In this Periodic Videos YouTube video by Brady Haran and the University of Nottingham, we see a piece of gold being dissolved in aqua regia, while Professor Poliakoff explains why aqua regia can dissolve gold even though its constituent acids can't.

The piece of gold was placed partway into the acid. I noticed that the acid appears to attack the gold the strongest at the liquid's surface, and the gold was first "dissolved through" along a line at the surface, leaving a little plate of undissolved gold dangling in the acid by one corner. You can see this best from about 4:10 to 4:30 into the video.

I was wondering if there's some effect at the surface of the liquid that causes this.

(I apologize if this is more physics than chemistry, and also for my Google-fu not being enough to find ready answers.)

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    $\begingroup$ 1. The bubbles formed rise to the surface and grow there until they are released into the air. 2. The piece of metal might be thicker at the bottom. $\endgroup$ – aventurin Jan 12 at 14:11
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    $\begingroup$ I do have a possible expl. that possibly I elaborate tomorrow. Rather than accumulation is right at the surface that the contact between metal and aqua regia is easily renewed plus potential. $\endgroup$ – Alchimista Jan 13 at 21:31
  • $\begingroup$ @aventurin Are you suggesting the bubbles cause an optical illusion, or are you more in line with Alchimista's answer? $\endgroup$ – Spencer Jan 14 at 17:02

Besides mechanical aspects

  • bubbles might form a sort of layer at the immersed part while at the air interface they evolve and detache easily renewing the metal/acid (oxidizer) contact

a more fundamental reason can be° surface excess. Normally we treat interfaces as very thin and therefore negligible (in mass or volume) as compared to the bulk.

However at an interface the chemical potential (and all related thermodynamic properties) differs from that of the bulk owing to the fact that a layer can be assumed to have the same number of entities per unit of surface.

While such a layer at depth is surrounded by the very same, at the interface it sees two different environments.

This aspect is quite familiar in physical phenomena, such as surfactivity, the shape of droplets, and even the tip effect in electrostatic. Another example possibly familiar to many chemists is the difference in product of solubility / solubility for solids at very different sizes (digestion of precipitates to attain bigger grains/crystals is based on this, tough intermixed with the merely geometric fact that small particles dissolve faster).

As for in water the interface has higher potential and this results in spherical droplets to minimize it, in your specific case you can see the aqua regia doing the same by being more reactive. It is like the oxidizer trying to escape the air interface (tough a new one is continually formed, of course) by reacting more energically than the layers below, if this pictorial helps you.

If you dig for "potential at acid/air interfaces" or "pH of an acid at the air interface" and so on, you can find some material dealing with example somehow related to your example. For instance I have found study of the dissociation of weak acids (ie pKa at the interface vs bulk), The vast majority is however studies in which the very interface is at focus, like in monolayer LB films, and the surface excess comes naturally at play whatever explicitly considered or not.

(For instance a title such as "On the dissociation constant of carboxylic acids in monolayer thin films" implicitly assumes that some difference exist as compared to the bulk of the solution).

° can be as for I do not know if the evident effect you have spotted is mainly due to the "stirring effect" of evolving bubbles or to the higher activity of the interface, or their mix.


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