# Why does agitating a hot liquid create steam/pressure? **URL:** \ **Category:** Factual Questions\ **Created:** [July 28, 2022, 5:30pm UTC](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727 "2022-07-28T17:30:12Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![iamthewalrus\_3](https://avatars.discourse-cdn.com/v4/letter/i/258eb7/32.png) [@iamthewalrus\_3](https://boards.straightdope.com/u/iamthewalrus_3)\ **Post date:** [July 29, 2022, 3:53pm UTC](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727/21 "2022-07-29T15:53:29Z")
> [@Dr.Strangelove](#): > > My blender has a specific soup-heating feature. There’s no separate heating element; it just blends until it gets steaming hot. My blender will heat things too, given enough time. It’s a beefy blender and uses maybe 1200W at peak. Some of that is transferred to the contents (and some warms the base or the air near the blender). But the effect I’m talking about takes place in a fraction of a second. Pour in soup, close blender, turn on blender, top blows off under pressure. _Way_ too fast for the power input from the blender to add appreciable heat to the system. If you poured hot soup into a sealed container with a 1000W heater at the bottom and flipped it on for a quarter of a second, it would not blow the top off. Both the fact that the air in the rest of the blender is rapidly heated and the fact that the agitation causes the blender contents to reach equilibrium vapor pressure faster by accelerating evaporation seem to be relevant here. Could be interesting to do an experiment to figure out which has the larger effect, but I’m satisfied with those dual explanations. ---
**Author:** ![octopus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/octopus/32/3716_2.png) [@octopus](https://boards.straightdope.com/u/octopus)\ **Post date:** [July 29, 2022, 4:23pm UTC](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727/22 "2022-07-29T16:23:34Z")
If I had to guess, I’d say cavitation since the heated liquid is close to boiling anyways the spinning blade will cause a pressure differential like an airplane wing. Lower pressure makes it easier to vaporize. ---
**Author:** ![oligArchie](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/oligarchie/32/21546_2.png) [@oligArchie](https://boards.straightdope.com/u/oligArchie)\ **Post date:** [July 29, 2022, 5:10pm UTC](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727/23 "2022-07-29T17:10:59Z")
I was asking myself this question this morning, as I do most mornings. When I make coffee, I will pour some into a thermos for drinking later. Before I add the coffee to the thermos, I’ll add a few ounces of recently boiling water to it, close the stopper, and shake. Then, when I crack the top to remove the warm water and add my coffee, there’s always a significant pressure build. How timely to have gotten an answer from you folks this afternoon. Thank you. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [July 29, 2022, 5:59pm UTC](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727/24 "2022-07-29T17:59:32Z")
> [@Mangetout](#): > > - Fill a bottle halfway with hot water > - Fit the lid > - Shake the bottle > - Carefully release the lid - you will hear gas escaping under pressure. Bold italic and large font _carefully_. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [July 29, 2022, 8:10pm UTC](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727/25 "2022-07-29T20:10:14Z")
Oh, if that’s been done, I stand corrected. I felt sure we’d had this conversation completely the other way around on this very board, and someone corrected me into the notion I expressed above. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 29, 2022, 8:41pm UTC](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727/26 "2022-07-29T20:41:47Z")
> [@iamthewalrus\_3](#): > > But the effect I’m talking about takes place in a fraction of a second. Understood–I was just making a side note that not only is frictional heating non-trivial (given enough time), it’s actually advertised as a feature for some blenders. Blending some hot (not too hot) oil would distinguish the two cases. If it still pops the top, then the air heating must be the dominant effect. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 1, 2022, 10:45am UTC](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727/27 "2022-08-01T10:45:37Z")
> [@iamthewalrus\_3](#): > > Both the fact that the air in the rest of the blender is rapidly heated and the fact that the agitation causes the blender contents to reach equilibrium vapor pressure faster by accelerating evaporation seem to be relevant here. Could be interesting to do an experiment to figure out which has the larger effect, but I’m satisfied with those dual explanations. You could certainly devise some experiments, but the thermodynamic properties of air and water are very well understood. The table below shows what happens if you start with dry air at 70F, and then heat it and/or moisturize it: ```auto Temperature, | Saturation vapor | Air pressure (amount above F | pressure of water, psi | above 14.7 psi) 70 0.365 0.000 80 0.504 0.278 90 0.688 0.556 100 0.930 0.834 110 1.242 1.112 120 1.644 1.389 130 2.154 1.667 140 2.798 1.945 150 3.603 2.223 160 4.602 2.501 170 5.832 2.779 ``` The table shows that starting at 70F, you can pour in a bit of room-temperature water, and once it gets to 100% relative humidity, the pressure will increase by 0.365 psi. For a 5"x5" blender lid, that’s over 9 pounds of force, just from water vapor pressure. That’s already enough force to pop the lid off of most blenders - no heating required. If you’re also adding heat, then the partial pressure of the air pressure increases, but water vapor pressure always increases faster. Even if you started with air that was at 50% RH, that just means you’d subtract (0.365\*50% =) 0.1825 psi from all of the numbers in the middle column to find the _increase_ in water vapor pressure beyond the initial conditions. At every point, water vapor would _still_ be providing a pressure increase far above what the warm air provides. [Previous page](https://boards.straightdope.com/t/why-does-agitating-a-hot-liquid-create-steam-pressure/968727.md?page=1)