The efficiency of an ideal heat engine working between the freezing point and boiling point of water, is

Freezing point of water \( T_2 = 0^\circ\text{C} = 273\text{ K} \). Boiling point of water \( T_1 = 100^\circ\text{C} = 373\text{ K} \). Efficiency \( \eta = 1 - \frac{T_2}{T_1} = 1 - \frac{273}{373} = \frac{100}{373} \approx 26.8\% \).