Why does agitating a hot liquid create steam/pressure?

Factual Questions
1

If you are like me and you once tried to blend soup in a blender without doing it in really tiny batches, you have probably noticed that when you agitate a hot liquid (let’s say something relatively close to its boiling point), a bunch of steam comes out and the top of the blender blows off and you end up with a kitchen and self covered in soup.

Why does this happen? What’s going on at a low level that causes so much steam to come out of the liquid? Is it similar to or the same effect as shaking up a soda causing a bunch of dissolved CO2 to come out?

2

I think what you’re describing is simply the result of heating the air in the container. When you agitate the hot liquid it quickly transfers heat into the air which expands, increasing the pressure.

3

How many Watts is your blender?
Because, most of that power is ending up in what you are blending.

4

You can experience this effect simply by partly filling a bottle or flask with boiling water and closing the lid. The contents will be under pressure and may spurt out when you loosen the lid, causing a risk of minor burns.

5

That makes sense!

6

The effect happens basically immediately. As soon as the blender spins up. Not enough time for the power from the blender to appreciably change things.

And this doesn’t happen with cold liquids or with no liquids in the blender.

7

Boiling often atarts from nucleation sites - rough spots on the container or imurities in the liquid - mixing may add nucleation sites making boiling more effective

8

I think you might see that if you used a stick blender in a really thick soup that’s still on the stove. But, if you pour soup into a blender you’ve removed all external sources of heat and it can’t suddenly boil unless it were super-heated. If the soup was super-heated you’d see it explode when you poured it, not when you turned the blender on.

9

Hot liquid is constantly evaporating, and turning some of the liquid water into water vapor (even when it’s not boiling). At the same time, water vapor is constantly turning back into liquid, and eventually, a closed system will reach an equilibrium where both of these are happening at the same rate.

When you pour soup into your blender, it’s not at equilibrium yet, so more is evaporating than is condensing. And when you agitate it, it approaches the equilibrium level faster. So a lot of liquid turns to vapor.

10

My blender has a specific soup-heating feature. There’s no separate heating element; it just blends until it gets steaming hot.

11

One of the reasons (I think) is that the agitation makes it a lot easier for the hot molecules in the liquid to bonk together in a way that ejects molecules into the air. As a comparison, think of one of those Newton Cradle toys. If the back weight hits just right, the front weight is shot into the air. It’s like that in liquid. All the molecules wiggling around will sometimes align just right to eject one into the air. If you agitate the liquid, there’s a lot more opportunity for these sorts of interactions to happen and the fast wiggling molecules in the liquid will be better able to transfer their energy to a molecule and shoot it out of the liquid. If the liquid is just sitting around, you have to wait for the random motions to work out just right to pop one in the air.

And for the reverse, you can sometimes get very still liquid to heat up past its boiling point. This sometimes happens with water in the microwave. The water is hotter than 212, but there’s not enough internal movement to pop the water molecules into the air. But if you move the cup just a little, the extra motion starts a vigorous boiling action.

But even if the liquid is cool, putting it in a blender will cause the liquid to mist up in the blender. But since it’s cool, it the molecules in the air don’t have the same energy and won’t be able to push the lid off.

12

yes, its the rate of growth of bubbles … the mechanics at the surface of the bubbles, whats happening at the edge between the liquid and the gas… means that gas leaves the liquid faster when there is a gas bubble to go into… so the more surface area of the bubbles, the faster the gas evolves out.

There’s a youtube video showing how shaking a bottle of softdrink/carbonated water doesn’t increase pressure, what it does is increase the rate that it can restore the higher gas pressure when the gas leaks out - all that surface area of bubbles can let CO2 out at the higher rate, thus create the situation that it grows the volume of bubbles really fast… thats why it bubbles up throth everywhere …its the total surface area of the liquid to gas interface that counts… so a straw can make a few bubbles, but shaking it makes lots and lots of tiny bubbles, a huge surface area…

13

Yep. The same effect happens in reverse if you have a vessel in which the contained air is hot, and you add cold water and agitate it. Newcomen atmospheric engine - Wikipedia

14

Answering the bolded part above

At 100 C (212 F) aka the boiling point of water at atmospheric pressure, the density of water is ~1000 kg/m3 (62.4 lb/ft3) while the density of saturated steam at the same temp/pressure is ~ 0.6 kg/m3 (0.037 lb/ft3).

