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Everything in chemistry often comes with a strong reason and facts behind it, but I stuck in definition of $1$ calorie.

My reference book define one calorie as amount of heat required to raise the temperature of one gram of water from $14.5$ degrees Celsius to $15.5$ degrees Celsius.

Why $14.5$ and $15.5$, why temperature such as $0$ to $1$ or $19$ to $20$ degree Celsius is not chosen?

Please forgive me if this question is too easy and I can't figure out the logic related to it.

I shall be glad to have your suggestions.

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  • $\begingroup$ Those are close to room temperature. Using 0C runs the risk of getting some enthalpy of the phase transition mixed in. $\endgroup$ – Jon Custer Oct 5 '16 at 16:32
  • $\begingroup$ i cant understand everything after room temperature $\endgroup$ – Vidyanshu Mishra Oct 5 '16 at 16:34
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    $\begingroup$ What happens to water at zero degrees C? How would that impact a thermodynamic measurement? $\endgroup$ – Jon Custer Oct 5 '16 at 16:36
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    $\begingroup$ Now, the question of why not other temperature ranges - the Gibbs free energy of water is not constant vs. temperature, so the heat capacity varies with temperature. Thus, you must specify a specific temperature range. $\endgroup$ – Jon Custer Oct 5 '16 at 18:08
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    $\begingroup$ That also is the reason it is so difficult to liquefy helium: at low temperatures the heat capacity drops, so that the slightest energy input warms it greatly. See en.wikipedia.org/wiki/Superfluid_helium-4#/media/… $\endgroup$ – DrMoishe Pippik Oct 5 '16 at 22:14
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why temperature such as 0 or 1 degree celsius is not chosen??

That has been chosen by some.

According to Physical Laboratory Experiments: Part III.--Heat, 4th edition (1912) :

The following different calories should not be confused:

  1. ZERO DEGREE CALORIE:- The quantity of heat required to raise one gram of water from 0 [degrees] C to 1 [degree] C.

  2. ORDINARY CALORIE: cal.-The quantity of heat required to raise one gram of water from 15 [degrees] C to 16 [degree] C, this being taken as the mean room temperature.

  3. MEAN CALORIE OR ICE CALORIMETER CALORIE: The one-hundred part of heat required to raise one gram of water from 0 [degrees] C to 100 [degrees] C.

  4. ...

Heat capacity of liquid water is a continuously varying function of temperature itself, so a particular temperature or temperature interval must be specified. Various intervals were chosen historically, and when a small interval near 15 degrees C is chosen it is because that was thought of as room temperature 100-200 years ago.

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Some measurements are easier than others. If you try to measure the heat capacity of water at $0\ \mathrm{^\circ C}$, you'll need to be awfully careful that ice melt doesn't alter the accuracy of your measurement. This is not a problem at a higher temperature like $15\ \mathrm{^\circ C}$.

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  • $\begingroup$ what if i go to 20 or 30 degree celsius?? $\endgroup$ – Vidyanshu Mishra Oct 6 '16 at 15:07
  • $\begingroup$ As @DavePhD pointed out, the heat capacity does vary as a function of temperature. You might get a different value. There's nothing that says you can't report that value or put it in a table, but it might be less useful if that doesn't fit your use case. $\endgroup$ – Zhe Oct 6 '16 at 17:50

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