.It is difficult to separate lanthanoids in pure state.

why?

It is difficult to separate lanthanoids in the pure state because they are very similar to each other in properties. In fact, till 1907, they were believed to be a single element. The lanthanoides are characterised by uniform +3 oxidation state, which is shown by all the metal of this series. Hence the elements are almost identical in size and their chemical properties are almost identical. It is because of this that the separation of one lanthanide from another is extremely difficult.