The correct statement that describes compounds is:
Compounds cannot be broken down into simpler substances by chemical means.
However, it should be noted that compounds can be broken down into simpler substances by chemical means, such as chemical reactions. Let's briefly evaluate the other statements for clarity:
- Compounds cannot be broken down into simpler substances by chemical means. (Incorrect)
- Each compound is composed of one type of molecule. (Correct, but can be misleading; while each compound is made of molecules, those molecules can consist of multiple types of atoms.)
- Compounds can be broken down into simpler substances by physical means. (Incorrect; they cannot be broken down by physical means but can be broken down by chemical means.)
- Each compound is composed of one type of atom. (Incorrect; a compound is made of molecules that consist of two or more different types of atoms.)
It seems there are multiple points of confusion with the provided options, as none of them accurately describe compounds as accepted in chemistry. A revised statement could be that "compounds are substances formed from two or more different types of atoms that can be broken down into simpler substances by chemical means."