Explain why a piece of aluminum or magnesium can be used to remove tarnish, which is mostly silver sulfide, on silverware.

When comparing the reactivity of aluminum or magnesium to the reactivity of silver, we will find that both aluminum and magnesium are more reactive than silver.
Therefore, magnesium or aluminum can both replace the silver in the silver sulphide.
That's why a piece of aluminum or magnesium can be used to remove tarnish, which is mostly silver sulfide, on silverware.

In order to balance this, you have to count each element where the elements in the reactants side and the product side should have equal number of molecules. The balanced reaction is as follows:

 2 Ca + O2 ---> 2CaO