Question

"In a pure substance all the particles must be identical; therefore pure substances are composed only of elements." Do you agree with this statement? Why or why not?

Answer 1

I do not agree with the statement.
The "substance" can be a compound.  It's "pure"
as long as there's nothing else in it but its name.

'Pure' water is 100% H₂O with nothing else in it.
'Pure' table salt is 100% NaCl with nothing else in it.
'Pure' carbon dioxide is 100% CO₂ with nothing else in it.

These example substances are all compounds, not elements.