Electron transfer reaction. Demonstration of an electron transfer reaction in a laboratory. At left is a test tube containing copper(II) sulphate solution, below this is a watchglass containing zinc powder. When the zinc is added to the sulphate, the solution loses its blue colour (right). This is due to the electron transfer between copper(II) ions (blue) and the zinc to form zinc(II) ions (colourless). The copper is deposited as a metal.