1 calorie is equal to ____________

Hint: To do this question, one must know the various units of calories could be defined into.
Calorie is a unit of energy. It can be defined as the amount of heat which is required at one atmosphere pressure to raise the temperature of one gram of water by one degree Celsius.

Complete step-by-step answer:

The quantity of heat represented by the calorie differs at different temperatures (~1%), it has therefore been necessary to define a temperature at which the specific heat of water is to be taken as 1 calorie.

So, there is the “15° calorie” which has been defined as the amount of heat that will raise the temperature of 1 gram of water from 14.5° to 15.5° C (difference = 10 C) equal to 4.1855 joules.

The mean calorie is defined as $\dfrac{1}{{100}}$ of the heat necessary to raise the temperature of 1 gram of water from 0° to 100° C ( difference =1000 C but on taking average it reduce to 10 C) 4.19002 Joules.

There is one more 20° calorie definition. The heat required to raise the temperature from 19.5° to 20.5° C ( difference = 10 C) is equal to 4.1819 joules

In thermo-chemistry the unit of heat energy used is the thermo-chemical calorie, equal to 4.184 joules. It is commonly used as the unit for heat capacities, heats of reaction and latent heats.

Another definition for calorie is the International Table calorie (IT calorie), defined as $\dfrac{1}{{860}}$ international watt-hour. It is used in engineering steam tables. It is equal to 4.1868 joules.

A kilocalorie is the amount needed to raise the temperature of 1 kilogram of water by 10 C.
Kilocalories (kcal) are generally used in assigning the amount of nutrition given by a small amount of food, often prescribed outside food packets.

Very simply, we say 1 calorie is equal to 4.2 Joules. This is the most accepted format.

Note: While defining a calorie, whichever system or unit we take the temperature difference is always equal to 10 C.