Month: February 2007

How can food catch fire in a microwave oven?

Lou Bloomfield Leave a comment

My wife makes blueberry pancakes for my daughter daily. Twice recently she noticed and brought to my attention a curious event in the Microwave oven. Frozen Blueberries placed inside a microwave oven to thaw, caused a popping sound and a small flame to appear amidst the blueberries. The flame self extinguishes. There is no apparent damage to the blueberries or the bowl they were contained in. — HA, New Jersey

I think that you’ve rediscovered an experiment in which people cut a grape almost in half, open the two halves like a book and lay it flat on a plate. In the microwave, the thin bridge between the halves carbonizes and than emits flames. Basically, the fruit pieces or berries are acting as antennas for the microwaves, which drive electric currents through the narrow bridges between parts. The berries aren’t great conductors, but they’re not true insulators either. Those bridges overheat (like an overloaded extension cord) and burn up. The flames come from the burning bridges.

If you let the flames go on long enough and enough carbon develops, you’ll probably start getting plasma balls in the oven (lots of fun, but not great for the oven… you can scorch its top surface because those plasma balls rise and skittle around the ceiling of the oven). Anyway, you can probably find the carbon areas if you look closely enough, but they’re no worse than a little burnt toast.

Keeping an open soda bottle fresh

Lou Bloomfield Leave a comment

My boyfriend and I are having this debate on whether or not to squeeze the air out of a 2 liter bottle of Coke after opening it. He thinks it will keep the Coke carbonated longer and I disagree. Who is right? — TN, Ft. Collins, CO

Yours is actually a complicated question. After you open the soda, the CO2 dissolved in the soda is no longer in equilibrium with the gas above soda. When you cap the bottle, CO2
will gradually escape from the liquid until it forms a dense gas so that CO2 molecules from that gas return to the liquid solution as often as they leave the solution for the gas. In other words, the equilibrium between dissolved CO2 and gaseous CO2 has to be reestablished.

By shrinking the volume of gas over the soda, your boyfriend reduces the number of CO2 molecules that must enter the gas phase in order to reestablish that equilibrium. BUT, when dense gas develops in the squeezed bottle, the high pressure of that gas will reinflate the bottle to its original size. The benefits of shrinking the gas volume will thus be lost.

To succeed in keeping more of the CO2 molecules in solution, you have to make sure that the squeezed bottle stays squeeze. That’s hard to do. You’re probably better off pouring the soda gently into a smaller bottle, one that just barely holds all of the liquid. That smaller bottle won’t expand as a dense gas of CO2 forms above the liquid soda and the soda will reestablish its equilibrium without losing too many of its dissolved CO2 molecules.