Objects float if they weigh less than the weight of the water they displace. Conversely, objects will sink if they weigh more than the weight of the water they displace. This principle is at the heart of a classic science demonstration: the differing buoyancy of Diet Coke and regular Coke. An unopened can of Diet Coke floats in a tank of water, whereas the same cannot be said for a can of regular Coca-Cola. This seemingly simple experiment opens a window into understanding density, buoyancy, and the scientific method.
Setting up the Experiment
To conduct this experiment, you'll need a clear tank (a plastic Rubbermaid tub or aquarium works well), unopened cans of Diet Coke and regular Coke, and optionally, salt, a beaker, and a stirring rod.
Place the diet soda at the bottom of the tank. While both cans displace the same volume, the diet soda will float and rise to the top. Start with both cans out of the water so that the audience has a chance to predict the outcome. Have towels ready.
It is fairly easy to fool the audience into thinking that both float on their own. The bottoms of each can are concave, so if one holds the Coke can perfectly upright when lowering it into the tank, enough air can be trapped under the can to make it buoyant. To debunk the prank, give the can a slight nudge and the air bubble will surface.
The Science Behind the Float
The key to this demonstration lies in the concept of density. Density is defined as mass per unit volume. If the immersed object is denser than the fluid, gravity exceeds the buoyant force, and the object sinks. If the object is less dense than the fluid, the object floats to a level where the buoyant force balances gravity.
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Although a volume of 355 mL is printed on the side of each can, we have found by simply pouring the contents into a graduated cylinder that they each contain about 352 mL of liquid. Since the mass and volume of the aluminum can and compressed gas are nearly the same for Coke and Diet Coke, the deciding factor becomes the specific gravity of the liquid contents. Our own rough experiment-weighing different volumes of liquid in a graduated cylinder-gives an average density of 0.911 g/mL for Diet Coke, and 1.026 g/mL for regular Coke.
Regular soda has approximately 40 grams of sugar that the diet soda does not contain. In regular soft drinks, there is more sugar packed into the same space as a diet. The diet uses artificial sugar. It takes less artificial sugars to create the same taste like regular sugar, so less is needed. The 39 g of sugar in 355 ml Coke is about 11%. According to the tables in the 51st edition of the CRC Handbook of Chemistry and Physics, an 11% solution of either fructose or sucrose has a density of slightly greater than 1.04 g/ml (1.045 for fructose and 1.044 for sucrose). We may reasonably expect that the sugars in Coke should bring its density near this. Dissolved carbon dioxide, in both Diet and regular Coke, probably also changes the density. If we use a density of 1.045 g/ml for the Coke, 355 ml has a mass of 371 g. Adding 13 g for the can brings this to 384 g, for an overall density of 1.01 g/ml, enough to sink the can. Any mass over about 380 g will sink the can, as noted above.
The soda cans are sold as containing 12 ounces, or 355 ml, of liquid. This leaves just under 25 ml head space. The can itself is about 13 g.
The total volume of a Coke can (of either variety) is 380 cm3. If the corresponding volume of water weighs just slightly more than the can does, then if we immerse the can in the tank of water, it floats. In other words, the mass of the can must be somewhat less than (1.0 g/cm3)(380 cm3) = 380 g. The can of Diet Coke shown above has a mass of about 370 g, as does a second spare can. Its overall density is 370 g/380 cm3, or 0.97 g/cm3. Each can of regular Coke currently on hand has a mass of about 384 g. The difference that this represents with respect to the density of water - about 1.0% - is enough to sink the can.
That is why regular Coke sinks while the diet can floats. diet soda is just under. This makes for a dramatic display of density differences. brand of diet soda floats. aluminum cans does the small density difference straddle the density of water. plastic containers sink. (nearly 8 teaspoons!) of sugar dissolved in the water of the regular soda. much smaller mass of aspertaine (Nutrasweet/Equal) to flavor the diet soda. densities to end up just over and under the density of water.
