Question

Zinc liberates hydrogen gas when reacted with dilute HCL acid,whereas copper does not. Explain

Answer 1

According
to the reactivity series copper is down while zinc is higher

Answer 2

According to the reactivity series, the reactivity of Zinc, Hydrogen and Copper is
$Cu\ \textless \ H\ \textless \ Zn$

In the reaction with Zinc, Zinc is more reactive than Hydrogen hence the metal displaces Hydrogen, forms its chloride and liberates Hydrogen
$Zn+HCl$ ⇒ $ZnCl+H_{2}$

In the reaction with Copper, Copper is less reactive than Hydrogen hence can't replace it.
$Cu+HCl$ ⇒ No Reaction