Fill in the blanks:
$(1)$ Since $V_2O_5$ acts as both an acid and a base,it is an ...... oxide.
$(2)$ The $Cr$ oxide which is basic is ......,and the $Cr$ oxide which is amphoteric is ...... .
$(3)$ The formula of chromite ore is .......... .
$(4)$ The formula of sodium dichromate crystal is .......... .

(N/A) $(1)$ $V_2O_5$ is an amphoteric oxide because it reacts with both acids and bases.
$(2)$ $CrO$ is a basic oxide,while $Cr_2O_3$ is an amphoteric oxide.
$(3)$ The formula of chromite ore is $FeCr_2O_4$ (or $FeO \cdot Cr_2O_3$).
$(4)$ The formula of sodium dichromate crystal is $Na_2Cr_2O_7 \cdot 2H_2O$.