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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?

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  • $\begingroup$ "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
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$\mathrm{kg~cal}$ 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.

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  • $\begingroup$ 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

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