A takeaway cup of coffee can catch fire in a microwave when it is not full due to a phenomenon known as "thermal runaway" or "superheating."
When water is heated in a microwave, it can reach temperatures above its boiling point without actually boiling. This happens because boiling requires the formation of bubbles, which provide nucleation sites for the vapor to escape. In a very clean or smooth container, there might not be enough nucleation sites for bubbles to form, causing the water to become superheated.
When you remove the cup from the microwave, the superheated water remains in a liquid state even though it is above its boiling point. At this point, any disturbance, such as adding a spoon, tapping the cup, or simply blowing on the liquid, can trigger a sudden and violent boiling reaction. This rapid boiling release of steam can lead to the contents of the cup erupting, potentially causing the cup to spill or even ignite if flammable materials are nearby.
Here are some safety tips to avoid superheating in the microwave:
Stir the liquid: Before microwaving liquids, such as water or coffee, it's a good idea to stir them to introduce some agitation and create nucleation sites for bubbles.
Use microwave-safe containers: Make sure you're using containers that are specifically labeled as microwave-safe and are suitable for heating liquids.
Be cautious when handling: Always handle hot liquids with care, even if they don't appear to be boiling. Use appropriate heat-resistant gloves or pot holders to protect your hands.
Allow for cooling: After heating liquids in the microwave, give them a moment to cool before handling or drinking to avoid accidental burns.
By following these precautions, you can reduce the risk of superheating and the potential hazards associated with microwaving liquids in a takeaway cup or any other container.