Blending Hot Liquids

Why is it that when you blend hot liquids they blow the lid off the blender if you aren't holding it in place?

--Dave

Actually, the science of this is pretty straight forward.  As you know, when you heat a gas, it expands.  So, you put hot liquid into the the jar of the blender and put the lid on.  At this point you have a near boiling liquid with some near room temperature air between it and the top of the jar.  When you start the blender, the air is pulled down through the liquid into the blades where it is broken into millions of tiny bubbles.  At the same time, all of that air gets heated up and expands to about a quarter again the volume it had before^*.  Because some of it is incorporated into the liquid, the effect of the expanding air is to push the liquid upwards.   Both the now hot air above the liquid plus the expanding liquid force the lid off the container.

In order to be safe, it is recommended when you blend hot liquids that you never fill the jar more than half full, that you put the lid on and then cover it with a dry towel and hold it down by hand.  Also, use as slow a speed as possible.

^* If you are interested in a more scientific explanation, the basic equation that describes the relationship between pressure, volume and temperature of a gas is PV=nRT, where:

P is the pressure of the gas;
V is the volume it occupies
n is the amount of the gas there is;
R is a scientific constant; and
T is the temperature of the gas in degrees Kelvin.

Kelvin is a temperature scale that has the same scale as Celsius, except that 0°K is the absolutely lowest temperature that anything can possibly reach.  0°K is the same as -459.67°F or -273.15°C.  What the formula says is that if you double the temperature, either the volume will double, the pressure will double (if the gas is tightly contained), or some combination of the two will occur.

So, the air in the blender jar is at room temperature, which is about 293°K.  You turn on the blender, and all of a sudden it is heated to around 373°K.  Assuming that the pressure tries to stay the same, which it will do if possible since the container has a lid that can be blown off, the gas expands by 373/293 = 1.273 times.

---

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Comments

[Aaron]

I wish I would have known this previously... all I can say is 'ouch!'.

[Bill Jenner]

What an excellent explanation! I've wondered about this for years and only now decided to do a search for the answer.

[g]

God I wish I'd known this before

[jtd]

Yeah, this is serious stuff. I just got back from the ER with 2nd degree burns on my face because of a hot liquid in blender mistake. I was even holding the lid down and it still blew off. Please, be careful!!!!!!

[melody]

You would think they would have this on the instructions for the blender instead of just saying that it is compatible with hot stuff. :) Glad to see this hint. I had mine less then half full with the lid held down and it still happened so next time I will make sure to have a towel. Thnx

[hockeypuck]

I just watched a re-run of the Food Network's show "Chopped". One of the contestant/chefs was burned after trying to blend a hot eggnog concoction in a blender. He thought he was holding the lid down sufficiently, but the lid completely blew off and he was hit in the face with hot eggnog. This 2007 article has fantastic info and great tips! ~~Thank you Kitchen Savvy!~~

[MinetTruly]

I blended hot stuff twice without any problem. Then my friend showed me this article. I wonder why I didn't have any problems? Both times, the soup was just-a-bit-too-hot-to-eat-comfortably, and filled the blender with less than an inch to spare.