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.