Question

A student sets up an apparatus to determine the melting point of ice. He takes a beaker half filled with crushed ice and dips a mercury thermometer with an initial reading of room temperature (25ºC) in such a way that the bulb of the thermometer is surrounded by ice. The correct observation obtained by the student is that :
(1) Mercury in the thermometer keeps on falling till it reads, −1ºC and becomes constant thereafter
(2) temperature falls, reaches 0ºC, then it remains constant even after the whole of the ice has melted
(3) the temperature falls in the beginning but starts rising as soon as the ice starts melting
(4) temperature falls, reaches 0ºC and remains constant only as long as both ice and water are present in it

Answer 1

When the student places the thermometer in a beaker filled with crushed ice, its temperature will decrease until it reaches 0º and then it will become constant. At 0ºC, all of the heat will get utilized in the phase transition of ice to water. So, the temperature of the thermometer will remain constant until all the ice melts.

Hence, the correct answer is option 4.