2.31 Deduce the position of a metal within the reactivity series using displacement reactions between metals and their oxides, and between metals and their salts in aqueous solutions

If a more reactive metal (A) is placed into a test tube (or other container) with a metal oxide (B) dissolved in water, it will displace that metal (B).

If a less reactive metal (C) is placed into the test tube with the metal oxide (B), no reaction will occur.

This applies to the solution of their salts as well. From this, you can see which is more reactive. I.e. if chlorine displaces bromine, you can tell that chlorine is more reactive. :)