Eli5 :Why you shouldn’t fill a blender with hot liquid ?

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« Eli5 »
Why does the lid come off when you use a blender with hot liquid but it doesn’t come off when you use it with cold liquid? Help me explain to my friend why it’s safer to let the steam come out and not over full the blender with hot soup.

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Hot liquids can expand when blended and cause the lid to pop off. Unless specifically designed, the spout and top may not be heat proof. They may deform and allow hot liquids to spray out.

When you blend hot liquid it aerates it creating more steam release which compounds with the thinner liquid rising further on the walls of the blender vessel. The fix is don’t seal the blender and don’t fill it more than half full you won’t have any issues.

[https://cookfasteatwell.com/how-to-puree-soup-in-a-blender/](https://cookfasteatwell.com/how-to-puree-soup-in-a-blender/)

Edit: as an FYI I have a Ninja Blender with two sets of blades that doesn’t have a totally sealing lid and I blend as much hot soup in it as I want without it doing anything dangerous.

Surface area, heat, and expansion.
When the blender is half full of hot water, and half air, the surface area is small, and the air temperature only slowly rises from the hot water. Once you hit blend, the surface area of the water and air is increase thousands of times, now the water can swiftly share that heat with the air, causing it to expand, and the lid blowing off.

The opposite can be done with cold water, and have the lid suck in.

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I blend hot things all the time.

Just don’t put a tight-fitting lid on top, and don’t overfill it.