The Displacement Reactions of Metals:
More reactive metals can displace less reactive metals from their salt solutions, oxides, etc. for example, solid iron displaces copper ions from a solution of copper (II) sulphate. Due to the action of iron, copper metal is precipitated out of the solution as a pink solid.
In other words, atoms of the more reactive metal become ions and form compounds while ions of the less reactive metal change back to atoms.
We see this behavior of metals in the reactions I told u about before this, too. In the reaction of metal with acids, only the more reactive ones were able to displace hydrogen and form a salt, while the less reactive metals like copper stayed unreacted.
In the decomposition reaction, we needed the reducing agent for more reactive metals. But metals that were more reactive than carbon stayed unreacted even when the reducing agent carbon was present. In case of the reducing agent hydrogen, it could not displace those that were more reactive, so the metal oxides of metals more reactive than hydrogen stayed as they were, and the reaction did not alter them.
For oxides of very reactive metals that do not get displaced and cannot be obtained as only the metal through combustion, a process called electrolysis is used. You will learn more about electrolysis in the next chapter.