Which of the following is a substance: table salt, seawater, or sand? explain how you know.

TABLE SALT IS A SUBSTANCE.
In chemistry, a substance refers to a matter which has specific composition and specific properties which do not change. A substance can be an element or a compound. A substance typically has a constant composition, which can not be separated by physical methods.
In the question given above, table salt is an example of a chemical substance because, it has a constant specific composition and its components can not be separated by physical methods. 
Seawater and sand are not substances, because they have varied compositions and their components can be separated by physical methods.

Yes you are correct use the scientific method