How is hardness compared among materials?

Hardness is primarily assessed through tests that involve the ability of one material to scratch another, known as scratch tests. This method is based on the premise that a harder material can scratch a softer one, allowing for a relative comparison of hardness between different substances. The most famous example of this is the Mohs scale of mineral hardness, which ranks minerals based on their capacity to scratch one another.

While measuring melting points, testing density, and determining boiling points are valuable properties of materials, they do not directly relate to the concept of hardness. Melting and boiling points are indicators of thermal properties rather than mechanical strength, while density measures mass per unit volume and does not provide information about a material's ability to resist deformation or abrasion. Thus, the relationship between materials and their respective ability to scratch one another effectively serves as the benchmark for hardness comparison.