How to know whether to use a dilute acid or concentrated acid in preparation of salts?

Dilute acids are usually used. However, if the metal is less reactive, e.g. copper, concentrated acid is then used.
if a metal is less reactive then use concetrated acid. hope u r satisfied with the ans. regards
for eg. cu is less reactive so
cu+hcl(concentrated)=cucl2+h2

regards