The Molecular Basis for the Heat Capacity and Thermal Expansion of Natural Waters

The high heat capacity of seawater has been cited as why 93% of the heat trapped by anthropogenic greenhouse gases is absorbed by the ocean. Specific heats (CP) are closely tied to molecular weight. The mean molecular weight of pure water over the range 0–40 °C is 86.1–80.7 and 89.4–84.5 for seawater. Warming of water increases the kinetic energy of the molecules and induces breaking of hydrogen bonds (8.364 kJ/mol); both effects increase the volume of the fluid. Warming pure water from 0–10 °C increases the single H2O molecular form by 1.64%, accounting for 36.3% of the energy consumed. The specific heat of pure water is thus attributable (63.7%) to increasing the kinetic energy of the water, and (36.3%) to the energy required to break hydrogen bonds. For seawater, 34.7% of the energy goes to breaking hydrogen bonds while the rest (65.3%) is attributable to increasing the kinetic energy of the molecules. Key Points The specific heat of water and sea water is a function of the molecular structure governed by hydrogen bonding. Warming water and sea water breaks hydrogen bonds and releases single H2O molecules accounting for 36% of the heat absorbed. The molecular weight of water ranges from 86 to 81 from 0 to 40 degrees C accounting for the remaining 64% of the specific heat.