Answer:
I think 1,300ft (400m) not entirely sure.
A formula unit is the empirical formula for an ionic compound. It is the lowest possible ratio of the cations and anions of a given ionic compound.
This would be a physical change because it can change back to its original form. This is like ripping paper. You can piece it back together and it still is paper.
The opposite of this is chemical change. Chemical change means the product has been changed completely like burning paper. The paper has now been turned to ash and it's impossible to change this back to its original form.