Monte Rompal- Density

Exploring density- How to float a coke.

This is the second part of a three part lab. The other two parts can be found here. The lab handout is attached below.

Principle(s) Investigated: Students should learn intuitively that Density is a ratio of "stuff" in a given "space." While learning "Density = Mass / Volume" is important, and that it has a lot to do with how/if a object floats, this lab is designed to make that concept more "real" to the students.

California Standards: From the Science Content Standards for California Public Schools Grade 8:

"Density and Buoyancy

8. All objects experience a buoyant force when immersed in a fluid. As a basis for

understanding this concept:

a. Students know density is mass per unit volume.

b. Students know how to calculate the density of substances (regular and irregular

solids and liquids) from measurements of mass and volume.

c. Students know the buoyant force on an object in a fluid is an upward force equal

to the weight of the fluid the object has displaced.

d. Students know how to predict whether an object will float or sink"

Materials

Hardware:

1 Large vase

1 12oz can Coke (regular)

1 12oz can Diet Coke

1 Stirring spoon

Software:

3L water (approx)

100g NaCl

C02 (solid- aka dry ice)

Procedure

Water should be in the vase, Other materials should be set aside in a organized easy to access manner. The dry ice should be broken up to increase surface area and speed the reaction. \

Students should be guided through a series of observations about the two cans of coke and the setup. Observations can include:

Both cans are the same size (roughly).

Both have roughly the same mass (thought the regular coke is slightly heavier)

They have different ingredients.

When placed in water, the regular coke sinks, and the diet coke floats.

Students should infer through discussion that both cans have a density close to that of water, and that their densities (relative to water) are the primary reason they float or sink.

At this point Students should be lead on a discussion to solve the question "how could I make the can of regular coke float?" While the coke is difficult to alter, the water is much easier. Using the idea that density is "stuff" per "space" students should come up with a way to add more "stuff" to the water's "space" such as salt. Stir in the salt until the coke can floats.

After this, the reverse problem should be considered. How can we make the diet coke can sink in the freshly salted water? Discussion should lead to finding a way to increase the amount of "space" the solution takes up without adding a lot of "stuff." Effervescent tablets are one solution, I find adding dry ice works more reliably. the Coke can should bob up and down, both sinking in the less dense water and rising when carried by the bubbles.

This video relates the concept to a much larger example.

Applications to Everyday Life:

- This demonstration shows how buoyancy can be used as a test for density. One popular word problem is sorting beverages without labels using this principle. This is also how tomatoes are sorted.

- Implications to boating are huge, as shown in the video. Engineering a boat to deal with gas releases or for travel in unusually dense water can benefit from this knowledge.

- Density is a significant factor in household cleaning... if Drain cleaners were not significant'y denser than the water they were put in, they could never make it to the clog they were meant to fix!