What is the hot chocolate effect? asks a reader.
Did you ever pour a packet of cocoa mix into a cup of hot water…and notice that the pitch of your spoon striking the cup seemed to rise or lower as the mix dissolved? If so, you’re familiar with the Hot Chocolate Effect. The strangely musical effect can also occur with instant coffee, or when we add powdered creamer to coffee or tea, spoon sugar into a hot drink, or even drop a scoop of ice cream into a mug of root beer.
To listen for the hot chocolate effect, you’ll need a metal or wooden spoon; use either end. (You can also use your knuckle, and rap against the outside of the cup.) After filling the cup with hot water or milk, quickly tap the bottom or side of the cup before adding cocoa. That way, you’ll tune into the mug’s powder-free sound.
Now add the cocoa, and keep tapping. You should hear the pitch of the tap first drop, and then begin to rise. How come?
Physicist Frank Crawford explained why in a 1982 article in the American Journal of Physics. Crawford dubbed it The Hot Chocolate Effect, and since then, researchers have expanded on his original explanation. (Read about engineer Kevin Kilty’s experiments with “the cheap instant coffee effect” at www.kilty.com/coffee.htm.)
Scientists say that when you add cocoa to your cup, some of the air dissolved in the hot water gloms onto the powder grains. The result is tiny bubbles, clinging to the powder. In other words, foam. The cloud of foam reduces the speed of sound through the liquid. Meanwhile, the frequency at which sound resonates inside the mug depends on the sound wave’s speed. The lower the speed, the lower the frequency.
So when you first add cocoa powder, sounds will actually decrease in frequency. Since the frequency determines a sound’s pitch, your tapping spoon will sound up to an octave lower than it did in plain hot water. (Think low thunk.)
But as the bubbles float to the surface and pop, the sound traveling through the cocoa quickly speeds up. The frequency of the sound begins to rise, and along with it, the pitch. And so we hear higher and higher notes with each new, now-tinkly tap. Astonishingly, the pitch can rise up to three octaves, or about the vocal range of a well-trained singer.
(Watch a video of the effect using instant coffee at www.youtube.com/watch?v=JCVaOzlOUfY.)
Experiments to try yourself: Does the composition (glass, ceramic, plastic) or thickness of the cup make a difference in the changing pitch? What about a taller or shorter cup or glass? How about a cup with or without a handle? (Try tapping on the cup’s bottom, then on or near the handle.) Does using different liquids (water, skim milk, whole milk) alter the sound? Finally, what about adding whipped cream or marshmallows? At the very least, it’s all a good excuse to make a second (or third) cup of cocoa on a cold winter’s day.