Question

For which of the following reactions are both reactants and products likely to be found when the reaction appears to be complete? How can you tell?
- Mg + O2 --> 2MgO
- HF + H2O <--> H3O(+) + F(-)

Answer 1

the answer is
HF + H2O <--> H3O(+) + F(-)

HF + H2O gives  H3O(+) + F(-)
and
 H3O(+) + F(-) gives HF + H2O
it is a reciprocal reacation, so
reactions are both reactants and products