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Why does coffee explode in the microwave?

Ever wondered why spurting and frothing are created when we make coffee in the microwave? The phenomenon is characterized by water exploding in a roiling form, causing it to spray all over our hands and body. Indeed, this can burn quite badly.

Many of us boil coffee in a microwave using a medium-sized ceramic cup of coffee. After two minutes, if you remove the cup of coffee and try to stir it, this can cause bubbles to rise and overfill the cup, leading to burns or at the very least a mess.

This is quite a common experience yet the question remains; why does coffee explode in the microwave? In this article, we will look to answer this exact question.


golden smoke taking away from coffee seeds

First, heating water in a microwave can make the temperature go beyond boiling point even before it has started to visibly boil. This is called superheating and will destabilize the liquid. For instance, superheating 1 liter of water by an extra degree beyond the boiling point makes it produce about 3 liters of steam. This makes it boil vigorously and explode.

If you were wondering why coffee explodes in the microwave, this superheating is the cause for the phenomenon. When a liquid is boiling, say in a glass, the molecules on the surface actually start moving and continue to evaporate and even disappear.

In other words, the molecules are moving into the vapor phase as the boiling continues whilst it is continually being heated. The transition to the gas phase is happening as the molecules hit the surface of the liquid and it must overcome the surface tension. To overcome, it must gain enough kinetic energy. In other words, the kinetic energy to escape.

So liquid is disappearing and molecules are evaporating into airspace in the container. The air gets trapped in the container whenever a liquid is introduced. When a liquid is put into a container, and the container into a microwave, the water is being heated but the container is not, and so the container has the effect of cooling the water as it is heated. It means the water is hotter at the center by as much as 10 degrees than the water in contact with the container. The liquid at the center will reach boiling point and become superheated before the water reaches the boiling point.

Introducing sugar and a teabag spurs vigorous boiling and the introduced sugar and tea are a form of surface imperfections. These imperfections will introduce trapped air bubbles and superheated liquid vaporizes into these air bubbles. The explosion happens as the superheated liquid comes into contact with air bubbles on the periphery.

The bubble of air doesn’t have to be on the surface, it can also be at the bottom. So when a molecule is escaping from liquid into the air, it can impact the bubble or crash into the bubble and cause it to expand. In other words, the bubbles are serving as central points for the water vapor to boil around. As it gathers more crashing or vaporizing molecules, the bubble becomes large until it is buoyant enough to detach from the bottom and move to the surface, and this is what we see as boiling. It produces a bubbling effect similar to blowing up a balloon. The elastic force compressing the bubble is greatest on the smallest bubble.

Thus the small imperfections in the container which trap the tiny air bubbles, are the starting points for the bubble-blowing process. Sometimes when the high energy molecules cannot have the weak points to collect in, they will stay dissolved in the water until they are disturbed. This disturbance can, for instance, happen if someone starts jostling the cup or putting a straw into the container.

Conditions favoring the explosion

cup of coffee putting inside the microwave

Some conditions that could favor this condition include using a container with a very smooth surface to boil the water in the microwave. A good example is a container that is unscratched or glazed.

When we heat water in the microwave for longer, this condition is more likely to occur than when water is heated for the adequate time. An example is when we put water in a new mug, and then leave it in the microwave for longer than necessary due to a distraction.

Reheating makes the explosion worse

A cup of coffee that was heated previously is more likely to undergo superheating when reheated again because gasses find it tough to stay dissolved in preheated liquids.

How to avoid exploding coffee in a microwave

You can avoid accidents by placing a plastic spoon in the mug or glass while heating in the microwave. The alternative is using a glass rod where there is concern about chemicals leaking from the plastic spoon.

A wooden stir-stick can also be used, dipping it into the cup of coffee before boiling happens and this can stop the problem.

We can also avoid the problem by using a container that has imperfections, maybe a few scratches or glazing. The most obvious way of avoiding this is actually to stop overheating the water by putting it in the microwave for the perfect amount of time!

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