Here's why:

* Chemical weathering involves changes in the chemical composition of rocks. Oxidation is a chemical reaction where oxygen combines with other elements, often metals like iron, to form oxides. This changes the composition of the rock.

* Mechanical weathering involves the physical breakdown of rocks into smaller pieces without changing their chemical composition. Examples include frost wedging, abrasion, and root wedging.

Example: When iron in a rock reacts with oxygen, it forms iron oxide (rust). This rust is a different substance than the original iron, and it is weaker, causing the rock to break down.