Here's why:

* Hardness: Hardness refers to a material's resistance to scratching. This is a physical characteristic and doesn't involve changing the chemical composition of the zinc.

* Density: Density is a measure of how much mass is contained in a given volume. It's also a physical property, as it doesn't alter the chemical makeup of the zinc.

Physical changes are alterations in the form or appearance of a substance, but not its chemical composition. Examples include melting, freezing, boiling, or crushing.

Chemical changes involve the formation of new substances with different chemical compositions. Examples include burning, rusting, or cooking.