Why does hot water help to remove a metal lid from a glass jar?

Chris puts this question to physicist Jess Wade from Imperial College London...

“My standard way of opening a stubborn metal lid on a glass jar for the first time is to run hot tap water over the lid. I think the jar opens more easily because the lid expands with the heat. My husband thinks the heat makes the air in the jar expand and this pushes the lid off. Why does the lid come off more easily?" So who’s right?

Jess - I think that you’re entirely right that the lid expands in the heat. The metal lid will expand much quicker than the glass jar that you’ve got, so that will expand and open up and then you can loosen it up. It also has a lower specific heat capacity so when it gets to a certain temperature that expansion happens quicker than the glass that it’s surrounded by.

As you increase the temperature of the gas, the gas will expand but it’s incredibly unlikely to break that seal. But that’s actually why on the top of pasta sauce when you buy it, it’s popped down and that’s because they seal it when it’s really, really hot. And then when it cools down when it’s on the aisle or whatever, that gas starts to sink down and get smaller again so the lid pops down and then you know that you’ve opened it when it pops up.

Chris - Because it does say if you cannot depress this lid you know it’s fresh.

Jess - I don’t buy an awful lot of pasta sauce.

Chris - You would think if the jar air was made hotter, it would then force the lid up and that would actually make it harder to untwist the lid, wouldn’t it if the gas is pushing on the lid and making more friction between the lid and the jar?

Jess - Yeah. I guess it would make it harder and harder to untwist. But what we have in the question, I think, is that the gas will expand so much that it would just cause it to pop up without any untwisting at all.

Kate, you had an interesting theory, right, about using a fork and whacking it?

Kate - Public service announcement! If you take a large knife and you invert it so that it’s the blunt edge of the knife and you just give a swift whack to the edge of a metal lid, it works almost every time - it’s perfect.

[giggles]