Question

Many jars have dimples in their lids that pop up when you open the jar. What holds the dimple down while the jar is sealed, and why does it pop up when the jar is opened?

Answer 1

The dimple in the jar lid is held down by the difference in pressure inside and outside of the jar. When the jar is opened, the pressure equalizes and the dimple pops up. Running hot water over a tight lid helps break the seal by increasing the pressure inside the jar.

Step-by-step explanation:

When a glass jar is sealed, the dimple in the lid is held down due to the difference in pressure inside and outside of the jar. The air pressure on the outside of the jar pushes the dimple down against the contents of the jar, creating a seal. When the jar is opened, the pressure inside the jar equalizes with the atmospheric pressure, causing the dimple to pop up.

The same concept applies to running hot water over a tight metal lid on a glass jar. Heating the lid with hot water causes the air inside the jar to expand, increasing the pressure inside. This increased pressure helps to break the seal and make it easier to open the jar.