For each of these reactions, indicate the property which might be observed in order to determine when equilibrium has been reached.
a) PCl5(g) <--> PCl3(g) + Cl2(g)
b) CaCO3(s) <--> CaO(s) + CO2(g)
c) H2O(l) <--> H2O(g)
d) Cl2(g) + 2H2(g) H2(g) + 2HCl(g)
e) 2HBr(g) <--> H2(g) + Br2(g)