Which one of the following is the amount of heat gi ven up when 20g of steam at 100°C is condensed and cooled to 20°C?

To calculate the total heat released, we must consider two distinct processes: condensation and cooling. First, 20g of steam at 100°C condenses into water at 100°C. The heat released during this phase change is calculated using the latent heat of vaporization (L = 540 cal/g), resulting in Q1 = mass × L = 20g × 540 cal/g = 10,800 cal [c1][t1]. Second, the resulting 20g of water cools from 100°C to 20°C. Using the specific heat capacity of water (c = 1 cal/g°C), the heat released is Q2 = mass × c × ΔT = 20g × 1 cal/g°C × (100°C - 20°C) = 20 × 80 = 1,600 cal [t2]. The total heat given up is the sum of both processes: 10,800 cal + 1,600 cal = 12,400 cal. This demonstrates how latent heat is released during phase transitions without temperature change, followed by sensible heat loss during cooling [c5][t1].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Explanation > p. 295
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Latent Heat > p. 294