We did an experiment with blue copper sulfate solution. On the left, we added iron, the middle was zinc and the right was magnesium. Notice that for the two on the right, the solution is no longer blue. It is no longer copper sulfate. On the right, the magnesium metal is more reactive than the copper, so the copper and the magnesium swap places.
Magnesium sulfate has no colour. The slightly rusty coloured stuff in the liquid is actually the displaced copper. It has been dumped out of the liquid.
The same applies in the middle reaction. Zinc sulfate is colourless and you can see the rusty coloured displaced copper.
There is some physics behind the reactivities that is usually taught with metals by how easily they can get rid of electrons to form complete electron shells. This becomes more difficult when you get to zinc and iron which are Transition Metals on the Periodic Table. I'm working on an explanation that satisfies my own curiosity. I found this lovely blog post whilst looking. It works at our level and shows the experiments really well. I'm going to have a go at the very slow reactions!
https://chemlegin.wordpress.com/2014/08/31/unreactive-metals/
