## Make a Coke Float

This is a lesson about buoyancy, density, and specific gravity.

Make a Coke Float: Lesson in Buoyancy, Density, Specific Gravity

Materials List:

Bucket large enough to submerge a soda can

water

Can of Coke Classic or regular soda

Can of Diet Coke or diet soda

Postal scale

Measuring cup

Tablespoon

Salt

2 pennies (change added August 2009)

Procedure:

Step 1: In the bucket, measure and place enough water to cover the unopened can of Coke Classic. Write down the volume of water used, call it Vwater. Make sure there is no air bubble trapped under the soda can. The Coke Classic should sink. If there is room in the bucket, put in an unopened can of Diet Coke. The Diet Coke will float. Remove the can of Diet Coke.

[August 2009 change: current soda cans are made with thinner aluminum. We found when we placed new cans of Coke and Diet Coke in the water, both cans floated! To fix the problem, we added a penny under the tab of each can. Be sure to use two pennies of the same vintage, pre-1982 pennies are different than post-1982 pennies.]

Step 2: Density, ρ, is the mass per unit volume, or,

Step 3: Calculate the density of the unopened can of Coke Classic in the units of gm/cm3. Repeat the calculation to find the density of the unopened can of Diet Coke in the units of gm/cm3.

Step 4: When studying density, it is very common to use specific gravity. Specific gravity, γ, is the ratio of the density of a liquid or solid relative to the density of water at 4°C, ρwater at 4°C = 1.0 gm/cm3. For example, the specific gravity of gold is γgold =(density of gold)/(density of water at 4°C). Calculate the specific gravity of a can of Coke Classic. Calculate the specific gravity of a can of Diet Coke. An item with a specific gravity greater than 1.0 will sink in water, while an item with a specific gravity less than 1.0 will float in water. Does your calculation support the results of the Coke Classic and Diet Coke demonstration?

Step 5: So, how do we make the coke float? The can of Coke Classic should still be submerged in the water. Since the density of the unopened can of Coke Classic is fixed, the only thing we can change is the liquid in which to float the Coke Classic. This liquid would need to have a specific gravity greater than the specific gravity of the can of Coke Classic. What liquid can we use? How about salt water? You can just add salt and watch what happens, or, let’s first predict (calculate) the amount of salt you will need to make that coke float!

Step 6: Choose a specific gravity greater than your calculated specific gravity of a can of Coke Classic. For example, γsaltwater=1.02.

Step 7: Recall the γsaltwater =ρsaltwater/ρwater at 4°C. Find the density of the saltwater you want to make using ρsaltwater=1.02*ρwater at 4°C.

Step 8: The volume of water in your bucket is Vwater. Convert this volume to cm3. Since the density of water is defined above, we can calculate the mass of the pure water using the relation, mwater=ρwater *Vwater.

Step 9: We will assume when we add the salt to the water, the volume of the water to saltwater stays the same, so Vwater=Vsaltwater. If you know the volume and density of the saltwater, you can find the mass of the saltwater, using msaltwater=ρsaltwater *Vsaltwater.

Step 10: But the mass of the saltwater is composed of the mass of the pure water plus the mass of the salt. So the mass of the salt can be found by msalt=msaltwater-mwater.

Step 11: One tablespoon of salt has an approximate mass of 18.0 gm. Add your predicted mass of salt to the water and watch what happens…you just made a Coke float! But, don’t drink the water!

Step 12: For questions or comments on this Do-It-Yourself Experiment, please e-mail us at

Sources:

1. Root Beer Float,

2.

Materials List:

Bucket large enough to submerge a soda can

water

Can of Coke Classic or regular soda

Can of Diet Coke or diet soda

Postal scale

Measuring cup

Tablespoon

Salt

2 pennies (change added August 2009)

Procedure:

Step 1: In the bucket, measure and place enough water to cover the unopened can of Coke Classic. Write down the volume of water used, call it Vwater. Make sure there is no air bubble trapped under the soda can. The Coke Classic should sink. If there is room in the bucket, put in an unopened can of Diet Coke. The Diet Coke will float. Remove the can of Diet Coke.

