Question: how long does it take a coin to dissolve and would it vary depending on where it is?

James Martin answered on 15 Mar 2017:

In water, it wouldn’t. But if in acid or something, you’d need to look at the rate of reaction, and then also make sure the neutralisation of the acid is compensated for. You can probably assume the pH will be spread uniformly over the whole medium, so independent of where in the beaker it is. However, you could make the situation more interesting by increasing the reaction rate somehow. Perhaps you could heat the acid up, or perhaps provide a flow within the liquid to expose the coin surface to fresh acid (H+) ions. You could also increase the surface area of the coin to increase surface exposed, so you could use a larger coin, or even squash (or drill holes in!) the current coin.