Final answer:
The statement is false. Pure substances have a constant composition but heterogeneous mixtures do not have exact ratios because their composition varies.
Step-by-step explanation:
The statement that pure substances have a constant ratio, and heterogeneous mixtures have exact ratios is false. Pure substances indeed have a constant composition; they can be either elements, which contain only one type of atom, or compounds, made up of two or more types of atoms in a fixed ratio. However, heterogeneous mixtures do not have exact ratios as their composition is not uniform. An example of a heterogeneous mixture is oil and vinegar salad dressing, while an example of a homogeneous mixture is a sports drink, where the composition is uniform throughout.