Re: How Much PSI would be inside a can of pop to cause it to explode?

Hi Terri. I don't know the exact amount of pressure required to make a can explode (the anser varies depending on the thickness of the metal, shape, etc) but it doesn't sound like exploding is what happened in your situation. Here is what I think happened, but first, I should point out that you should not open a can that has bowed out. Bowing is a great sign that the contents of a soda can have been heated or frozen in an extreme way, and often coincides with damage of the inner lining of the can, making the soda taste very metallic. Bowing in tin cans is typically a sign of microbiological contamination and that the contents are not safe to eat. But back to your question...

You mentioned that the can was very cold but not frozen. I am guessing that you had it in the freezer before opening it? Did you know that water freezes at a lower temperature when it is under pressure? Let's start with a phase diagram for water (at the bottom of the ChemistryCoach link).

Soda cans are usually under ~3 atmospheres of pressure before you open them. And since soda is 99% water, let's assume that the phase diagram for water is the same for soda. If you draw a vertical line starting from the "normal freezing point" temperature, up to where 3 atm of pressure might be, you will notice that you have entered the liquid region. The soda is not frozen before you open the can.

When you open the can, you are releasing the pressure on the liquid instantaneously from 3 atm to 1 atm. Moving your pencil back down along the same vertical line you drew, you are now back at the solid region. The soda froze as you opened it! Water expands as it freezes and the ice formed in that instant needs somewhere to go. Since you didn't completely open the tab, the expanding ice exerted a tremendous amount of pressure on the can, bowing it out, while also shooting out the pinhole opening you made.

My suggestion - do NOT place soda cans in the freezer to cool them down quickly. Depending on what temperature the freezer is at (freezers typically cycle between -10F and 10F to prevent frost build-up), you run the risk of exploding cans. Placing the soda in an ice bath or ice bucket will accomplish the same thing in a much safer way. :)

Thanks for your question!

Kieran