Kaitlin Lockhart and Rob Hirsch

September 31, 2007

Professor Hartlaub

Effect of Temperature on Diet Pepsi / Mentos Liquid Displacement

Introduction:

Mixing diet cola with Mentos is a classic science experiment that involves a physical reaction, producing an eruption. This eruption occurs because the Mentos candies have thousands of tiny cracks along their surfaces, providing pores for the carbon dioxide to quickly adhere to, forming many bubbles. These bubbles are forced to the surface as the candy sinks to the bottom of the bottle. This increase of pressure pushes the liquid up and out of the bottle, like a geyser. We were curious to see whether the temperature of the cola affected the force of this pressure, and therefore, the amount of liquid that was displaced. Since gases expand less the colder they get, the carbon dioxide in colder soda would have less energy and thus cause less pressure to build up in the bottle. This would force less liquid out of the bottle than at higher temperatures. Our experiment compared a control group of room temperature Pepsi bottles to a treatment group of chilled soda. The response variable was the liquid remaining in each bottle, while the explanatory variable was the temperature of the soda. The chilled Pepsi was expected to displace less liquid than the room temperature Pepsi.

Materials and Methods:

We separated twelve 710ml diet Pepsi plastic bottles using a random number generator into two groups (Fig1). The first group of six bottles was left at room temperature (19C) while the second group was refrigerated at approximately 4C for 45 minutes. While the second group was chillin, we emptied 8 packs of mint Mentos into a bowl. Out of this group of 56 candies, we randomly picked Mentos one at a time until we had 12 groups of four candies. We then used a random number generator to randomly assign each group of Mentos to a bottle (Fig 2). The use of randomization in grouping soda and Mentos, and in pairing the groups, helped remove sources of bias and allow for a more accurate collection of data. We conducted the experiment indoors to minimize temperature differences. One at a time, one person removed the cap and held the bottle still while the other person quickly released four Mentos through a paper funnel into the bottle. The bottle was kept in position while the reaction was occurring, and then capped when complete. This process was repeated for each of the six room temperature bottles. We then measured the liquid remaining in each bottle using a glass measuring cup on a level surface. We did not allow the Mentos to be poured into the cup. Then we recycled and cut the plastic wrappings so squirrels would not get caught. We repeated this process with the refrigerated bottles, removing half at a time from the refrigerator to keepthe temperature as consistent as possible.

Results:

The mean amount of liquid left in the six cold bottles was288.0 ml (±7.55, n= 6), while the standard deviation was 18.49 ml. The six control bottles (room temperature) had a mean amount of 249.17 ml left(±0.749, n= 6), and a standard deviation of 1.83 ml. The room temperature bottles were observed to explode quicker and with more force (higher geyser) than the chilled bottles did. The cold bottles had 13.48% more liquid remaining in the bottle. A Minitab two sample T-test revealed that the means were significantly different (T-test, T= -5.12, p= 0.004). Figure 3 shows the lower mean and variance of the control group compared to the experimental group.

Discussion:

In this experiment, we hypothesized that chilled bottles of diet Pepsi would expel less liquid in a Mentos induced eruption than diet Pepsi at room temperature. Our data showed that chilled bottles of diet Pepsi retained more liquid than the room temperature group. Due to random sampling, both groups had low bias but differed in variability. The greater variation in the data for the cold bottles may be due to the greater difficulty in controlling variables in the experimental group versus the control group. For example, slight temperature discrepancies between individual bottles in the experimental group were larger than in the control group. While the room temperature bottles were always in the same environment together, the amount of time each experimental bottle was refrigerated varied slightly. This variance occurred because the chilled bottles waited outside the refrigerator for slightly different lengths of time before being exploded, and may have regressed toward the mean temperature of the room. Also, the final two chilled bottles had the greatest remaining liquid, which most likely corresponds to longer refrigeration and less interval time between refrigeration and reaction.Also, the handling of each bottle varied and hand temperature may have increased bottle temperature slightly. Another source of variation may include the observation that the chilled geysers were much shorter, thus having a greater chance of liquid falling back into the bottle. Some lurking variables may include inconsistency in the time between the bottle un-capping and the insertion of the Mentos, and the timing of the individual Mentos as they were dropped in; longer time between the insertions of individual candies might have reduced their combined effect on the soda. Future studies might examine the effect of different sodas, different types of Mentos, and different amounts of candy and soda on the force of the explosion. Despite various sources of error, the overall conclusion of the experiment affirmed our hypothesis that chilled soda dispels significantly less liquid than room temperature soda in a Mentos reaction.

Amount of Soda Remaining in Bottles:

Control(ml) Cold(ml)

248262

250299

248280

252275

250302

247310

Descriptive Statistics: Data (combined), Cold, Control

Variable N N* Mean SE Mean StDev Minimum Q1 Median Q3

Data 12 0 268.58 6.88 23.84 247.00 248.50 257.00 294.25

Cold 6 0 288.00 7.55 18.49 262.00 271.75 289.50 304.00

Control 6 0 249.17 0.749 1.83 247.00 247.75 249.00 250.50

Variable Maximum

Data 310.00

Cold 310.00

Control 252.00

Figures:

Figure 1:

Randomized bottle assignment

Group 1 (control):

Group 2 (Experimental):

Figure 2:

Randomized pairing of bottles and Mentos groups:

Mentos Group:Bottle:

11 / 8
1 / 5
6 / 2
5 / 9
2 / 4
9 / 1
10 / 11
8 / 3
4 / 12
7 / 6
12 / 7
3 / 10

Figure 3: