This week's experiment is a favorite of mine, which won't be a surprise to anyone that has been on my experiment list for long. To try this tasty experiment, you will need:
- 2 glasses
- carbonated soda
- ice cream
For this experiment, we will make two ice cream sodas. Although both will contain exactly the same ingredients, they will be very different.
Start by filling one glass half-full of soda. Then add a scoop of ice cream. In the other glass, start by adding a scoop of ice cream, and then pour the soda into the glass.
You should notice a big difference between the two mixtures. The glass where you added the ice cream first will have lots of thick, long lasting foam, while the glass where the ice cream was added after the soda has very little foam. Why is there such a big difference?
There are two things that contribute to the difference in the foam. First, the carbonated soda contains quite a bit of carbon dioxide gas dissolved in it. It may seem strange to think of a gas dissolving in a liquid, but it is quite common. If you watched the Watched Pot video, you may remember the bubbles of gas that appeared before the water started to boil. Those bubbles formed from gases that were dissolved in the water.
The soda is supersaturated with carbon dioxide, which means that it contains more of the gas than would normally dissolve in it. As long as the soda is undisturbed, the carbon dioxide escapes very slowly, but you can speed up the process by adding bubbles. The bubbles provide more surface area for the gas to escape from. As the gas escapes, the bubbles grow larger, providing even more surface area.
You can see a very good example of this with an unopened bottle of soda. If you give it a hard shake, and then open it, you are in for a mess. Shaking the soda introduces lots of tiny bubbles into the soda, providing plenty of places for the carbon dioxide to come out of solution. On the other hand, if you shake the soda, and then let it sit for a while before opening it, the bubbles will have time to float to the surface and pop. In that case, when you open the soda, it stays in the bottle.
Another example of bubbles causing foam is the classic Mentos and Coke experiment. The candy has a porous surface, which produces many tiny bubbles. You can get similar results by dropping pieces of chalk into your soda, as it also has a very porous surface, but I don't recommend drinking the soda afterwards.
Now back to our ice cream. Ice cream contains a LOT of bubbles. In fact, a carton of ice cream may be as much as half air. Those bubbles are important, as they keep the ice cream soft and smooth, instead of hard and crunchy like an ice cube.
When you pour the soda into the glass first, microscopic bubbles from irregularities in the sides of the glass serve as a starting place for the foam. Most of the carbon dioxide bubbles form and pop before the ice cream is added.
OK, so why is it any different when you add the ice cream first? The ice cream has lots of tiny bubbles of air, so much more of the carbon dioxide comes out of solution. If you taste the soda afterwards, you will find that it is quite flat, with no fizz left. But, ice cream also contains chemical thickeners, to make it smoother and creamier. As you pour the soda over the ice cream, some of the ice cream melts, letting the thickeners mix with the soda. Just as they thicken the ice cream they thicken the foam, making it much firmer, and much longer lasting. Instead of quickly popping, this time the foam stays long enough for you to enjoy eating your tasty treat.
So although both recipes use exactly the same ingredients, the order you add them makes a big difference, although both turn out quite tasty. And keep in mind that an important part of science is that experiments should be repeatable, so you might want to repeat the test several times, just to be sure of your results.
Have a wonder-filled week.
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