Draw a graph of the variation of specific heat capacity of water with temperature and define calorie.

(N/A) The old unit of heat was the calorie, and one calorie was earlier defined as the amount of heat required to raise the temperature of $1 \; g$ of water by $1^{\circ} C$.
Variation of specific heat capacity of water with temperature:
The specific heat of water varies slightly with temperature; hence, for a precise definition of a calorie, it was necessary to specify the unit temperature interval.
Precise definition of a Calorie: The amount of heat required to raise the temperature of $1 \; g$ of water from $14.5^{\circ} C$ to $15.5^{\circ} C$.
The specific heat capacity of water is approximately $4186 \; J \; kg^{-1} \; K^{-1}$, which means $4.186 \; J \; g^{-1} \; K^{-1}$.
From the relation $W = JH$, the amount of work needed to produce $1 \; \text{cal}$ of heat is called the mechanical equivalent of heat.
Therefore, $W = J$ (where $H = 1 \; \text{calorie}$).
Thus, there are two units of heat, Joule and Calorie, and for conversion, $1 \; \text{calorie} = 4.186 \; J$ of heat is needed.