Evaporation. Molecules. Humidity. And how to cool soup fast.
It’s a universal practice: we blow on hot soup (also called broth) and hot drinks to cool them.
But wait… at ~37°C/99°F, our human breath is usually warmer than the ambient temperature (not more than 35°C/95°F in summer in most places around the world). So why does blowing on hot liquids cool them more quickly than simply leaving them to cool in the ambient temperature?
It’s not our breath per se. It’s evaporation. Blowing makes it evaporation easier, and this cools our soup or drink down.
Evaporation cools down a liquid because when the most energetic molecules leap off the surface, what is left in the soup or drink are the slower-moving molecules. This means a lower temperature, and that is how the liquid begins to cool down.
But there is one thing that prevents the molecules from jumping off: the “smoky” layer above hot liquids. We like to call this “steam”, but it is really fog.
It is a humid layer that prevents evaporation from taking place (not unlike what we experience during humid weather!). Blowing on a spoonful of hot soup, or anything hot and moist (such as soup noodles) removes this humid layer, allowing for evaporation to take place more rapidly.
The faster we blow the “smoky” layer away, the faster our hot drink or soup will cool. The physical world has given us a simple visual cue to let us know how we are doing. How’s that for a neat nature design?
Do you have a special trick to cool your hot soups down? What are your favorite soups? Share them with me!
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