The name calorie is used for two units of energy.
- The small calorie or gram calorie (symbol: cal) is the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.
- The large calorie, kilogram calorie, dietary calorie, nutritionist's calorie, nutritional calorie, Calorie (capital C) or food calorie (symbol: Cal) is approximately the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius. The large calorie is thus equal to 1000 small calories or one kilocalorie (symbol: kcal).
Although these units are part of the metric system, they have been superseded in the International System of Units by the joule. One small calorie is approximately 4.2 joules (so one large calorie is about 4.2 kilojoules). The factor used to convert calories to joules at a given temperature is numerically equivalent to the specific heat capacity of water expressed in joules per kelvin per gram or per kilogram. The precise conversion factor depends on the definition adopted.
In spite of its non-official status, the large calorie is still widely used as a unit of food energy in the US, UK and some other Western countries. The small calorie is also often used for measurements in chemistry, although the amounts involved are typically recorded in kilocalories.
The energy needed to increase the temperature of a given mass of water by 1 °C depends on the atmospheric pressure and the starting temperature. Accordingly, several different precise definitions of the calorie have been used.
The pressure is usually taken to be the standard atmospheric pressure (101.325 kPa). The temperature increase can be expressed as one kelvin, which means the same as an increment of one degree Celsius.
|Thermochemical calorie||calth||≡ 4.184 J||the amount of energy equal to exactly 4.184 joules |
|4 °C calorie||cal4||≈ 4.204 J
≈ 0.003985 BTU ≈ 1.168×10−6 kWh ≈ 2.624×1019 eV
|the amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure.|
|15 °C calorie||cal15||≈ 4.1855 J
≈ 0.0039671 BTU ≈ 1.1626×10−6 kWh ≈ 2.6124×1019 eV
|the amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure. Experimental values of this calorie ranged from 4.1852 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.|
|20 °C calorie||cal20||≈ 4.182 J
≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.610×1019 eV
|the amount of energy required to warm one gram of air-free water from 19.5 to 20.5 °C at standard atmospheric pressure.|
|Mean calorie||calmean||≈ 4.190 J
≈ 0.003971 BTU ≈ 1.164×10−6 kWh ≈ 2.615×1019 eV
|1⁄100 of the amount of energy required to warm one gram of air-free water from 0 to 100 °C at standard atmospheric pressure.|
|International Steam table calorie (1929)||≈ 4.1868 J
≈ 0.0039683 BTU ≈ 1.1630×10−6 kWh ≈ 2.6132×1019 eV
|1⁄860 international watt hours = 180⁄43 international joules exactly.[note 1]|
|International Steam Table calorie (1956)||calIT||≡ 4.1868 J
≈ 0.0039683 BTU ≈ 1.1630×10−6 kWh ≈ 2.6132×1019 eV
|1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).|
- The figure depends on the conversion factor between international joules and absolute (modern) joules. Using the mean international ohm and volt (1.00049 Ω, 1.00034 V), the international joule is about 1.00019 J, using the US international ohm and volt (1.000495 Ω, 1.000330 V) it is about 1.000165 J, giving 4.18684 J and 4.18674 J, respectively.
- 2. The two definitions most common in older literature appear to be the 15 °C calorie and the thermochemical calorie.
The calorie was first defined specifically to measure energy in the form of heat, especially in experimental calorimetry.
In these contexts, confusingly, the word "calorie" and "kilocalorie" refer to equivalent units (the former to the large calorie and the latter to 1000 small calories). Sometimes, in an attempt to avoid confusion, the large calorie is written as "Calorie" (with a capital "C"). This convention is not always followed, and not explained to the average person clearly.
These quantities are often used for the total amount of food energy (e.g. in a meal) and for the specific energy, namely amount of energy per unit of mass (e.g. "calories per gram", "calories per serving"). Nutritional requirements or intakes are often expressed in calories per day.
In scientific contexts, the term calorie almost always refers to the small calorie. Even though it is not an SI unit, it is still used in chemistry. For example, the energy released in a chemical reaction per mole of reagent is occasionally expressed in kilocalories per mole. Traditionally, this use was largely due to the ease with which it could be calculated in laboratory reactions, especially in aqueous solution: a volume of reagent dissolved in water forming a solution, with concentration expressed in moles per liter (1 liter weighing 1 kg), will induce a temperature change in degrees Celsius in the total volume of water solvent, and these quantities (volume, molar concentration and temperature change) can then be used to calculate energy per mole. It is also occasionally used to specify energy quantities that relate to reaction energy, such as enthalpy of formation and the size of activation barriers. However, its use is being superseded by the SI unit, the joule, and multiples thereof such as the kilojoule.
- Merriam-Webster's Online Dictionary Def 1a http://www.merriam-webster.com/dictionary/calorie
- Conn, Ph.D., Carole; Len Kravitz, Ph.D. "Remarkable Calorie". University of New Mexico. Retrieved 30 April 2014.
- Hargrove, James L (2007). "Does the history of food energy units suggest a solution to "Calorie confusion"?". Nutrition Journal 6 (44). doi:10.1186/1475-2891-6-44. Retrieved 31 August 2013.
- International Standard ISO 31-4: Quantities and units, Part 4: Heat. Annex B (informative): Other units given for information, especially regarding the conversion factor. International Organization for Standardization, 1992.
- FAO (1971). "The adoption of joules as units of energy".
The 'Thermochemical calorie' was defined by Rossini simply as 4.1833 international joules in order to avoid the difficulties associated with uncertainties about the heat capacity of water (it has been redefined as 4.1840 J exactly).
- Rossini, Fredrick (1964). "Excursion in Chemical Thermodynamics, from the Past into the Future". Pure and Applied Chemistry 8 (2): 107. doi:10.1351/pac196408020095. Retrieved 21 January 2013.
both the IT calorie and the thermochemical calorie are completely independent of the heat capacity of water.
- Lynch, Charles T. (1974). Handbook of Materials Science: General Properties, Volume 1. CRC Press. p. 438. Retrieved 8 March 2014.
- International Union of Pure and Applied Chemistry (IUPAC) (1997). "1.6 Conversion tables for units". Compendium of Analytical Nomenclature (3 ed.). ISBN 0-86542-615-5. Retrieved 31 August 2013.
- "Prospects improve for food energy labelling using SI units". Metric Views. UK Metric Association. 24 February 2012. Retrieved 17 April 2013.
- Chemistry Daily.[dead link]
- Zvi Rappoport ed. (2007), "The Chemistry of Peroxides", Volume 2 page 12.