Sunday, April 21, 2013

Pressure Cookers: Put a lid on it

The Boston Marathon bombers used pressure cookers. Before Patriot’s Day 2013 I had no idea that many of the IEDs used in Iraq and Afghanistan were fashioned from these wonderful cooking devices.
Every year when we discuss boiling points and vapor pressure in my chemistry class, I talk about pressure cookers. My mom had a pressure cooker when I was growing up, and many of my Indian students’ families routinely use pressure cookers.
It was common for my mom’s generation to can or “put up” fruits and vegetables. They had been children of the depression and had been teenagers during the Victory Garden years of World War II; growing and preserving vegetables and fruits were expected practices in the summer and fall. The problem was that non-acidic vegetables like green beans and corn could not be canned safely without risk of botulism contamination. Botulism spores can survive boiling water; they need to be heated to 250 degrees F or 120 degrees C to render them harmless. (You could pickle the veggies: the acid in thevinegar kills the bacteria.) 
The heavy-walled pot has a lid that can be sealed tightly with a rubber gasket keeping the steam from escaping. Excess steam can be released through a release valve. Pressure builds up inside to the pot. Water boils when its vapor pressure (a measure of the amount of forces holding water together) is equal to the surrounding pressure. Since the pressure is higher in the pot, the water boils at a higher temperature and food cooks faster and botulism bugs are killed. Dentists and hospitals use them to sterilize instruments between patients. These are truly marvelous inventions.
Now, when I talk about boiling point and vapor pressure, I will have to add comments about pressure cookers being used as bombs, just as I discuss the Oklahoma City bombing when I discuss nitrogen-based fertilizer. Sometimes connecting chemistry to current events just stinks.

No comments:

Post a Comment