Here's why:

* Monomers to Polymers: Polymers are formed by joining together many smaller molecules called monomers. This joining process involves the formation of covalent bonds, which are strong chemical bonds.

* Chemical Reactions: The process of forming polymers from monomers is called polymerization. This is a chemical reaction where monomers react with each other, releasing small molecules like water or methanol as byproducts.

* Changes in Properties: The resulting polymer has significantly different properties compared to the original monomers. For example, individual ethylene molecules (monomers) are gases at room temperature, while polyethylene (a polymer of ethylene) is a solid plastic.

Therefore, the formation of polymers is a clear indication of a chemical change.