Therefore, 1 cup of water will give you 1000/0.6 ~ 1667 cups of steam.

This is why even a small amount of vaporizing water, gives a lot of steam.

15

It’s not that agitation causes steam pressure from vapour leaving the hot liquid, it’s that agitation causes the hot liquid to quickly heat the existing air inside the container, and the heated air expands.
You can show this as follows (taking all reasonable safety precautions):

  • Fill a bottle halfway with hot water
  • Fit the lid
  • Shake the bottle
  • Carefully release the lid - you will hear gas escaping under pressure.

Now:

  • Allow the pressure to escape
  • Tighten the lid
  • Shake again
  • Release the lid - this time, there will be very little pressure escaping, because the air inside the vessel was already hot.
16

There is the heating of the air above the soup in the blender, and there is evaporation of liquid water from the soup that humidifies said air.

When you first pour hot soup into the blender and put the lid on, things are not at equilibrium: the air above the soup is relatively cold and dry. If you secured the lid tightly in place and came back a couple of minutes later, the contents of the blender (the air and the soup) would be at equilibrium with each other: the air would be as hot and humid as it’s ever going to get, and if the blender lid were sealed and secure, you would have achieved some pressure above ambient because of the increased temperature of the air and the added partial pressure of water vapor.

When you turn on the blender, you are speeding up the journey to equilibrium by agitating the air, agitating the soup, and also by increasing the total free surface area of the soup across which water vapor and heat move from the soup to the air. Instead of the contents of the blender reaching equilibrium in a couple of minutes, it will reach equilibrium in a few seconds, and the pressure you achieve with a sealed blender lid would be pretty much the same as in the earlier scenario (in which you just let things sit for a couple of minutes).

In either case, it’s possible to generate significant pressure if the blender is fully sealed. Imagine the air in the empty blender is initially at room temperature (~70F) and essentially dry. Pour in boiling hot soup, and quickly slap the lid in place. Suppose the soup manages to heat the air to 175F. What happens to the pressure of the air above the soup? According to this ideal gas law calculator, the absolute air pressure will increase from 14.7 psi to 17.6 psi, a gain of 2.9 psi.

As mentioned, the soup also humidifies the air, up to basically 100% relative humidity. Here’s a plot showing the vapor pressure of water as a function of temperature. At 175F, 100% relative humidity means the partial pressure of water vapor in the air is about 8 psi.

So the grand total pressure increase inside your blender would be about 8 + 2.9 = 10.9 psi, with the vast majority of that gain coming from humidification. If your blender lid is 5x5 inches, that’s 25 square inches, for a total force of 272.5 pounds. Good luck keeping that in place with your free hand.

The soup is off the stove and being poured into a blender. Boiling is not happening here.

The reverse situation can happen if you fill a container with hot humid air (or just steam, 100% water vapor), seal it, and let it cool. The pressure drop inside the container can be so great that it gets crushed by the weight of the atmosphere.

17

Not the question the OP is asking, but it’s relevant:

as anyone who has done agitation in a separatory funnel in chemistry lab knows, agitating COLD liquids (especially mixtures) will create pressure, which must be regularly vented. It’s because of vapor pressure that evolves due to the shaking.

Not as dramatic as with the hot liquid example, but I think that if you don’t regularly vent the separatory funnel by opening the stopcock every now and then you can actually crack the funnel.

18

Try the same experiment with hot sand, and you’ll see a much smaller effect. Hot water and hot sand will both heat the air, but hot sand won’t evaporate.

19

I learned this lesson trying to wash out a salad dressing container with hot water. I put in the hot water, and everything was fine until I shook the bottle, and the pressure expanded instantly.

20

Your last line didn’t fully convey the situation. It should read, “this time, there will be very little pressure escaping, because the air inside the vessel was already hot and at 100% relative humidity.”