Why did a pound coin change colour after soaking in vinegar?

June 13, 2021 2:24 AM

My 7yo child is into chemistry. As an experiment we tried to clean a tarnished UK pound coin by soaking it in vinegar. The colour changed from gold to copper. Can someone explain the chemistry of this in terms suitable for a 7 year old?
posted by TheophileEscargot to Science & Nature (7 answers total)

I only have secondary school chemistry (and that almost two decades ago) but some quick googling suggests that the reason your pound coin looks like copper is because it is copper.

The outer ring of a pound coin is made of brass, which is an alloy of copper and zinc. (Technically it's nickel brass, which is brass with a little extra nickel thrown in.) Because the zinc in brass reacts more easily with a wide variety of chemicals than the copper does, brass is vulnerable to a process called "dezincification" where the zinc on the outer layer of metal is carried away by reactions with the environment, leaving the copper behind.

So, either as a result of whatever process caused the tarnishing in the first place, or as a result of being left in an acidic solution (which vinegar is), the zinc has been carried away, leaving behind the copper, which is naturally copper-colored. (Technically, there's probably also a little bit of cuprite on the outside as a result of the copper reacting with oxygen. Pure copper is sort of a salmon-pink color, which tarnishes to red and then reddish brown as cuprite forms.)
posted by firechicago at 3:20 AM on June 13, 2021

+ a specific reference to vinegar, among other conditions: "Dezincification also occurs when brass is cleaned with a mixture of salt and vinegar."
posted by away for regrooving at 9:18 AM on June 13, 2021

Incidentally, not all brass is equally vulnerable to dezincification.

For fittings used on boats, which spend time in seawater, we use DZR brass ("dezincification resistant"), sometimes also known as CR brass ("corrosion resistant"). It's slightly more expensive than regular brass but otherwise pretty similar.

So if you really want, you can buy a small DZR part for a couple of quid and the matching plain brass version for slightly less, and drop them both in vinegar. Although they start off looking identical, one should start to deteriorate well before the other.

The difference is made by having less than 2% of other metals in the alloy allong with the copper and zinc!
posted by automatronic at 3:08 PM on June 13, 2021

The interesting chemistry is that brass is just a mixture of the elements copper and zinc -- an alloy is two metals mixed together, but they're not a chemical compound of two elements, like table salt (sodium & chlorine) or water (hydrogen and oxygen). The chemical reaction here is the acid dissolving away the zinc, leaving behind the copper.
posted by Rash at 4:08 PM on June 13, 2021

If you want to do a related experiment, you can put nuts and bolts (most of which are zinc plated these days) into vinegar and the zinc coating will bubble off, leaving them dark grey. If you take them out and coat them with oil, they'll stay nearly black but if you let them dry, they'll rust over in minutes.
posted by bonobothegreat at 6:50 PM on June 13, 2021

Thanks folks! That's really helpful!
posted by TheophileEscargot at 3:00 AM on June 14, 2021

Showed this thread to the kid, he was very satisfied with the explanation. Thanks again!
posted by TheophileEscargot at 12:21 PM on June 14, 2021