Thermodynamics of water superheated in the microwave oven

Abstract

A simple visual demonstration is proposed that provokes thinking about the elementary thermodynamics of heating and boiling. Water is conveniently heated above its normal boiling point in a microwave oven in a glass microwave oven teapot. Water stops boiling soon after heating is interrupted, but subsequently added rough particles can still act as nucleation centers for a brief, spectacular burst of steam bubbles. The heat to make those steam bubbles obviously comes from the water itself, so that one can conclude that the boiling water was superheated, which is confirmed with a thermometer. Besides illustrating chemical thermodynamics, the demonstration also shows why safety precautions are usually taken in the laboratory to prevent superheating. Details of the observations are discussed by giving background on the nucleation of steam bubbles.