pure substance & mixture

The pure substance is built up of only one kind of particles. If the pure substance is built up o molecules, then all molecules in that substance are equal. The substance can be represented with one molecular formula. As soon as other molecules do mix, then the whole in impure. Then you have got a mixture of substances.
The pure substance could be built up of ions. In that case different ions are present in the pure substance (positive and negative ones). Are we then talking about a pure substance?
Take for example kitchen salt. It is built up of two kinds of ions, nicely arranged in an ionic lattice (Na+ and Cl-). The substance had one clear formual: NaCl (the ions are close toghether in the proportion 1:1. If the substance is built up like that, and can be indicated with one formula, then we say: this substance is pure. As soon as strange ions come in, or molecules, then you've got a mixture. The substance has become impure.