[August 2009 change: current soda cans are made with thinner aluminum. We found when we placed new cans of Coke and Diet Coke in the water, both cans floated! To fix the problem, we added a penny under the tab of each can. Be sure to use two pennies of the same vintage, pre-1982 pennies are different than post-1982 pennies.]

Step 2: Density, ρ, is the mass per unit volume, or,

**ρ= m/V**, with m being mass and V meaning volume. The can of Coke Classic is denser than the water, while the can of Diet Coke is less dense than water, the difference being the sugar versus sugar substitute used in the soda. By definition, the density of water at 4°C is, ρwater at 4°C = 1.0 gm/cm3. The density of water will vary slightly depending on its temperature. Use a postal scale to weigh the soda cans. Convert the mass of each soda can to grams, if necessary. Note: the volume of the unopened soda can is actually extremely difficult to measure with adequate precision (using displacement or physical measurement techniques). Our tests have produced an approximate volume of Vsoda can= 378 cm3. Go ahead and use this volume in your calculations.Step 3: Calculate the density of the unopened can of Coke Classic in the units of gm/cm3. Repeat the calculation to find the density of the unopened can of Diet Coke in the units of gm/cm3.

Step 4: When studying density, it is very common to use specific gravity. Specific gravity, γ, is the ratio of the density of a liquid or solid relative to the density of water at 4°C, ρwater at 4°C = 1.0 gm/cm3. For example, the specific gravity of gold is γgold =(density of gold)/(density of water at 4°C). Calculate the specific gravity of a can of Coke Classic. Calculate the specific gravity of a can of Diet Coke. An item with a specific gravity greater than 1.0 will sink in water, while an item with a specific gravity less than 1.0 will float in water. Does your calculation support the results of the Coke Classic and Diet Coke demonstration?

Step 5: So, how do we make the coke float? The can of Coke Classic should still be submerged in the water. Since the density of the unopened can of Coke Classic is fixed, the only thing we can change is the liquid in which to float the Coke Classic. This liquid would need to have a specific gravity greater than the specific gravity of the can of Coke Classic. What liquid can we use? How about salt water? You can just add salt and watch what happens, or, let’s first predict (calculate) the amount of salt you will need to make that coke float!

Step 6: Choose a specific gravity greater than your calculated specific gravity of a can of Coke Classic. For example, γsaltwater=1.02.

Step 7: Recall the γsaltwater =ρsaltwater/ρwater at 4°C. Find the density of the saltwater you want to make using ρsaltwater=1.02*ρwater at 4°C.

Step 8: The volume of water in your bucket is Vwater. Convert this volume to cm3. Since the density of water is defined above, we can calculate the mass of the pure water using the relation, mwater=ρwater *Vwater.

Step 9: We will assume when we add the salt to the water, the volume of the water to saltwater stays the same, so Vwater=Vsaltwater. If you know the volume and density of the saltwater, you can find the mass of the saltwater, using msaltwater=ρsaltwater *Vsaltwater.

Step 10: But the mass of the saltwater is composed of the mass of the pure water plus the mass of the salt. So the mass of the salt can be found by msalt=msaltwater-mwater.

Step 11: One tablespoon of salt has an approximate mass of 18.0 gm. Add your predicted mass of salt to the water and watch what happens…you just made a Coke float! But, don’t drink the water!

Step 12: For questions or comments on this Do-It-Yourself Experiment, please e-mail us at

__seekalaska@aol.com__.Sources:

1. Root Beer Float,

__http://littleshop.physics.colostate.edu/Root_Beer_Float.html__, 9/18/2003.2.

*Physics for Science and Engineering*, by John P. McKelvey and Howard Grotch, Harper & Row, New York, NY, 1978.*3. Introduction to Physics for Scientists and Engineers*, by Frederick J. Bueche, McGraw-Hill Book Company, New York, NY, 1975.