Mixing silver nitrate and copper is one of the most famous chemistry experiments, with it starring in many a school science lesson around the globe.
The experiment involves introducing copper – typically a copper wire – to a silver nitrate/ water solution and suspending it there for a couple of hours.
The combining of both triggers a single replacement reaction, where copper is changed from its elemental form (Cu) to its blue aqueous ion form (Cu2+ [aq]), while the silver ions (Ag+ [aq]) in the silver nitrate solution will be changed into their elemental metallic form (Ag) and deposited onto the wire.
These silver deposits continue to grow off the copper in a series of fractal-like crystals until all reactable copper in the solution is exhausted, leaving the end products of silver and copper nitrate.
The reason this replacement reaction occurs is that the atoms in the copper are oxidised when introduced to the silver nitrate solution, losing electrons and forming copper ions, while the silver ions in the nitrate solution are reduced (ie they gain electrons) into elemental silver.
What’s really cool is that once the silver crystals have grown they can be removed from the copper, dried off and then displayed as funky pieces of fractal art.