At the request of one of the loyal readers of Oculon, I have done some research on the chemistry or lack thereof on Pop Rocks, the flavorful candy that makes a funny popping noise when you put it in your mouth.

Before I started (mainly on a long walk home one night) I reasoned that some rather begein gas was used and that it was either produced from a simple (and common) chemical reaction or it was dissolved/trapped in the candy somehow and was released when you put it in your mouth.

After a quick brief from the wizards over at howstuffworks.com I found that there really is no chemical reaction that occurs in your mouth when you eat pop rocks. The popping that you hear is the release of CO2 that is trapped in tiny bubbles on the order of 300 micrometers or 3 millimeters and smaller. In fact, the entire process of pop rocks has been patented since 1981 (US Patent #4,289,794) and its rather ingenious at that.

When hard candy is traditionally made, sugar is melted and heated until unwanted impurities either separate or boil away from the mixture, (things like O2, CO2, and Water). Other flavorings, colors, and ingredients are added and the candy is cooled and it hardens. Depending on the level of impurities and amount and type of ingredients added the candy produced is either brittle, or hard. Much like glass production where certain molecules or elements will strengthen or weaken a particular type of glass by adding or removing more structure to its atomic or molecular matrix.

In either case Pop Rocks are produced with a twist. The sugar is melted at 600 psi (which is more than 40 times Earth's atmosphere) most likely in the absence of Oxygen (to prevent the sugar from burning). The sugar is heated and melted and CO2 (carbon dioxide) is bubbled through the sugar. Liquid sugar is very viscous so much of the CO2 remains suspended in the sugar in small and large bubbles alike.

As the mixture cools the pop rocks shatter from the pressure of the CO2 bubbles inside. As the temperature decreases the pressure also decreases, however its the lack of atmospheric pressure (and thus reinforcement) from the outside that causes the candy to shatter, much like a balloon breaking. However in some instances, the bubbles are small enough that the candy can withstand the difference in pressure between inside and outside and the bubble is maintained. The candy is then further broken down into tiny bits and packaged for sale.

So what happens when you put it in your mouth? The candy dissolves in your saliva (it does not melt contrary to popular belief), and when the side of a bubble dissolves the gas rushes out and cases the popping noise that you hear.

Now for some dorkiness through unnecessary approximation and calculation:

If you have a bubble that is 3 millimeters in diameter and is at 41 atm (atmospheres) and we say that a single piece of pop rock is about 10 milligrams, we can make a very rough approximation of the theoretical speed of a pop rock in your mouth.

Atmospheres is a unit that's more useful in chemistry than in physics so we'll convert it to Pascals. One Pascal is one Newton of force per square meter and there are 101,325 Pascals in 1 atm. So there are 101,325 Newtons of force per square meter under standard atmospheric conditions. Our bubble is forty times that so it comes out to 4,053,000 N/M2 .

The total surface area of our sphere is 4r^2pi=28.3 mm2 so we'll say that when the pop rock goes into your mouth is opens up a hole about 10 mm2 i n the bubble. So we convert our pressure number so that its N per mm2.

4,053,000 / 1000^2 = 7.053 N per mm2

Since we hav e 10 mm2 its about 70.053 N and that's how much force is exerted from our hole and we'll also say that this force occurs over .1 milliseconds.

If you recall that F=ma or force = mass x acceleration, we can substitute that acceleration is the change in veloc ity divided by time so we get

F = mv/t or
Ft = mv

We have approximated F, t, and m so we solve for v.

70.053 * .0001 = .001 * v

v = 7.0053 m/s

This of course doesn't actually happen because your saliva adds mass to the pop rocks and also holds it down. In any case this number is essentially useless since its based on wild approximation but you get the idea.

So the next time you eat pop rocks, you'll know why they pop!