Ah, summertime! Time to get away from it all; to sit in the shade with a cold soft drink or a bowl of delicious homemade ice cream (such as the batch I made Saturday). Doesn’t seem very conducive to thinking about science, does it?
Consider that soft drink (or any other bubbling brew) you’re holding. The obvious question is, where do all those bubbles come from?
Soft drinks bubble because carbon dioxide is dissolved in them. The bottlers do that by chilling the liquid, then letting it flow over a series of plates in an enclosure that contains carbon dioxide under pressure. The lower the temperature and the higher the pressure, the more gas will dissolve in the drink.
As long as the drink is in its bottle or can, it’s in equilibrium. The empty space at the top of the container contains carbon dioxide at two or three times the pressure of air at sea level. Some of its molecules are constantly diving into the liquid, while some of the molecules of carbon dioxide in the liquid are leaping free, but it all balances out.
When you pop the cap or tab, the pressurized carbon dioxide expands rapidly outward. To move, molecules require energy, which they take from their surroundings. The result is a brief drop in temperature at the mouth of the container–down to -28 degrees Celsius. That makes the water vapor at the mouth of the container condense, which is why you see a brief wisp of fog.
Before the top was popped, the pressure was too high for bubbles to form; however, “protobubbles,” tiny clusters of carbon dioxide molecules, had gathered at “nucleation sites,” microscopic pits in the container wall or floating particles. With the pressure reduced, these protobubbles collect more carbon dioxide, expand and ascend, growing larger as they gather more of the dissolved gas.
Every child knows that if you shake up pop, it fizzes. That’s because the agitation causes miniature whirlpools. At the centre of each whirlpool is an area of low pressure, which permits microscopic bubbles to begin to expand. The harder you shake the pop, the more whirlpools–and bubbles!–you create.
If you do this before the can or bottle is opened, you create millions of protobubbles that erupt into full-size bubbles the moment you release the pressure holding them in check. And “erupt” is exactly what they do, as many a soggy victim can attest.
Of course, the victim may appreciate such a cooling shower if he or she has been cranking away at a good old-fashioned ice-cream maker. (Yes, I know electric ice cream makers exist, but purists such as myself insist on cranking our ice-cream makers ourselves. The ice cream tastes better that way. Honest!)
Ice cream is frozen by putting a mixture of milk, sugar, flavoring and other ingredients (depending on the recipe) in a tub and rotating it in a bath of salt and ice while a paddle keeps the mixture stirring.
You put salt on the ice to make the resulting ice water colder. In ice water, as in the unopened pop, there’s an equilibrium. Some water molecules from the ice break free, while other molecules in the liquid attach themselves to the chunks of ice. If more molecules break free than attach, the ice melts.
To break free, a molecule requires energy. As salt dissolves in water it breaks into charged particles (called ions) that prevent molecules in the liquid from attaching to the ice, but don’t interfere with molecules breaking free from the ice. The energy for them to break free comes from the surrounding water and ice, which makes the temperature of the mixture drop below freezing. Because the salt acts as an antifreeze, the ice continues to melt until a new equilibrium is reached 10 degrees below freezing–a good temperature for making ice cream.
The sugar within the ice cream mixture also acts as an antifreeze. As the water in the mixture freezes, the concentration of sugar in the remaining liquid increases, which drops its freezing point lower and lower–eventually, lower than the temperature of the ice-salt mixture. As a result, the ice cream never freezes completely solid.
Constantly stirring the mixture insures that the ice crystals that do form don’t grow too big and the ice cream chills uniformly. It also whips air into the ice cream, which increases its volume and also helps keep it soft
You see? Science is not only fun–it’s downright delicious!