This lab is about reacting the penny with hydrochloric acid. I was give following data:

Pre-1982 penny:

Mass of penny at timestamp 0.55: 3.081 grams

Mass of penny at timestamp 4:26: 3.09 grams

which timestamp 0.55 is before and 4:26 is after placing the penny into the HCl solution.

Does this data suggests that the penny is a pure substance (made of copper only)? But how and why does the mass of the penny increase after putting it into HCl solution?