This reaction teaches us that the substances on the left side of the arrow in a chemical equation are known as reactants.
A reactant is a substance that exists at the start of a chemical reaction.
And the substances to the right of the arrow are referred to as products.
A product is a substance that results from a chemical reaction.
The reactants in this case are zinc and sulphur, which can be chemically combined to form the zinc sulphide product.
Final Answer:
Hence, a reactant is a substance that exists before a chemical reaction begins, whereas a product is a substance that results from a chemical reaction.