Question

why magnesium ribbon cleaned before burning

Answer 1

I'm not sure but I can say if their is a impurity in the ribbon then explosion can be the result ... once u check it in the internet ull be fine

Answer 2

because it is a highly reactive metal
thus reacts in open oxygen
and forms mgo magnesium oxide
it is quite stable
so don't react further on burning
so it is cleaned to remove magnesium oxide
so to carryout reaction