Four students did their experiment on studying the cooling caused by evaporation of alcohol and recorded the following as their observations: A. After dipping in alcohol, the cotton plug absorbs alcohol and becomes pale blue. B. After attaching the cotton plug dipped in alcohol to the bulb of the thermometer and exposing the thermometer to air, the temperature starts falling C. After evaporation is complete, temperature starts increasing D. Tiny bubbles of alcohol escape from the surface of the cotton plug. The correct observations are those of students (1) B and D (2) B and C (3) A and C (4) A and D



Answer: (2) During evaporation, the liquid particles absorb energy from the surroundings to regain the energy lost. As a result of this absorption of energy, the surroundings become cooler. Therefore, when exposed to air, the alcohol absorbed by the cotton plug starts evaporating and the temperature starts falling. Hence, the observation made by student B is correct. When evaporation is complete, there is no more absorption of energy from the surroundings. Consequently, the temperature starts increasing. Hence, the observation made by student C is also correct.