How much is 1 calorie in joules?
4.186 joules
The calorie and the joule are both units of energy, so one can be expressed in terms of the other by a fixed conversion factor.
This factor comes from the mechanical equivalent of heat, the experimentally established link between heat energy and mechanical work, first measured by Joule.
The calorie is defined as the heat needed to raise the temperature of one gram of water by one degree Celsius, and this quantity of energy equals a definite amount of mechanical work.
By that definition, one calorie equals \(4.186\) joules, so \(1\ \text{cal} = 4.186\ \text{J}\).
The other choices do not match this standard value: 1 joule and 10 joules are arbitrary round numbers with no physical basis for the conversion, and 2.5 joules is simply incorrect.
Only the value fixed by the mechanical equivalent of heat is right.
Hence, the answer is 4.186 joules.
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