* Pure substance: A pure substance is made up of only one type of particle. Lye is made only of sodium hydroxide molecules.

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio. Lye is formed from the chemical combination of sodium (Na), oxygen (O), and hydrogen (H) in a 1:1:1 ratio.

Therefore, lye is not a mixture, but a pure compound.