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Exploring Further: Salt Water
If the fluid in the tank were dense enough, both cans would float. To this end, this demonstration includes a beaker of salt and a large stirring rod. By adding salt to the tank, you can increase the density of the water (which is now salt water). A 1.5% (wt/wt) salt solution has a density of 1.0107 g/cm3. For the 15-liter tank, this would be approximately 225 g of salt. So any salt concentration greater than this should be enough to raise the can of regular Coke. To be safe, the beaker contains about 275 grams of salt, which is more than enough to raise the heaviest Coke can listed above. The dissolved salt raises the density of the water
Turning Sinkers into Floaters (and Vice Versa)
Tell students that sinkers can be "taught" to float. whispering "coaching instructions" to it. / container wall or into a corner so it is perpendicular to the water. This traps an air bubble under the can and it should float. 'swimming lessons' before class. Students will be amazed. and challenge them to make floaters out of the sinkers. is it possible to reverse the process and turn a floater into a sinker? there is a bit of deception involved here, too. plasticene to the bottom of the can. remove the clay without being detected.
Variables to Consider
As the measured masses of the Coke cans indicate, there is some variation in either the density of the Coca-Cola itself, or the volume delivered to each can. Also, the mass of the can may vary somewhat (the ones I measured were somewhere between 12 and 13 g), and it is also possible that the overall size of the can could vary slightly.
invite students to participate. the floating fate of each. Let them experiment in their groups. way to systematically test the variables. keep track of the concepts they test. interpret the data they collect. diet drinks vs. "no fair." Students often scream "no fair" during their school day if someone gets a head start in a race; or more dessert in the cafeteria; or an easier class assignment. experienced. caffeine content. how the can is placed in water. amount of carbonation. mass of the can. even though all the cans may say 354 ml or 355 ml - do they contain that? Measure their contents. or 340 ml. filling process. These cans float significantly higher than others. below is a chart showing the mass of 60 cans. among the average mass : 389 g for Sprite; 388 g for Coke and Pepsi. minted to have a mass of 5.00 g. Pepsi and Sprite equals more than five nickels. only become skilled at it through practice. be tested. are the liquid volumes of all the cans identical ? have a greater mass than the same brand name that is caffeine-free? the empty, rinsed cans have the same mass? performed to rule out that . . . some of their hypotheses would be impossible to test. with white paint have a greater mass than the same can with orange paint? be tested is part of the learning process. perhaps the most important variable is the drink's sugar content. drinks. aluminum pie tin and compare the residue. the cafeteria staff who will supervise the boiling process on their stove. The liquid can be boiled away directly or using a double boiler method. oven when the cherry pie spills over. a deposit of cola syrup. and the school's all-important support staff. While overseas I have collected diet colas that SINK. responsible for floating/sinking. sinks. Students are stunned. variable -- the can. In many parts of the world they are still using iron-based containers. A magnet determines this rather quickly.
Buoyancy and Beyond: Real-World Connections
Understanding density is important for understanding weather. Warm air is less dense than cold air. Additionally, as the moisture content of the air increases, the less dense it becomes. In both cases, the less dense air rises easier. A Severe Thunderstorm Warning is an urgent announcement that a severe thunderstorm has been reported or is imminent and warns you to take cover. Mobile homes are especially vulnerable to the high winds of a thunderstorm and are subject to overturning and rolling if not properly anchored to the ground. Professional advice should be obtained from a reputable mobile home contractor. At a minimum, though, the frame should be secured with heavy steel straps.
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Conclusion
The Diet Coke vs. Regular Coke buoyancy experiment is a memorable and engaging way to introduce the concepts of density, buoyancy, and the scientific method. It highlights how small differences in composition can lead to observable differences in physical behavior. By encouraging students to ask questions, form hypotheses, and test variables, this demonstration fosters critical thinking and a deeper understanding of the world around them.