Here's why:

* Physical properties can be observed or measured without changing the chemical composition of the substance. Hardness is measured by how resistant a substance is to scratching or indentation. This measurement doesn't alter the substance's chemical makeup.

* Chemical properties describe how a substance reacts with other substances or changes its composition. Hardness doesn't involve any chemical reactions or changes in the substance's identity.

Think of it this way: you can test the hardness of a piece of metal by scratching it with a knife. The metal might get scratched, but it remains the same type of metal. You haven't changed its chemical makeup, just its physical appearance.