When you heat soup in a microwave, the heating process is not uniform throughout the liquid. Several factors contribute to the phenomenon you described, where the soup gets very hot initially but cools down quickly to a lukewarm temperature. These factors include:
Uneven heat distribution: Microwaves work by emitting electromagnetic waves that are absorbed by water molecules in the food, causing them to vibrate and generate heat. However, this process can be uneven, leading to pockets of hotter and cooler areas within the soup. Some parts of the soup may become very hot, while others remain cooler.
Lack of convection: In conventional heating methods like stovetop heating, the heat is transferred through conduction and convection. In a microwave, convection currents don't play a significant role, leading to less effective heat distribution. As a result, the heat doesn't circulate well within the soup, contributing to temperature variations.
Standing time: After you remove the soup from the microwave, it continues to cook due to residual heat. During this standing time, the heat within the soup equalizes, with the hotter parts releasing heat to the cooler parts, causing an overall temperature drop.
Loss of steam: When heating soup in a microwave, steam builds up within the liquid, which can contribute to initial temperature spikes. Once you remove the soup from the microwave, some of the steam may escape, taking away heat energy from the soup and cooling it down.
Evaporation: The microwave heating process can lead to some evaporation of water from the soup, which can also cause cooling as it takes away heat energy.
To minimize temperature variations, you can try stirring the soup halfway through the microwave cooking time to distribute the heat more evenly. Additionally, letting the soup sit for a minute or two after microwaving can help the temperature even out before consuming it.