Lab Safety: Heating a Closed System & Exploding Eggs

Explosions and science labs don’t make a happy mix, so we try to avoid them at all costs. Still, the unexpected sometimes happens in the least likely place. In this case, there was an unexpected “explosion” in our break room and a teachable moment emerged.

The other day, our office intern tried to heat a peeled hard-boiled egg in the microwave. When Dani removed the egg from the microwave and touched it with a fork, the egg blew up. The eggsplosion (see what we did there?) sprayed egg white and yolk everywhere and literally brought our safety manager running. It turns out an exploding egg sounds eggsactly (sorry, but it had to be done) like a chemical explosion.

After Alan, our safety manager, got his heart beating again, he took the opportunity to remind our team— and teach Dani — about a basic chemistry safety principle for wet chemistry work: Do not heat closed systems.

A closed system is any assembly which is completely sealed and not open to the air. In the lab, this is often a stoppered flask, glass vial or metal vessel. In our break room, the closed system was the egg white surrounding the yolk. Heating the system creates pressure inside it. If too much pressure builds up, it has to release somehow. The stopper may pop out of the flask or vial — if you’re lucky — or the container itself may rupture, which is what happened with Dani’s egg. Fortunately, even hot egg white is pretty soft and harmless, but if you were heating flammable materials, a rupture could cause a very large fireball and shrapnel. There would also be the risk of dangerous chemicals getting on the scientist doing the testing.

“When microwaving a peeled hardboiled egg, it’s possible to generate significant steam pressure in the yolk, which is temporarily contained by the white,” Alan says. “If the egg is in such a state, any disturbance can lead to a really loud explosion as the white ruptures and the yolk flies everywhere. It sounds almost exactly like a 4-litre bottle of acid exploding, and has been known to cause heart palpitations in the safety manager until the cause is identified.”

Because this was an important reminder — and we’re a curious bunch — Alan decided to replicate Dani’s results in a more controlled setting. Wearing proper safety equipment, he nuked eggs for 30, 45 and 60 seconds. Each test yielded slightly different results. At 1 minute, the egg didn’t even make it out of the microwave; it exploded before Alan even opened the microwave, and the force of the explosion pushed the microwave door open.

“We do a wide variety of work and sometimes you’ll be doing something new, or [work] that you’ve not run for a while, so it’s important to keep basic safety in mind,”he says. “Although it’s not an absolute, generally you shouldn’t heat closed systems unless you’ve done a detailed Eggsplosionanalysis to show that the pressure after heating will be within the known safety margins of the container, or have taken specific precautions to prevent injury or fire should the system rupture.”

Sometimes, it’s necessary to heat a closed system. Acceptable closed system heatings include parr bombs, which are certified able to withstand 2,000 psi of pressure, have pressure gauges and safety rupture discs, which vent at 1500 psi. “We’ve also done work with heating sealed glass vessels with ethanol, but we worked behind the building in a place where a ruptured vessel would not cause injury to the worker or damage to vital equipment,” Alan says. “The vessels did rupture and blew out the walls of the oven.”

Bottom line — it’s rarely a good idea to heat a closed system, but if you must do so, use a blast shield and face protection.

In the end, no one but the eggs was any the worse for wear because of the great exploding egg caper— although Dani did spend the rest of the day picking egg white out of her hair and finding it embedded on her person. And while she won’t forgo warming up her hardboiled eggs — because, let’s face it, warm eggs are yummy — next time she’ll cut it in half before putting it in the microwave!