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.
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.
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.
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.
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.
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:
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.