Question

why does the size of naphthalene balls put in open keep on the reducing with time

Answer 1

napthalene balls when not kept in closed vessel undergo a process known as sublimation... in this process the solid directly converts into vapours...
that's why it's size decreases ...

hope it helps !! ✌️❤️

Answer 2

napthalene balls keeps reducing due to a process called sublimation which involves transformation of solidd directly into gaseous state
As these balls are left in air, it starts converting into gaseous form and hence keeps reducing with time