Concentrated nitric acid HNO3 and copper Cu, forms a visible brown gas of nitrogen dioxide NO2 gas.
Cu + 4HNO3 --> Cu(NO3)2 + NO2(g) + H2O
Dilute solution (about 3 M molar solution) of nitric acid HNO3 with copper form the clear gas of nitrogen oxide NO gas (g).
3Cu + 8HNO3 --> 3Cu(NO3)2 + 2NO
A solution in between concentrated and dilute can produce both of these NOx gases, and in reality several of the NOx gases can form from these reactions.
The clear gas of nitrogen oxide NO with oxygen O2 (or oxygen in the air) can form the visible brown gas of nitrogen dioxide NO2.
2NO(g) + O2 --> 2NO2
Note this oxygen can come from the H2O2 in the water solution used to dilute the acid, or from the hydrogen peroxide and water we use to bubble our NOx gases through when we make nitric acid.
Note also the NO gas is really not soluble in water, but the NO2 gas is soluble in water.
The brown gas of nitrogen dioxide NO2 gas mixed with water and some oxygen will form nitric acid.
4NO2(g) + 2H2O + O2 --> 4HNO3
Nitric acid is a strong oxidizer, concentrated nitric acid can be so strong it can form a layer of the passivation of metal oxide onto the surface of the metal, making it hard for them to be attacked, or dissolve into solution, being protected by this oxide coating, you will even notice this with copper or silver where with using concentrated nitric acid there is a delayed reaction as the metal forms a layer of passivation, until gas of nitrogen dioxide NO2 gas. Until the nitric acid begins to break down into solution forming HNO2 and NOx gases which we can then begin to attack the metal in a vigorous reaction, compounded by the heat evolved from this reaction, while we begin to see our nitric acid foam and boil over out of the beaker taking much of our metals with the foaming boil over.
We can see here were dilution of the nitric will actually dissolve the metal easier, with less chance of a boil over.
We can see also where even just water alone will make our nitric acid go further in the reaction where the NO2 in solution can reform making more nitric acid in solution instead of evolving, or leaving the solution as wasted gases, we can also see from the reactions above where the oxygen in solution provided by a little added Hydrogen Peroxide can supply oxygen in solution to convert the insoluble invisible NO gas back into solution as NO2, and then with the water to more HNO3 to react further with the metal, instead of being wasted as dangerous gases.
Diluting the nitric acid, I will normally dilute mine more that the recommended 50/50 mix, and add small additions of H2O2 as the reaction proceeds, I do this with heat.
Note heating also can have a reverse effect of pushing gases out of solution, as well as concentrating the acid in solution as the heat evaporates water, so we keep this in mind.
The heat helps to speed the reaction of the dilute nitric, the H2O2, can provide oxygen and water to solution, which can convert the NOx gases formed in solution back into nitric acid with the water and oxygen in solution to make your nitric acid go further at making metal nitrates instead of being evolved as gas.
Heating also concentrates the remaining nitric in solution, as the process proceeds.
You may have noticed that towards the end of the reaction that the silver (or copper) does not seem to dissolve in the remaining nitric acid, well with water forming from the reaction, (see the reactions above), this water dilutes the remaining acid that would take a very long time to attack the remaining metal, this is where we can still have free nitric acid, even after we think it has all reacted or is used up, heating towards the end of this reaction can evaporate the water concentrate the remaining nitric acid in solution with the metals, and dissolve more metals into the silver or copper nitrate solution, again dissolving much more of the metal than we could have without using heat in the process, you can be surprised by how much more silver can be dissolved into this remaining solution even after you think the nitric has depleted or has stopped reacting with the silver.
By diluting the acid in the beginning and using heat in a controlled manner, along with your H2O2 additions, to dissolve your silver or copper, you can keep gases in solution making your nitric go further, and heating the solution during the process to slowly evaporate the excess water added, and the water formed from the reaction,
( the water formed from the hydrogen gas that is set free from the acid as the acid reacts with metals forming water in this case with these metal less reactive than hydrogen in the reactivity series of metals, see below).
Then with this heat evaporating the solution to get the most from the nitric acid concentrating it in solution to make it stronger to dissolve more metal into solution without using up your nitric acid as wasted gas, or having to use it up later with a metal like copper when trying to cement silver from solution later.
If we look at the reactivity series of metals we will see hydrogen in this series (hydrogen is not a metal) and there as a reference, reactive metals above hydrogen, will displace hydrogen (from the acid as gas from solution as it dissolves, this hydrogen gas leaves the reaction as H2gas.
With unreactive metals like copper and silver the hydrogen is also set free from the acid when the metal is oxidized by the strong oxidizing acid, but in this case the hydrogen does not leave the solution as a gas, but hydrolyzes the water to form H3O and then more H2O in solution as the HNO3 helps to provide oxygen to form water in the reaction.
3Cu + 6HNO3 --> 3Cu(NO3)2 + 6H
then
2HNO3 + 6H --> 4H2O + NO (gas)
overall of the above reactions
3Cu + 8HNO3 --> 3Cu(NO3)2 + 2NO +4H2O
Here we see water forming in this reaction of the unreactive copper metal and the strong oxidizing acid of nitric acid.
