Whereas in everyday language strong is used to denote what in
the language of chemistry is referred to as concentrated, for
instance a strong coffee.
–That
means a coffee (the drink) with a relatively high concentration of its ingredient.
A strong substance in chemistry (such as an acid) may not
necessarily be concentrated, it may be dilute.
Similarly, weak in everyday language means low concentration
while in chemistry it denotes an acid or base that does not
dissociate completely.
–You
can easily have a concentrated solution of a weak acid!