The vacuum effect observed when covering a bowl of hot soup in a plastic container is primarily due to the cooling of steam, which condenses into water, significantly reducing the volume and pressure inside the container. This phenomenon is explained by the ideal gas law, where the decrease in temperature leads to a decrease in air volume. Additionally, the initial increase in pressure from the steam plays a minor role compared to the effect of condensation. This principle is analogous to the implosion of a sealed tin can when steam condenses.
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Storm8 said:Hi this is probably very easy for you guys, but I can't figure this out. Heated air rises right? Then why is it that when I put a bowl of soup in the microwave and heat it in a plactic Tupperware-like container, then remove and cover it -- does a vacuum like effect occur? It would seem to me tha the hot, steamy air would want to push the cover off. Instead it sucks the cover down. Why?
This is the same reason as to why airbeds can be completely inflated at first but seem to loose air throughout the night. I strangely worked this out in a semi-dream state recently (before looking it up to confirm it). I went away for the weekend and stayed at a friend's on the Friday, his house was well heated and the airbed was fine all night. On Saturday I stayed at a different friend's on the same bed but this friend's house was much colder, halfway through the night I woke up on a partially deflated bed. In my half-dream state I concluded that if it had a puncture it would have deflated all the way and that the inbuilt pump must heat the air it pumps through, thus because of the cold air it had seemed to deflate.mathman said:Once you remove it no more air is added. As the air cools down the volume reduces (ideal gas law) - so it sucks down.Storm8 said:It would seem to me tha the hot, steamy air would want to push the cover off. Instead it sucks the cover down. Why?
mathman said:Once you remove it no more air is added. As the air cools down the volume reduces (ideal gas law) - so it sucks down.
I suspect both factors (water condensation and air cooling) play a role. I'll leave it to others to quantify.Lsos said:That's what I thought at first, but I think a larger reason for the effect is that as it cools, the steam inside the container condenses into water. The volume of water is something like 1000x less than steam, so it greatly reduces the pressure inside the container.
When the soup is heated, the fact that hot air rises plays a small part trying to lift the lid. However, a larger part is played by the increased pressure inside, again due to water turning into steam.
Lsos said:That's what I thought at first, but I think a larger reason for the effect is that as it cools, the steam inside the container condenses into water. The volume of water is something like 1000x less than steam, so it greatly reduces the pressure inside the container.
When the soup is heated, the fact that hot air rises plays a small part trying to lift the lid. However, a larger part is played by the increased pressure inside, again due to water turning into steam.