Tue, 30 May 2006

I encountered the "don't boild water in a microwave" urban legend today.
Strictly speaking, it isn't an urban legend, but it is unlikely you will see it in real life. I was onsite at a client, and I saw this sign in the break room:

I had heard about this before, and I checked Snopes to see if it was real. What Snopes said is that is just barely possible, but unlikely. Basically, when you boil water in a clean cup in a microwave, the water can be superheated, but the cup will not get hot. Because of this, no bubbles form on the inside of the cup, and the superheated water is just waiting for any kind of nucleation sites that will trigger boiling. Nucleation can be supplied by a spoon or tea bag, as the sign says.

What the sign doesn't say, and Snopes does, is that this nucleation can be triggered during the microwaving by leaving a non-conductive (wooden or plastic) spoon in the cup. This will give the bubbles a place to form. Why not say that on the sign, rather than banning all microwave heating of water?

When I first saw this, it reminded me of the signs on gas pumps that said "don't use your cell phone because it can spark and start a fire". Actually, there are no recorded instances of this happening (see Snopes again), but I guess somebody figured you can never be too safe. Much like the prohibition on boiling water.
posted at: 19:44 | permalink |