I see heats of combustion given in kg [dot] calories per gram molecular weight (Handbook of Chemistry and Physics, 1969-1970). I don't know how that makes sense, as I don't know what a kg cal might be. I would have expected units of energy per mass (calories per gram) or energy per mole. What is a kg cal, and what is wrong in my thinking?

  • $\begingroup$ "gram" in the title of this thread should be "gram molecular weight". I previously misread the units in the table in the Handbook. $\endgroup$ – John Surname Jun 22 '15 at 22:15

$\mathrm{kg~cal}$ probably represents the kilogram calorie (especially given that it is an old book). Nowadays it is usually just known as the Calorie, usually abbreviated $\mathrm{kcal}$. It is roughly the amount of energy required to raise the temperature of $1~\mathrm{kg}$ of water by $1~\mathrm{^\circ C}$. Since this is temperature and pressure dependent, there are slight variations in the actual definition depending on the context in which it is used.

| improve this answer | |
  • $\begingroup$ Okay, thanks. Then a kg cal is the same as a "food calorie". Back in the day, we just called a calorie a calorie and a kilocalorie a kilocalorie. BTW I find that paraffin wax has about the same energy content as food fat. $\endgroup$ – John Surname Jun 21 '15 at 20:49
  • $\begingroup$ I sincerely doubt that kilogram-calorie is "nowadays known as the Calorie"... the two don't have the same dimensions. Perhaps you meant to say that the kilogram-calorie-per-gram is now known as the Calorie? $\endgroup$ – user541686 Jun 22 '15 at 0:48
  • $\begingroup$ The kilogram calorie is a unit of energy, not energy times mass, despite the name. It is named 'kilogram' to distinguish it from the 'normal' calorie which is the energy required to heat $1~\mathrm{g}$ of water by $1~^\circ C$. However, the kilogram calorie ($\mathrm{kcal}$) has become the 'normal' calorie due to its extensive usage in food labelling and is now colloquially referred to just as the calorie. $\endgroup$ – bon Jun 22 '15 at 20:19
  • $\begingroup$ Presumably the "kilogram" in the name "kilogram calorie" refers to heating a kilogram of water, rather than heating a gram of water. $\endgroup$ – John Surname Jun 22 '15 at 22:13
  • $\begingroup$ @JohnSurname That is correct. $\endgroup$ – bon Jun 24 '15 at 13:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.