Displacement reactions

Displacement reactions involve a metal and a compound of a different metal. In a displacement reaction:

  • a more reactive metal will displace a less reactive metal from its compounds

Displacement reactions are easily seen when a salt of the less reactive metal is in the solution. During the reaction:

  • the more reactive metal gradually disappears as it forms a solution
  • the less reactive metal coats the surface of the more reactive metal

For example, magnesium is more reactive than copper. When a piece of magnesium is dipped into blue copper sulfate solution:

  • the blue colour fades as colourless magnesium sulfate solution forms
  • brown copper coats the surface of the magnesium
Magnesium powder is added to blue copper sulfate solution and when stirred, they change into a colourless magnesium sulfate solution and copper powder.

Here are the equations for the reaction:

magnesium + copper sulfate → magnesium sulfate + copper

Mg + CuSO4 → MgSO4 + Cu

No reaction is seen if you do things the other way round – in other words, if you put copper powder into magnesium sulfate solution. This is because copper is not reactive enough to displace magnesium from magnesium sulfate.

Investigating displacement

You can investigate the reactivity of metals using displacement reactions. The table shows the results from a series of experiments involving four metals and solutions of their salts. A tick shows where there is a visible reaction and a cross shows where there is no visible reaction.

Magnesium sulfate
Zinc sulfate
Iron sulfate
Copper sulfate
Reactions seen3210

Magnesium displaces three metals, zinc displaces two metals, iron displaces one metal and copper does not displace any of the other three metals. So, the order of reactivity, starting with the most reactive first, is:

  • magnesium
  • zinc
  • iron
  • copper

Displacement reactions can also involve metal oxides.