When a glass of water is heated by a microwave, the water undergoes a process known as "heating by dielectric heating." In this process, microwave radiation excites the water molecules, causing them to vibrate rapidly and generate heat. As a result, the water temperature increases.
During the heating process, some of the water molecules will convert to vapor, leading to evaporation. This can cause the overall volume of the water to decrease slightly. However, the loss of water through evaporation is typically minimal unless the heating time is significantly prolonged or the power of the microwave is very high.
It's worth noting that water can become superheated in a microwave, which means it can exceed its boiling point without actually boiling. Superheated water is in a metastable state and can undergo a sudden and violent boiling eruption when disturbed, such as when a spoon or other object is placed in the container. This is why it's important to exercise caution when heating water in a microwave and to avoid superheating by using microwave-safe containers and stirring the water before consuming or adding anything to it.
In summary, while some water loss may occur due to evaporation, it is generally not significant when heating a glass of water in a microwave for a typical duration.