How do natural processes represent physical or chemical changes

A "natural" chemical change would be burning wood, cooking eggs, baking a cake and milk going sour. These changes can't be changed so therefore they are chemical changes.

A natural physical change would be mixing salt or sand with water, melting or boiling water, cutting wood, and shredding paper. The salt and sand can be removed from the water. Hot and cold water are still water. Wood and paper didn't change their composition. All of these examples are reversible.