Q: When I was eating rice recently a grain of rice fell into a glass of fizzy lemonade and sank to the bottom of the glass. It then went up again and down, and kept doing this for about ten minutes. Why?
A: The grain of rice sinks initially in the lemonade because it is denser than the liquid. However, once it has reached the bottom of the glass, bubbles of carbon dioxide begin to collect around the grain, because bubbles in fizzy drinks tend to form preferentially around rough surfaces.
Eventually the bubbles of carbon dioxide begin acting as if they were airbags inflated under water. The grain of rice becoems buoyant and starts floating towards the surface. When the rice reaches the surface, the bubbles burst and the grain sinks again, restarting the process. This effect will eventually die down as the lemonade goes flat.
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A: Extensive research into this phenomenon was conducted by students of Aberdeen University between 1989 and 1991, using a variety of drinks and bar snacks.
The most consistent effects were usually obtained using salted peanuts and cheap lager. In fact, a handful of peanuts rising and falling in a pint can produce an effect similar to the lava lamps popular in the 1970s, but the amount of salt on the nuts, usually renders the lager undrinkable.
Newcastle Brown Ale and most draught ciders are usually so fizzy that bubbles form as fast as they burst, so the peanut never sinks twice. Dry roasted peanuts are far too dusty to work properly, while pork scratchings and crisps do not sink, they just float on the top and go soggy.
The tests were all conducted in controlled laboratory conditions (the bar) by students from a range of subjecs were were all members of the Aberdeen University Scottish Country Dance Society.
A: This phenomenon forms the basis of the 鈥淎mazin鈥� Raisin鈥� demonstration which our junior chemists have used successfully in chemistry club competitions and which gets a lot of parental attention on open days.
We use a cylindrical chromatography tank into which are put bicarbonate of soda and dilute acid. After the initial fizz there is still an evolution of bubbles. Drop in the raisins. They solemnly go up and down for ages.