Displacement and double displacement

Put a clean iron nail in blue copper sulphate solution and wait. After about 20 minutes the nail is covered with a brownish coat of copper, and the blue colour fades.

Fe + CuSO₄ → FeSO₄ + Cu

"Why did the iron take copper's place?" Iron is more reactive than copper. A more reactive element can push a less reactive one out of its compound, like a stronger player taking the place of a weaker one. The weaker one can't push the stronger one out again: copper will not displace iron from iron sulphate. This is a displacement reaction.

Zinc and lead are also more reactive than copper, so they displace it too: Zn + CuSO₄ → ZnSO₄ + Cu. You will meet the full order of metals, the reactivity series, in Chapter 3.

In a double displacement reaction, two compounds swap their partners (their ions). No one is pushed out; both change places.

Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

Mix the two clear solutions and a white solid appears: barium sulphate, which does not dissolve in water. An insoluble solid formed in a reaction is a precipitate, and the reaction is a precipitation reaction. Lead nitrate with potassium iodide gives a yellow precipitate of lead iodide.