As you can observe in the above series that zinc is more reactive than the hydrogen whereas copper is less reactive than hydrogen.
More reactive the metal is, more easily it will displace hydrogen atoms. Hence,
i. When zinc reacts with hydrochloric acid, it liberates hydrogen gas (hydrogen is displaced)
Zn + HCl → ZnCl2 + H2
ii. On the other hand, when copper reacts with hydrochloric acid, it is unable to displace the hydrogen gas. As a result, no hydrogen gas is liberated.
Cu + HCl → No reaction
