• Assess students’ understanding of the relationship between mass, volume, and density by collecting their written explanations about the demonstration with the soda cans.
• Assess students’ understanding based on their responses to the warm-up questions on Day 2, and review the concept of density as needed.
• Collect and assess the Liquid Layers Lab worksheet and the Densities of Common Substances worksheet.

To prepare, fill the aquarium at least half full with water. Place a can of cola and a can of diet cola into the aquarium. The diet cola will float and the cola will sink. Be sure to test the cans first. The diet soda should float because it contains aspartame instead of sugar, and less aspartame is needed to sweeten soda than sugar; it will be less dense than the regular cola.

Day 1: Instruction

Show students the cans of soda in the aquarium. Discuss why one soda floats and the other sinks. Encourage students to ask questions and make hypotheses. Accept all explanations at this point. After five minutes, have students write down their explanation as to why the diet cola floats and the cola sinks. From students’ explanations, make a statement such as, “We have determined that the reason an object floats has something to do with how much mass it has or how much space it takes up. We will revisit this demonstration at the end of the lesson.”

Show students the Density PowerPoint presentation (see S-5-3-3_Density PPT in the Resources folder). Have students write down the definition of density and the density equations. As you progress through the presentation, guide students step-by-step through each of the example problems.

Hand out copies of the Density Worksheet (see S-5-3-3_Density Worksheet and KEY in the Resources folder). Have students work in pairs to complete the worksheet. Then, go over each of the problems step-by-step with the whole class.

Ask students to write a new explanation about the cola and diet cola cans, using the words mass, volume, and density. Have volunteers measure the mass of each can and calculate the density (the volume of each is 355 mL). The density of the diet cola will be less than 1.0 because it floats, and the density of the cola will be greater than 1.0. Have several students share their explanations. Then, explain that water (at room temperature) has a density of 1.0 g/cm³. Objects and liquids that are denser than water will sink and objects that are less dense than water will float.

Day 2: Liquid Layers Lab

To prepare, gather the materials for each group of students for the Liquid Layers Lab. Students will not know the identities of the three liquids, so prepare unlabeled containers of oil, corn syrup, and colored water.

As a warm-up, have students answer the following questions:

Which has greater density…

• Ice or water? (water)
• A lake full of water or a cup full of water? (The density is the same.)
• One pound of lead or 100 pounds of feathers? (lead)

Go over the answers to the warm-up, and correct misunderstandings as needed.

Divide students into lab groups and hand out the materials and copies of the Liquid Layers Lab worksheet (S-5-3-3_Liquid Layers Lab.docx). Go over the lab procedure with students, and then monitor the groups as they conduct the lab. Have students clean up all materials, and then go over their conclusions, revealing the identities of the unknown liquids. Collect the lab worksheets.

As a follow-up to the lab, assign the worksheet Densities of Common Substances for homework (S-5-3-3_Densities of Common Substances and KEY.docx).

Extension:

• Students who might need opportunities for additional learning can practice measuring the mass and volume of various objects and then determine the densities.
• Students who may be going beyond the standards can be asked to explain why helium balloons float in air, in terms of mass, volume, and density.
• Students can conduct tests of other regular and diet sodas to determine if they float or sink, and create a data chart to compare various brands of sodas.