Calorie
Definition and meaning of Calorie in chemistry.
Calorie is a unit of heat energy, originally defined as the amount of heat needed to raise the temperature of 1 gram of water by 1°C. In modern usage, the thermochemical calorie is fixed by definition as exactly 4.184 joules.
In more detail
Two related units share the name: the small calorie (cal), used in chemistry and physics, and the large Calorie (capital C, also called a kilocalorie, kcal), used in nutrition, which equals 1000 small calories. Because the heat capacity of water varies slightly with temperature, early calorie definitions differed (the 15°C calorie, the mean calorie, and the thermochemical calorie); today the thermochemical calorie's fixed value of 4.184 J is the accepted conversion factor. In modern SI-based scientific work, the joule is the preferred unit of energy, but the Calorie persists on nutrition labels and in everyday dietary contexts.
Key facts
| Field | Physical Chemistry |
|---|---|
| Conversion | 1 cal = 4.184 J (thermochemical calorie, exact) |
| Nutritional Calorie | 1 Cal (kcal) = 1000 cal = 4184 J |
| SI status | Non-SI unit; joule is the SI unit of energy |
Heating 1 gram of water from 14.5°C to 15.5°C at standard atmospheric pressure requires about 1 calorie (4.184 J) of energy; a food item listing "200 Calories" actually supplies 200,000 small calories, or about 837 kJ.
Frequently asked questions
What is the difference between a calorie and a Calorie?
A calorie (lowercase, cal) is the small unit historically defined for 1 g of water and used in chemistry; a Calorie (capitalized, equivalent to a kilocalorie) is used on food labels and equals 1000 small calories.
Why do scientists prefer joules over calories?
The joule is the SI-derived unit of energy and integrates directly with other SI units, whereas the calorie is a legacy unit retained mainly for historical, nutritional, and some thermochemical applications, related to the joule by the fixed factor 4.184 J per calorie.