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Why do equal masses of different substances require different amounts of heat to warm up?

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Joseph Black Joseph Black, detail of an engraving by J. Rogers after a portrait by Sir Henry Raeburn

The premise that equal masses of different substances require different amounts of heat to warm up is a fundamental observation that led the Scottish scientist Joseph Black to establish the concept of specific heat in the 18th century.1

While Black identified this phenomenon through experimental observation, modern science explains that these differences arise because heat capacity depends on the internal energy levels available to the atoms within a material, such as their specific quantum states. For most materials, the amount of energy required to raise the temperature varies based on these atomic properties.2

In the early 19th century, this relationship was further clarified by the Dulong-Petit law, which demonstrated that for many elements, the heat capacity per atom tends to be consistent at sufficiently high temperatures, allowing scientists to calculate atomic weights based on specific heat measurements.21