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A pure substance is something that is only 1 substance. It has not been mixed or combined with anything.
Homogeneous mixtures are samples that contain more than one single substance, but all of them are in a single phase and they're mixed perfectly so that whatever sample of it you take, no matter from where, it will be the same as any other sample. It simply contains more than one pure substance. Salt water is such an example.
Heterogeneous mixtures are not the same throughout the sample. It contains more than one substances and also all the substances are not in the same phase. You can take identical amounts from different parts of the mixture, and get different results. Pizza is an example. When you eat a pizza, the toppings aren’t evenly distributed throughout the pizza, neither is the cheese or sauce, which means that it too is a heterogeneous mixture.
Si I too will go with pure substance. :)