You have surely noticed that it can sometimes be extremely difficult when open a jam jar with a screw-on lid on it (even though the jar may contain other foods, such as pickles, for example). One option to reduce the effort is to warm the jar in hot water, which makes the lid easier to open. Another option is to make a hole in the lid with a knife or to pry between the lid and the jar and let air get inside.
The reason why the lid is often hard to open is not usually because the lid was screwed on tightly. For example, when we make jam, we don't screw the lid on the jar with great force. Yet, the lid often comes off with great effort when we want to open the jar later. Why is this so?
The reason is that jam (and other foods) is usually put into the jar hot, as this helps preserve it longer. But when the jam in the jar is hot, the air above it is also hot. Once the lid is screwed on, the jam and the air gradually cool down. As the air cools, its pressure decreases because, at lower temperatures, the speed of the molecules is slower, and there are fewer collisions against the jar lid. Thus, the air pressure inside the jar becomes lower than the external air pressure. The external pressure then presses the lid tightly against the jar, increasing the friction between the lid and the rim.
But would it help if the jar were filled to the brim with jam so that no air is left above the jam? It turns out, that wouldn't help either. When the jam cools down, it contracts, leaving a vacuum above it, and the external pressure has an even greater effect!
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