Question

Which element from Mg, Zn, Cu, Ag liberates hydrogen on reacting with an acid?

Answer 1

$Zn$ and $Mg$ liberates hydrogen on reacting with an acid as $Zn$ and $Mg$ are more reactive than hydrogen.
$Zn+2HCl\to ZnC{l}_2+H_2\uparrow$
$Cu+2HCl\to CuC{l}_2+H_2\uparrow$
$Cu$ and $Ag$ cannot liberate hydrogen on reacting with an acid as $Cu$ and $Ag$ are less reactive than hydrogen.