Cleaning two pence coins

I produced six coins, one nice and new and shiny and five dull ones (all 2 pence) and asked the toddler which one was different and why.  He immediately told me one was shiny and he took it to have a closer look.  He was fascinated - he doesn't handle coins very often as we rarely pay cash for anything, and his brother eats whatever he can get his hands on so we keep coins away from him!  Once the boy had finished admiring the gleaming two pence, he wrinkled his nose at the other 'dirty' ones so I asked if he would help me clean them.  He rarely says no to helping with anything, so I was onto a winner!

I took two clean plastic pots out of our recycling pile.  I added a little water to one then asked the small boy to squirt about the same amount of white vinegar into it.  We mixed it, then poured a little into the second pot for later.  I asked him to add the dirty coins carefully to the main pot, and to make sure they weren't overlapping.  This done, we went to play in the garden for a while.

After about 20 minutes, we came back inside and had a look.  Whilst not entirely shiny, the dull coins were definitely shinier, and he seemed pleased that his cleaning was working!

I found him two steel nails which had large heads (to make it easy to stand them up).  I got him to stand one on its head in the tub with the coins, and one in the other tub with some of the vinegar/water mix that we'd saved from earlier (as a control experiment).  We left them for another 15 minutes or so.

Coins and nails in vinegar

I asked the boy to look at the nails and tell me if he could see if anything had happened.  He was quite excited to tell me that the part of the nail that was submerged in the tub with the coins had gone black.  Nothing had happened to the other nail.  I suggested that maybe something had come off the coins, gone into the liquid and then had gone onto the nail.  He didn't question this, or really seem that interested in what it might be so I didn't push the science!

The left hand nail was partially in the liquid with the coins

As he wanted to look at the coins, I let him take them out of the pot.  He chose 3 to put onto kitchen roll and 2 for me to wash before he put them on the kitchen roll.  We left them for a couple of hours - the kitchen roll under the unwashed ones was turquoise and there was a little blue/green colouring on the unwashed coins themselves.  He wasn't very interested in this, I think because it was quite hard to see, so I didn't try to explain that this was malachite like he'd seen in the science museum earlier in the week.

 A little malachite on the coin

So what is the science involved in this activity?  The dark coating on the two pence is copper oxide, formed when the copper in the coin reacts with oxygen in air over time.  The vinegar is an acid (acetic acid) and dissolves the copper oxide.  The reaction makes water and copper acetate.  When you put a steel nail in the vinegar solution, some of the iron dissolves to make iron acetate, and the some copper ions replace the iron ions on the nail and it changes colour.  For the pennies that are left with a little of the acid solution on their surface, they turn turquoise because of the formation of malachite.  You should be able to get more malachite if you add a little salt (sodium chloride) to the mix at the start, but we will try this another time when he's interested.

Cleaning the pennies and making the nail change colour were sufficiently interesting for the not-quite-3 year old, but there's more we could do with this activity in the future when he's a bit older.  You'd also be able to do this with UK pennies, but I didn't fancy having smaller coins around with the little one on the loose. For now, I'll keep the shiny two pence coins for another activity I've got in mind soon...