Assertion (A) : Water in an open pond remains cool even on a hot summer day. Reason (R) : Water on heating evaporates and heat energy gets converted into latent heat.

The assertion that water in an open pond remains cool on a hot summer day is true due to the process of evaporative cooling. Evaporation is a phase change where water transforms from a liquid to a gaseous state [c1]. This process requires energy, known as the latent heat of vaporization, which is absorbed from the water body itself and the surrounding environment [c2][c4]. As water molecules at the surface absorb this heat to overcome molecular attraction and escape as vapor, the average kinetic energy of the remaining liquid decreases, leading to a drop in temperature [t3][t5]. Therefore, the reason correctly explains the assertion: the heat energy from the sun or the water is converted into latent heat during evaporation, effectively carrying heat away and keeping the pond cool [c4][t3].

Sources

1. [1] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 10: Water in the Atmosphere > EVAPORATION AND CONDENSATION > p. 86
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Explanation > p. 295
3. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Latent Heat > p. 294