Curiosity Corner: Blowing on Hot Soup

Curiosity Corner

By

Dr. Jerry D. Wilson,

Emeritus Professor of Physics

Lander University

Question: Why does blowing on a spoonful of hot soup cool it? (Asked by a young, curious soup eater.)

Reply: Don’t burn your mouth! Blow on it. That’s the axiom we all learn from eating soup. Cooling, or a reduction in temperature, requires energy transfer. In the case of cooling soup by blowing, this involves convection and the movement of molecules. Basically, this is evaporative cooling.

When you heat a soup or other liquid in a pan to boiling, heat is passed to the molecules by conduction. The energetic molecules collide with each other, transferring energy. Given enough heat, the molecules near the bottom of the pan have enough energy to become a gas and form bubbles. The rise to the surface and the gaseous molecules escape, taking away energy which we see as water vapor. (No, not steam! It’s an invisible gas.) When boiling water occurs, the added heat and escaped energy equal each other, and the temperature of the liquid is constant. This is why boiling water cannot be heated above 212 degrees Fahrenheit under normal atmospheric pressure. (Pressure cookers allow for higher temperatures, because the boiling point of water increases with pressure.)

Now, back to the soup cooling and blowing. Prepared soup in restaurants is kept at 145 degrees Fahrenheit or above to prevent bacteria growth. Let’s say you have a spoonful of soup at that temperature, which is pretty hot. The soup in the spoon willhave some evaporation and cooling, but eventually a cloud of vapor will form above the liquid that limits the ability of surface molecules to vaporize and allow for cooling. This is mainly because of vapor pressure, which is the pressure the vapor molecules exert back on the liquid, keeping the liquid molecules from evaporating and cooling. (See good ol’ Newton’s law of equal and opposite forces.)

When you gently blow on the soup, and this removes the vapor cloud above the liquid and allows more molecules to evaporate. With a blow or two, you may get enough cooling so that the soup doesn’t burn your mouth.

This evaporative cooling doesn’t work too well on solids, though. For example, a hot pizza cheese may take a lot more than a blow or two.

C.P.S. (Curious Postscript): “I have not failed. I have just found 10,000 ways that won’t work.” -Thomas Edison

Curious about something? Email your questions to Dr. Jerry Wilson at curiosity.corner@yahoo.com. Selected questions will appear in the Curiosity Corner.