Answer: 400 kcal
The heat needed is \(Q = mc\Delta T\), where m = 5 kg, c = 1 kcal/kg ºC and \(\Delta T = 100 - 20 = 80\) ºC.
\[Q = 5 \times 1 \times 80 = 400\ \text{kcal}\]
In joules, \(400\ \text{kcal} = 400 \times 4180 = 1.67 \times 10^6\ \text{J}\).