Which of the following statements are true for pure substances?

i) Pure substances contain only one kind of particle.

ii) Pure substances may be compounds or mixtures.

iii) Pure substances have the same composition throughout.

iv) Pure substances can be exemplified by all elements other than nickel.

A pure substance contains only one type of particles which can either be atoms or molecules or elements or compounds. It has fixed composition as well as fixed melting and boiling points.

For example, $\mathit{NaCl}$ is an example of pure substance. 

Pure substances can be either elements or compounds. Elements are composed of only one type of atom, while compounds are composed of two or more different types of atoms chemically bonded together.

Pure substances include elements like nickel as well as compounds. Nickel is an element.