Calorie (thermochemical)

The calorie was defined as the heat needed to raise one gram of water by one degree Celsius, but water's specific heat varies with temperature, so the definition had to specify which degree — producing a small family of incompatible calories. The thermochemical calorie was fixed at exactly 4.184 J to end the argument, while the international-table calorie used in steam work is exactly 4.1868 J, and the 15 °C calorie is about 4.1855 J.

The unit persists in three places: chemistry, where bond and reaction energies are still often quoted in kcal/mol; nutrition, where the capital-C Calorie on a food label is a kilocalorie, so a 500-Calorie meal is 2.09 megajoules; and older HVAC and refrigeration literature. SI has deprecated it, but a kcal/mol is such a convenient size for chemical energies — about 4.184 kJ/mol, roughly the strength of a hydrogen bond — that it refuses to die.