I reheated a mug of instant coffee in the microwave, but left it too long and it was boiling over when I took it out. I ran a little cold water into the mug to cool the coffee, and it boiled again. The tap water ran in gently, and large active bubbles appeared on top of the coffee. How could this happen?
鈥� Boiling depends on bubbles of water vapour being able to form within the liquid that is being heated. That process is helped by the presence of nucleation sites 鈥� for example, sharp points on surfaces, suspended particles and existing small bubbles 鈥� all of which help new bubbles to form and grow.
If the pressure inside a bubble is less than that of the atmosphere on the liquid鈥檚 surface, the bubble collapses. It is the noise of collapsing bubbles that makes a kettle 鈥渟ing鈥� just before it comes to the boil. Once the liquid has reached its boiling point, the vapour pressure inside the bubbles is sufficient for them to persist and rise to the surface, carrying molecules of the liquid into the air.
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This is where things get less straightforward. The pressure required to stop a bubble of vapour collapsing is actually slightly greater than that on the liquid鈥檚 surface.
Perhaps surprisingly, the excess pressure is inversely related to the bubble鈥檚 radius. This means that the smaller the bubble, the higher the pressure needed, and since pressure rises with temperature, so the hotter the bubble needs to be to persist.
This helps to explain why a liquid can be heated well above boiling point without boiling, provided no bubbles or sharp edges exist inside. Such a liquid is said to be superheated.
Microwaves heat a cup of coffee unevenly. So even though the coffee was boiling over when taken out of the microwave, parts of it may remain superheated after the existing bubbles have escaped. Indeed the average temperature may stay above boiling point.
鈥淢icrowaves heat a cup of coffee unevenly, and parts of it can be superheated even if no longer bubbling鈥�
The cold water that is then added to the mug immediately warms and gives up small bubbles of air (air is less soluble in warm than in cold water). These bubbles bring the coffee to the boil once again, by allowing new bubbles to form within the superheated regions.
The coffee can now boil vigorously, which is potentially dangerous as spitting and foaming of the hot liquid may occur. This is why microwave ovens and microwaveable meals often carry warnings to let food stand for a minute or so after cooking before being moved or stirred.
Alan Goodwin, Cheadle Hulme, Cheshire, UK
鈥� The coffee was probably not actually boiling; rather, dissolved air was escaping.
Take a pan of tap water, put it on the stove and start heating it. Very quickly, tiny bubbles will form on the surface, even when the water is quite cool. You can test this (very cautiously) by dipping your finger in.
Tap water is often saturated with air. With most gases, the solubility in water decreases as the temperature rises, so the dissolved air escapes when the water is heated. The air bubbles expand as the temperature rises and they gradually detach from the pan鈥檚 sides and rise quickly to the surface of the water, where they burst.
What has any of this to do with the coffee miraculously appearing to come back to the boil? When the cold tap water was added to the cup, the dissolved air in this water would come out of solution very quickly as it heated. As a result, these bubbles would rapidly expand and burst.
It is very difficult to visually distinguish between air bubbles forming, expanding and bursting and steam bubbles doing the same 鈥� but only the latter counts as boiling.
鈥淚t is difficult to distinguish between air bubbles forming and steam bubbles doing the same鈥�
Peter Borrows, Amersham, Buckinghamshire, UK
This article appeared in print under the headline 鈥淏oil foils鈥