On reacting copper oxide with hydrochloric acid and filtering it, the filtrate is of which colour?

Q. The salt formed on reaction of copper oxide with hydrochloric acid is :

Q. Copper does not react with dilute hydrochloric acid even on heating but it reacts with a highly concentrated solution of _____.

Q. A coloured solution known to contain two metal ions was treated with excess cold sodium hydroxide solution. When filtered a whitish solid, slowly changing to brown, was retained on the filter paper and a colourless solution collected as the filtrate. The dropwise addition of hydrochloric acid to the filtrate produced a white ppt. which dissolved in excess acid. Treatment of the residue on filter paper with a solution of a strong oxidiser produced a reddish-violet solution.