Discovery

Science at Home

A web site that would be of interest to science people is the Amateur Scientist page found at: http://www.thesphere.com/SAS/
You can find some great ideas and explanations surrounding everyday events. Much of the content could be used in a science classroom. Below is an edited example from the site. Further Amateur Scientist material can be obtained from the Scientific American magazine.

Choral Comestibles

Fill a coffee mug with hot water. Pour in a package of hot chocolate powder and stir. Now place the spoon in the cup and repeatedly tap the bottom while holding the cup up off the table.

You'll hear a low ringing tone while striking the cup, and the frequency of the tone will slowly ascend! Stop tapping and wait, then resume. The tone will have risen.

Now stir the liquid well, then resume tapping. The tone will have fallen to nearly the initial frequency. The tone will increase with time, and will decrease with stirring.

With some practice it is possible to stir and tap, resulting in a descending tone.

If water is slowly poured into the chocolate, the tone falls, then rises again.

Other instances of the phenomenon:

While beating meringue, the sound of the beaters changes as the volume of egg white grows, and if the beaters are knocked against the bowl, a low resonant tone is sometimes heard.

When sugar is poured into superheated water and stirred, then the spoon is tapped on the side, a weaker version of the effect ensues. A bit of detergent can help form the small bubbles.

If you strike rocks repeatedly together underwater, then whenever a cloud of bubbles is created there will be a deep thrumming tone coming from the resonant cavity formed by the bubble cloud, and the tone will rise as the bubbles rise. This sound is surprisingly loud for having been generated underwater, no doubt because of the better impedance match between foam and air, rather than the poor match provided by water and air.

When the hot chocolate is poured into the water, lots of tiny bubbles appear. This radically alters the speed of sound because, although the density of the water remains nearly the same, the stiffness of the medium drops dramatically because the air bubbles are so squishy. That means that the speed of sound becomes the square root of the ratio of a small number (stiffness) over a large number (density) and so the speed of sound in the hot chocolate is smaller than the speed of sound in either water or air!

The bubbles are not added by the chocolate. The air is already dissolved into the water. Tap water contains a certain amount of dissolved air (that is, after all, how fish breathe.) Hot water can not hold as much air as cold water. So when tap water is heated (by heating in a microwave oven, for example) it becomes supersaturated with air. Any little disturbance causes the air to come out of solution in tiny bubbles. You can see this for yourself by doing this experiment with a handful of salt or sand substituted for the hot chocolate. Tea bags or coffee work as well.