Advanced #10

Liquid water can be easily superheated

Water can be heated above its boiling point without boiling.

Scientific Explanation

Water boils at 100 degrees Celsius — at least in theory. In practice, liquid water can be heated significantly above this temperature without forming vapor bubbles. This phenomenon is called superheating and arises from the same principle as supercooling: the absence of nucleation sites where the new phase — in this case, vapor bubbles — can form.

For a vapor bubble to survive inside the liquid, it must be large enough to overcome the pressure of the surrounding liquid and the surface tension. Small bubbles are crushed by surface tension and collapse immediately. Only above a critical size does a bubble grow spontaneously. In very pure water without impurities and in smooth containers, the sites where these critical nuclei can form are absent.

Under ideal conditions, water can be superheated to approximately 300 degrees Celsius at atmospheric pressure — a metastable state. The absolute limit is the spinodal temperature of about 330 degrees Celsius at atmospheric pressure, where the liquid spontaneously and explosively transforms into vapor. Any disturbance — a tiny air bubble, a speck of dust, a vibration — can trigger the sudden and violent bubble formation known as bumping.

Superheating Curve of Water Heating curve showing how water can be heated above 100 degrees Celsius without boiling (superheating) until a disturbance causes sudden violent boiling (bumping). The diagram shows temperature versus time with the superheating region highlighted. Time Temperature 20 °C 100 °C 130 °C Liquid heating Normal boiling Superheated! Bumping (violent boiling) Superheating of Water
Superheating of water: the temperature rises above the boiling point until a disturbance triggers bumping.

Everyday Relevance

Superheating is a well-known hazard particularly associated with microwave ovens. When water is heated in a smooth container, the nucleation sites necessary for normal boiling are absent. The water reaches temperatures above 100 degrees Celsius while appearing perfectly calm. The moment you insert a spoon or add instant coffee, the water can erupt violently, causing severe burns.

In the chemistry laboratory, boiling chips — small porous bodies whose rough surfaces serve as nucleation sites for vapor bubbles — provide protection against bumping. In the food industry and industrial processes, superheating is carefully avoided to prevent uncontrolled steam explosions. Water’s tendency to superheat makes it a substance that demands respect.