Final answer:
When a substance A is heated and produces substances B and C, A must be a compound; it consists of more than one type of atom and was chemically pure before decomposition. Thus, the correct answer is option c.
Step-by-step explanation:
After heating, if a pure substance A is found to produce both B and C, we can say that the substance A is a compound. This is because an element cannot be broken down into other substances by chemical means. Since A decomposes into B and C upon heating, it must have been composed of more than one kind of atom, making it a compound.
An element is defined as a substance that cannot be separated into two or more substances by ordinary chemical or physical means and has a constant composition. A compound, on the other hand, contains two or more elements, in definite proportion by weight, and can be separated into its elements. A homogeneous mixture has uniform composition throughout, while a heterogeneous mixture does not. A pure substance will always be either an element or a compound.
Therefore, for examples given:
Air is a homogeneous mixture because it has a uniform composition but is made up of several components like nitrogen, oxygen, and other gases.
Iron is an element because it cannot be broken down into simpler substances by ordinary chemical means and has constant composition.