Whoever developed this etching procedure really knew his chemistry! After all, a reaction of the type Cu+2HAc=CuAc2+H2 cannot take place because copper is more noble than hydrogen (HAc I use here as a general short form for any non-oxidizing acid, whether this is hydrochloric or acetic (=vinegar) acid .

In order to make it possible to dissolve Cu with HAc, the reaction needs to be changed to

Cu+HAc+O=CuAc2+H2O

This is done by adding an oxidiser. This can be H2O2 (commonly used for bleaching hair). This works because, in the presence of heavy metals such as copper or iron ions, H2O2 becomes unstable and decays into H2O (water) and O, activated oxygen, which I use here without reference to the intermediate steps. An alternative would be to use household bleach (Hypochlorite), which is just another oxidiser, or potassium permanganate, which is used in some countries as a fungicide and thus available from gardening shops / nurseries.

But even then, the reaction would not proceed because the reaction Cu+HAc+O=CuAc2+H2O ignores the formation of compounds with very limited solubility in water (the so-called gunk), which simply isolate the copper metal from the acid. To improve that solubility, sodium chloride (common salt) is added.

And this is where the color of the final solution comes into play, as well as good stirring. Up to now, In order to understand what happens we could afford to just take a global look and ignore the details. As soon as solubility matters, we need to look at the reaction step by step. First we need to understand valence. Since not all of you will have university level training in chemistry, I will keep it as simple as possible, i.e. you need not read and understand the Wikipedia article on it. It suffices here that aluminum has valence 3 and copper either valence 1 or 2. Valence is always expressed by writing the number in ROMAN numerals, with the numerals enclosed in round brackets, i.e. as aluminum(III), copper(I), or copper(II). Other valences we need not consider. Thus, the reference to aluminum(II) above by videoschmideo is a typo.

Another number to understand is oxidation state. Oxidation is the removal of electrons from a chemical element, and oxidation state thus tells us how many electrons have been removed. Thus Cu+ corresponds to oxidation state +1 and valence copper(I), Cu2+ to oxidation state +2 and valence copper(II), and Al3+ to oxidation state +3 and valence aluminum(III). Unlike valence, oxidation states can also be negative numbers: -2 for oxygen in H2O and -1 for oxygen in H2O2.

Splitting the bulk Cu+HAc+O=CuAc2+H2O into its components, we obtain

Cu=Cu++e- (copper is oxidised from oxidation state 0 to oxidation state 1)

Cu+=Cu2++e- (copper is oxidised from oxidation state 1 to oxidation state 2)

H2O2+2e- = 2OH- (oxygen is reduced from oxidation state -1 to oxidation state -2)

2HAc+2OH- = 2Ac- +H2O (the OH- ions are neutralised)

If this were all, no salt needed to be added.

Unfortunately, basic copper acetate Cu(OH)Ac, commonly known as verdigris (green), is poorly soluble in water, and thus forms a layer between the copper metal and the acid that effectively stops reaction progress. Similarly, copper(I)chloride CuCl (white) is poorly soluble in water, and effectively stops reaction progress. And CuCl must be formed before CuCl2 can be formed.

Thus, another step is needed.

We need another reaction partner, and before we can introduce it, we need to understand color.

Color is the response of an electric charge upon stimulation by light, or, more accurately, how much energy must be contained in a single photon to be able to break an electron out of its place. This leads to selective absorption of some of the frequencies contained in white light and thus to color. How strongly the electron is bonded to copper depends to a substantial degree to the other molecules / ions that are close by. If there are a lot of NH4- ions close by, as in Cu(NH4)4Cl2, the copper(II) ion appears dark blue or purple, with water forming the bulk of its neighbors, the copper(II) ion appears just blue, and the more Cl- ions are present, it becomes increasingly green through the formation of CuCl43- ions. Anhydrous CuSO4 is colorless, but the hydrated CuSO4.2H2O is colorless / white.

And that is what the addition of salt does: it adds chloride and thus promotes the formation of Na3CuCl4, which readily dissolves in water. And before any of you start a flame war, my source only states that solubility is drastically improved by the addition of chloride, it does not state whether there are four or six chlorine ligands, i.e. whether the ligands are arranged in a square around the copper atom, or whether they form an octahedron. These are the two configurations that Ligand Field Theory permits.

