| MadSci Network: Physics |
Hello Julie,
This is a very good question, and one I have often asked myself, especially when I was trying to impress a girl with my cooking, but couldn't open a jar. For some reason, an explanation about "vacuum" doesn't quite cut it in such a case. I am not 100% sure about the answer, but I'll give you my best shot.
I can think of two possible reasons why a jar becomes stuck, and I'll explain both. One has to do with the lid, and the other with the contents of the jar.
A normal jar consists of a glass pot and a metal lid. Often, it's best to keep a jar cool, so it is in the refrigerator. In this case, the glass, contents and the metal lid are cooled from some 21 degrees C to about 5 degrees C. When you cool a substance, it (almost) always shrinks a little. Grabbing my trusty high school table book 1, I find that glass shrinks by 8 millionth per degree C and mild steel (the lid) shrinks by 7.8 millionth per degree C. This means that if I have a glass object, and I cool it by 1 degree C, it becomes 8 millionth of a centimeter smaller for each centimeter it is in size. So, assuming our lid has a width of 8 cm, the glass jar shrinks by 0.0102 mm and the steel lid shrinks by 0.00998 mm. The lid actually shrinks less than the jar, so if anything, it will become a little bit looser. Besides, the difference is less than a thousandth of a millimeter.
Jars that are not cold also can be quite difficult to open, so there must be a different explanation! The contents of a jar are generally kept under vacuum to preserve freshness. If you want to open the jar, this vacuum is preventing you from doing so. The air around you is at a pressure that is roughly equal to 10 tonnes per square meter. Now, you normally don't feel this, because there is air everywhere, including in the microscopic spaces in your body, so wherever air pushes, there is other air pushing back-except when there is a vacuum somewhere, like in our jar. Our 8 cm lid has a surface area of 0.005204 square meters. This means there is a force of 50 kilos pushing on the lid! No wonder you can't open it!
When tapping the bottom of a jar, you normally put the jar upside-down and then tap the bottom. A good hard smack will cause the contents of the jar to get loose from the bottom and pile up on the lid. This moves the contents to the lid of the jar. If you now try to open the jar, there is some force on the other side of the lid due to the contents. Opening it will be a lot easier.
I suspect (but am not sure) that tapping the bottom might also release water vapor (or other gases) from the contents (just like hitting a soda will cause carbon dioxide to be released), further decreasing the pressure difference between inside and outside the jar.
Sometimes, a jar previously opened might also be a little stuck. This can happen if you put it in the refrigerator. The air that is in the jar cools and therefore contracts. This effect is typically about 5% of the effect of having a true vacuum. You can compute this using Gay-Lussac's Law2.
Two other very good ways of opening a jar are heating the lid (try hot tap water), causing the metal to expand and breaking the vacuum seal, and putting a screwdriver under the lid, bending it a little, also breaking the seal.
In summary, we have investigated two possible explanations for stubborn jars: shrinking of the lid and the vacuum in it. The vacuum can explain why it can be very difficult to open a jar. Tapping the bottom can break the vacuum, so it makes the jar easier to open.
I hope this answers your question. Regards,
Bart Broks.
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