Bronze swords are harder than pure copper swords due to the addition of other elements in the bronze alloy. Bronze is primarily composed of copper, but it is often alloyed with other metals such as tin, arsenic, or lead to create a stronger and more durable material.

The addition of these alloying elements enhances the hardness and overall strength of the bronze. Tin, in particular, is commonly used as an alloying element in bronze, as it forms a solid solution with copper and contributes to the material's hardness. The exact composition of the bronze alloy can vary depending on the desired properties and the specific time period or culture in which the sword was made.

In contrast, pure copper is a relatively soft metal, making it less suitable for weapons like swords. It is more malleable and ductile, which means it can deform and bend more easily under stress. Copper swords would not hold an edge well and would be prone to deformation or breakage during combat.

By alloying copper with other elements, such as tin, bronze swords gain improved hardness, strength, and durability. This allows the swords to maintain their shape and sharpness, withstand the forces of combat, and provide a more effective weapon for cutting, slashing, and thrusting.