Acids to be careful with
The pattern "metal + acid → salt + hydrogen" is a useful one. But it does not always hold — and both exceptions are dangerous.
1 — concentrated acids
Different gaseous products
The reaction of concentrated sulphuric acid and of nitric acid (dilute or concentrated) with metals may give other gaseous products instead of hydrogen.
2 — highly reactive metals
Not to be done in the laboratory
Metals such as potassium (K), sodium (Na) and calcium (Ca) react violently with dilute acids. Ignition with an explosion may even occur. These reactions must therefore not be carried out in the laboratory.
What both of them show
Both show that the reactivity of metals with acids varies with the metal. At one end stands copper — no reaction at all. At the other stands potassium — too violent to attempt.
Arranged by reactivity
Violently → rapidly → slowly → not at all. This is already the shape of the activity series.
Notice that this order matches the one we saw with water and the one we saw with air — which is no accident.