Final answer:
The temperature of water drops quickly after the immersion heater is turned off due to increased heat transfer to the cooler surroundings, until the temperature difference decreases, which subsequently slows down the rate of cooling.
Step-by-step explanation:
The temperature of the water falls at the fastest rate just after the immersion heater switches off because heat transfer to the surroundings is the greatest at this point. When the heater is on, it supplies heat to maintain the water's temperature.
Once the heater switches off, no additional heat is supplied to counteract the cooling effect of the surrounding environment. Thus, the water begins to cool rapidly until thermal equilibrium with the surroundings starts to be re-established, after which the rate of temperature decrease will slow down.
Heat moves from the hot water to the cold surroundings. Initially, the temperature difference between the water and the surroundings is at its maximum, resulting in a high rate of heat transfer. As the water temperature decreases, the temperature differential between the water and its environment diminishes, therefore slowing down the rate of heat loss.