How does a copper piece added in nitric acid change after nitrogen dioxide starts appearing? There's an experiment I have no way of performing but want to know its results. The instructions (in short) are:

Dip a piece of copper in a glass of concentrated nitric acid, $\ce{HNO3}$, using a pincette. When a brown gas starts forming, take out the piece of copper and rinse it with water. Right after that dip it into $\ce{HgNO3}$ solution. Leave it in there for some time and then take it out of the solution. Rinse it with water and wipe it with a napkin.
How does the piece of copper look?

What I've worked out is that the first part can be written as:
$$\ce{Cu (s) + 4HNO3 (aq) -&gt; Cu(NO3)2 (aq) + 2NO2 (g) + 2H2O(l)}$$
and that $\ce{NO2}$ is the brown gas.
What I'm  not sure about is why exactly I'm dipping the copper piece into a solution of $\ce{HgNO3}$ following the above reaction. And why am I rinsing it with water before dipping it into the $\ce{HgNO3}$ solution?
Metals often have corrosion on their surface, such as oxides.  By dipping in nitric acid and then rinsing with water you would achieve a true copper metal surface free of oxide or other corrosion products.