl3}$ and $\ce{NH4SCN}$, «suddenly» disappearing by addition of $\ce{NaF}$ as in youtube.com/watch?v=Q4OfkdyMLRQ or with a nail: youtube.com/watch?v=2cknYNl5v6Y. Who knows what they have at their disposition. $\endgroup$
    – Buttonwood
    Apr 7, 2020 at 20:12
  • $\begingroup$ The question does request a precipitate though. I don't think ink droplets water are forming a precipitate. $\endgroup$
    – JMac
    Apr 8, 2020 at 17:20
  • $\begingroup$ @JMac True, this does not yield a solid, crystalline precipiate one could separate e.g., by gravity filtration. It is clouding the water, though. $\endgroup$
    – Buttonwood
    Apr 8, 2020 at 17:29
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This is not a chemistry answer, but it is not a comment either? Please remove if it doesn't fit here.

Since you are not familiar with chemistry, why risk it?

I would recommend just dripping dark food coloring or ink into the water, and using post-processing to either replace the drop with another shot of plain water dripping into the beaker, or modifying the color. There is likely to be a very clear cutoff (especially if filmed parallel to the surface of the water) and a limited time range. This could be done without expensive software or special skills.

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  • $\begingroup$ @Ciero The thread becomes fuzzy. But indeed, CGI is advanced enough to simulate the ink droplets in water; a tutorial post by 2017 (blendernation.com/2017/04/20/…) for the freely available blender. There even is a relevant entry on blenderSE, blender.stackexchange.com/questions/135244/… for a «chemistry free» version. $\endgroup$
    – Buttonwood
    Apr 8, 2020 at 20:36
  • $\begingroup$ I wasn't even suggesting simulation, merely composing a water drop falling into a glass with an ink drop falling into a glass. Worst case this can be no special effects whatsoever, just a cut between the two shots at the moment of impact $\endgroup$
    – Cireo
    Apr 10, 2020 at 21:54

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