A metal M does not liberate hydrogen from acids but reacts with oxygen to give a black colour product. Identify M.

Let's analyze the properties given in the question using the metal reactivity series:Metals located below hydrogen in the activity series (such as Copper, Gold, and Silver) cannot displace hydrogen ions from dilute acidic solutions to release ${\text{H}}_2$ gas. This rules out highly reactive alkaline metals like $\text{Na}$, $\text{Mg}$, and $\text{Ca}$.When Copper ($\text{Cu}$) is heated in the presence of atmospheric oxygen, it undergoes oxidation to form a dull black colored product known as copper(II) oxide ($\text{CuO}$):
$$2\text{Cu}+{\text{O}}_2\stackrel{\Delta }{\to }2\text{CuO}(\text{Black})$$
Thus, the metal M is copper ($\text{Cu}$).