It is either: a pure substance (such as water, sugar, iron, or salt) or a mixture of two or more pure substances (say a glass of salt water).
(It; is; a pure substance (such as water, sugar, iron, or salt) or a mixture of two or more pure substances (say a glass of salt water)
It
is
a pure substance (such as water, sugar, iron, or salt) or a mixture of two or more pure substances (say a glass of salt water