Book 14, Question 75
Why Does a Jar Lid Pop When You Open It?
A jar lid pops because air rushes in when the seal breaks! Inside a sealed jar, the air pressure is lower than outside. The outside air pushes the lid down tight until you open it.
More fun facts
- In home canning, air is pushed out of the jars while they are heated, and the food shrinks, so the food level looks lower after cooling.
- Canners are told not to retighten hot lids, because that may cut through the gasket and spoil the seal.
- Freshly canned jars cool at room temperature for 12 to 24 hours before the seals are tested.
- One test is to press the middle of the lid: if it springs up when you let go, the jar is not sealed.
- Tap a sealed lid with the bottom of a teaspoon and it makes a ringing, high pitched sound, while a dull sound can mean it is not sealed.
- A sealed lid is concave, curved down slightly in the center.
Sources: UGA
Questions kids ask
Why is a jar lid hard to open?
Air outside is pushing down on the sealed lid harder than the air inside pushes up.
Should you eat from a jar whose lid is bulging?
No. Ask a grown up, because a bulging lid can mean the food has spoiled.