Here's why:

* Flame Tests: Flame tests are a common way to identify certain elements. When a metal salt is heated in a flame, the electrons in the metal ions absorb energy and jump to higher energy levels. When they fall back down to their ground state, they release this energy as light, which we see as a colored flame.

* Copper: Copper ions (Cu²⁺) emit green light when excited in a flame.

* Copper(II) chloride: Copper(II) chloride is a common and soluble salt that readily produces copper ions in solution.

Note: Other copper salts, like copper(II) sulfate (CuSO₄) can also produce a green flame, but copper(II) chloride is often the most readily available and used for this purpose.