Here's why:

* No new substances are formed: Salt (NaCl) and pepper (a complex mixture of compounds) remain the same chemically. They are simply mixed together.

* Reversible: You can easily separate the salt and pepper by using a sieve or other methods.

* No change in chemical composition: The chemical makeup of the salt and pepper doesn't change during the mixing process.

Let me know if you'd like to explore other examples!