Final answer:
The dimple in jar lids is held down by a vacuum seal which is released when the jar is opened, causing the dimple to pop up as atmospheric pressure equalizes inside the jar.
Step-by-step explanation:
The dimple in jar lids is held down by a vacuum seal, which is created during the manufacturing process to preserve the jar's contents by preventing the growth of bacteria due to the absence of air. When a jar is sealed, the contents inside are heated, and as they cool, the air inside the jar contracts and creates a vacuum, which exerts less pressure than the atmospheric pressure outside, keeping the lid dimple depressed. When the jar is opened, the seal is broken, allowing air to rush in and equalize the pressure inside and outside the jar, causing the dimple to pop up as the vacuum is released.
Understanding why and how this happens connects with topics such as atmospheric pressure, gas behavior, and the principles of vapor pressure and dynamic equilibrium. It's similar to when you open a sparkling water bottle and the carbon dioxide forms bubbles because the high CO2 pressure that kept the gas dissolved is released, eventually making the water go flat if left open. The concept is also similar to what happens when a container of liquid is heated and the vapor pressure increases, potentially lifting the lid or, in extreme cases, causing the container to burst if not vented.