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The Hot Chocolate Effect

pexels-photoWinter Storm Jonas gave our house a miss and we are left with some ice and a dusting of snow. But that is not going to stop us from filling up on hot chocolate! Hot chocolate, the official drink of winter, also provides many opportunities for warm, delicious science experiments.

One of these, the Hot Chocolate Effect (HCE) is a fairly new discovery – first observed in 1982. You can do it yourself quite easily with a mug, a spoon, some milk or water and hot chocolate mix.

First, tap the rim of your mug several times with your spoon and listen to the pitch (how high or low the sound is). Did it change while you were tapping? Of course not!

Next pour some hot milk or water into the mug and tap again. Any change? Not likely. Try stirring the milk and tapping while the milk is still swirling. You probably won’t hear any changes in the pitch here either.

Now for the fun part! Pour a packet of hot chocolate (or a tablespoon or two) into the mug of hot milk. Give it a couple of quick stirs and start tapping again. What do you hear?? You should notice that the pitch of your tapping gets higher and higher. Give it another stir and tap again. The pitch will go down and then rise again!

What’s happening?

The answer is bubbles. Adding powdered hot cocoa doesn’t just add a sweet chocolate flavor to your milk, it adds tiny bubbles of air as well. As you stir in the chocolate you also mix the air bubbles into the liquid. All of those bubbles slow down the speed of sound in the hot chocolate. This lowers the frequency or the pitch of the sound waves that travel through the mug when tapped.

As the milk spins around in the mug, the bubbles rise to the top and pop. With fewer bubbles in the hot chocolate, the speed of sound increases and the frequency does too so you hear a higher pitch.

Giving another stir dissolves more bubbles and it starts all over again!

Try this!

Now that you’ve observed the HCE you can experiment with other liquids and powders. Here are a couple of ideas to try and you might come up with your own. If you find another liquid or powder that works, share it in the comments!

  • Does it matter what liquid you use? Try water, skim milk and whole milk. Or get crazy and try other liquids you have in the kitchen like vinegar or orange juice.
  • Does it matter what powder you use? Try salt, sugar, baking soda or anything else you can find.
  • Does the liquid need to be hot or will it work with cold milk as well?
  • Soda has bubbles. Does stirring soda in a mug give the same effect? What if you add a powder?
  • Some other combinations to try are sugar in coffee and alkaseltzer in water.

More Hot Chocolate Experiments

Does the color of the mug you use to drink your hot chocolate make a difference?
These scientists think so. Have your friends and family repeat the experiment and see if you get the same results.

How can you keep your hot chocolate hot?
Investigate the effect of

  • different types of cups (coffee mug, Styrofoam cup, plastic cup),
  • different types of insulators around your cup (paper towel, wool sock or bubble wrap)
  • or even the effect of marshmallows (with and without or different amounts)!

Set up a few cups for each variable and use a thermometer to measure the temperature of each cup every couple of minutes.