Here's why:

* Physical properties can be observed or measured without changing the chemical composition of a substance. Hardness is a measure of how resistant a material is to scratching or indentation. This is purely a physical characteristic.

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

For example, you can test the hardness of a piece of diamond by scratching it with another material. This test doesn't alter the diamond's chemical structure, only its physical form.