Which substance is hardest to heat up gram for gram by one Kelvin?

Water in its solid state (ice) possesses a particularly high specific heat capacity, meaning that it requires a significant amount of energy to raise its temperature, gram for gram, by one Kelvin. This characteristic is largely due to the strong hydrogen bonds that form between water molecules, which must be broken for the temperature of the ice to increase.

In contrast, metals such as copper and iron typically have lower specific heat capacities, allowing them to heat up more quickly under the same conditions. Air, being a gas, has a relatively low specific heat compared to solids and liquids, which means it also requires less energy to achieve a temperature change when compared to ice.

Therefore, among the listed options, ice requires the most energy per gram to increase its temperature by one Kelvin, making it the hardest substance to heat up in this context.