Boiling water can "explode" out of a cup when microwaving it due to a phenomenon known as "superheating." Superheating occurs when a liquid is heated beyond its boiling point without forming bubbles. This can happen in a clean cup with very smooth surfaces, where there are no nucleation sites (such as scratches or impurities) for bubbles to form.
Here's a step-by-step explanation of the process:
When you place a cup of water in the microwave, the microwave energy heats the water evenly, and it begins to reach its boiling point.
Normally, when water boils, bubbles of water vapor form at specific points within the water, called nucleation sites. These bubbles rise to the surface, releasing steam and preventing the water from further superheating.
However, in a smooth cup without any nucleation sites, the water can heat past its boiling point without forming bubbles. This means the water can exist in a superheated state.
If you disturb the superheated water or introduce a nucleation site (by, for example, adding something to the cup or tapping it), the water can suddenly and violently boil, causing an eruption of steam and hot water that can splash out of the cup. This sudden release of built-up pressure is what causes the "explosion."
To prevent this from happening, you can take some precautions:
Place a non-metallic object (e.g., a wooden stick or a microwave-safe stirrer) in the cup before microwaving. This will provide nucleation sites and help prevent superheating.
Alternatively, you can heat the water in shorter intervals and stir it between each interval. This will distribute the heat more evenly and also reduce the chances of superheating.
It's essential to be cautious when heating water in the microwave to avoid burns or accidents due to sudden steam eruptions. Always use microwave-safe containers and follow the manufacturer's instructions for safe microwave use.