Why is bronze harder than pure copper?

Bronze is harder than pure copper because it is an alloy created by combining copper with other metals, most traditionally tin.12

While pure copper is a highly ductile metal that is not particularly strong or hard on its own, the process of alloying it with tin or other elements—such as aluminum, manganese, or phosphorus—significantly enhances its mechanical properties. These additions alter the internal structure of the metal, resulting in a material that is stronger and harder than the base copper. In addition to increased hardness, these alloying processes often improve other characteristics, such as castability and resistance to corrosion.132