Specific heat capacity often varies with temperature, and is different for each state of matter. Liquid water has one of the highest specific heat capacities among common substances, about 4184 J⋅kg ⋅K at 20 °C; but that of ice, just below 0 °C, is only 2093 J⋅kg ⋅K. The specific heat capacities of iron, granite, and hydrogen gas are about 449 J⋅kg ⋅K, 790 J⋅kg ⋅K, and 14300 J⋅kg ⋅K, respectively. While the substance is undergoing a phase transition, such as melting or boiling, its specific heat capacity is technically undefined, because the heat goes into changing its state rather than raising its temperature.