If you are a forgetful soda drinker, it may have happened to you.
You return to your car on a hot summer afternoon to find a can of your favorite beverage puffed up like a balloon. It may have even exploded — its top peeled off like someone had used a can opener.
It can be disconcerting. After all, you had left the can unopened and intact.
Was someone in your car going through your stuff? Shaking your cans?
No, you realize. It simply got so hot inside the car, that the can somehow blew up. Upon further reflection this may be more frightening than the potential intruder.
But how, exactly, does it happen?
To answer this question you have to look at three things: The car, the carbonated beverage, and the aluminum can that contains it.
Let’s start with the car
“It’s really, really hot inside of your car,” says Kate Biberdorf, a UT Austin chemistry professor, who hosts the Seeking a Scientist podcast.
Biberdorf says that in the span of one hour, the inside of a car can become 43 degrees hotter than the temperature outside.
So, if it’s 100 degrees outside on a Texas summer day, the inside of a vehicle can quickly reach 143 degrees.
That heat makes carbon dioxide molecules move a lot faster and separate from liquid. Carbon dioxide, or CO2, is what makes the bubbles in carbonated beverages, so there is a lot of CO2 in soda cans.
“As [CO2] slams into the side of your container, it is providing more force,” Biberdorf says “As the temperature increases, the pressure is going to increase because we’re having more collisions with more force inside that can.”
Biberdorf says the pressure pushing out from inside the can can grow to five or six times greater than the normal pressure of our atmosphere.
Simply put, there’s much more force pushing out from inside the can than pushing at the can from outside.
And here’s where we get to soda can engineering.
Soda can engineering plays a role in these explosions
A typical soda can is designed to withstand an internal pressure about six times greater than external pressure without warping.
But, not all cans are manufactured perfectly.
As the pressure builds on a soda can’s weak spots it can lead to the can warping. Typically it happens at the top part of the can, where you’ll notice the metal ballooning out or even popping.
Biberdorf says that is the part of the can designed to give way first.
“That top part [is] accordioned in when they manufacture it so it can push out once the carbon dioxide is popping,” she says “It’s a safety thing because these are like little bombs, essentially.”
In most circumstances, the can will not actually explode. The top will balloon out, relieving some of the pressure.
But, if the seam at the top of the can is particularly weak, or if the can is under an extreme amount of pressure, the entire top will give way with a loud bang.
It will often look like it has been peeled off.
“They are designed so that hopefully it doesn't fly off. It kind of opens up more like a [tin] can,” she says. “But, again, it’s not perfect. We make so many of them there are going to be flaws."
Biberdorf says moving a hot can will add even more pressure to an already volatile situation, potentially causing it to pop.
Exposed aluminum can metal can be sharp, so don’t touch or shake a can left in the car. Instead, run the AC or open the windows, and let that can cool down before you move it.
Ruined soda cans are also a frightening reminder never to leave any living thing inside a hot car.
Biberdorf says it does not matter whether car windows are left cracked open, the heat inside a car will quickly become deadly for children and pets.
