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The Hot Chocolate Effect is a phenomenon of wave mechanics first documented in 1982 by Frank Crawford, where the pitch heard from tapping a cup of hot liquid rises after the addition of a soluble powder. It typically arises in the making of hot chocolate or instant coffee, but also occurs in other situations such as adding salt to supersaturated hot water or cold beer.

It can be observed by pouring hot milk into a mug, stirring in chocolate powder, and tapping the bottom of the mug with a spoon while the milk is still in motion. The pitch of the taps will increase progressively with no relation to the speed or force of tapping.

The phenomenon is explained by the effect of bubble density on the speed of sound in the liquid. The note heard is the frequency of a standing wave whose fixed wavelength is the distance between the base of the mug and the liquid surface.

This frequency f is equal to the speed v of the wave divided by the wavelength λ (lambda) of the wave:

Upon initial stirring, entrained gas bubbles reduce the speed of sound in the liquid, lowering the frequency. As the bubbles clear, sound travels faster in the liquid and the frequency increases.

[edit] References

• The hot chocolate effect (Abstract only) Frank S Crawford, American Journal of Physics, May 1982, Volume 50, Issue 5, pp. 398-404
• Hot water, fresh beer, and salt, Frank S Crawford, American Journal of Physics, November 1990, Volume 58, Issue 11, pp. 1033-1036
• Sound of a Cup With and Without Instant Coffee: A Foam-Filled Acoustics Demonstration, Andrew Morrison and Thomas D. Rossing, 143rd ASA Meeting, Pittsburgh.
• The cheap instant coffee effect Account by Kevin T Kilty
• The hot chocolate effect: introducing the scientific method PowerPoint presentation by Bradley Carroll and Michelle More
• Hot Chocolate Experiment In an educational format by Michael Piette. Includes recorded sounds.