Coins, like everything around you, are made of chemical substances. We can make them react with other things, and they’ll change how they look. If we’re lucky (and use the right chemical reaction), we can make a copper five-cent coin look like it’s made of silver or gold!
Goals
To change how a copper coin looks so that it seems to be silver or gold.
Things You’ll Need to Make a Copper Coin Look Silver
- Two five-cent copper coins
- Two small glass containers (about 100 mL each)
- Sodium hydroxide (NaOH)
- Zinc powder (Zn)
- A hot plate or a Bunsen burner
Let’s Do It!
Part 1
Carefully mix three small spoonfuls of sodium hydroxide (NaOH) with 50 mL of water. Be careful! Sodium hydroxide can cause burns, so you must not touch it with your skin. Make sure to wear gloves and safety glasses to protect your eyes. Then, add half a small spoonful of zinc powder to this mixture.
Now, wash one of the 5-cent coins with soap and water until it’s really clean. Rinse it well with water.
It’s important that the coin is very clean.
Using tongs (like the ones you might use for cooking), put the coin into the container where you mixed the zinc and sodium hydroxide.
Gently heat the mixture on the hot plate, but don’t let it boil. After about four or five minutes, the coin should look silvery.
Put about 150 mL of regular tap water into the other clean glass container.
Using the tongs again, move the silvery coin to the container with the cold tap water and then wash it a little more under the tap.
Part 2
If you want your coin to look like gold, you just need to heat the silvery coin on a hot plate or over the flame of a Bunsen burner for about two or three minutes.
It will turn a bright gold color! Be careful not to heat it for too long, or the gold color might go away!
What Happened and Why
The zinc reacts with the sodium hydroxide, and some of it dissolves. This creates a balance between the zinc metal and the zinc that’s dissolved in the liquid. When you put the copper coin in the container, some of the dissolved zinc sticks to the copper, and at the same time, the same amount of zinc metal dissolves into the liquid. What looks like silver on the coin is actually a very thin layer of zinc covering it.
When you heat the coin, its color changes. This happens because the zinc atoms on the surface of the coin move and mix with the copper atoms, creating a new substance called an alloy (an alloy is a solid mixture of two metals). The alloy of zinc and copper is called brass, and it has a pale gold color. People have used brass for many things throughout history.
Actually, the five-cent coins only have copper on the outside. Inside, they are made of steel. This doesn’t change what happens in this experiment, and you would get the same result with a coin made entirely of copper.
