Purpose of the Discussion

The purpose of the discussion is for students to use data to answer the investigation question, Same weight, same volume?. They make claims and describe their supporting evidence. Their discussion helps them to solidify the difference between weight and volume and to recognize that some materials are heavier for their size than others.

Engage Students in the Focus Question

All five containers weighed 40 grams. Were their volumes the same? How did they compare?

Have the groups share their results. Do all groups agree on the order of the materials? When there is consensus, write the order on the board or post a sketch of the five labeled containers, arranged from least to most.

The order is: sand or gravel, water, mineral oil, organic material. The sand and gravel are very close in volume; groups may differ on which material takes up more space.

Do the results match the students' predictions? Are there any surprises?

Did students expect gravel and sand to be similar? Did anyone think the gravel might take up more space because the spaces between the pieces are so big?

Summarize the Discussion

Reinforce the idea of heavy for size by asking some probing questions:

• Would the order change if we had just 5g of each material? What if we have 500g of each material? Why not?

• Do any of these earth materials have similar volumes for equal weights?

• yes, gravel and sand

• Do you think this is a coincidence, or might there be a reason why sand and gravel are similar?

• Sand and gravel have different grain sizes but the minerals of which they are made are very similar.

Allow time for students to write responses to the reflection questions in their notebooks [Volume reflections].

Recap the Section

Wrap up this section of the curriculum by returning to the investigation questions that began and ended this unit:

Same weight, same volume?
Same volume, same weight?

Students have seen, with their own eyes, that materials that have the same weight can have different volumes — just as materials that have the same volume can have different weights.

Check that students are taking away the important points of this unit:

• Weight and volume are not the same thing.
• Weight and volume are different properties of a sample of earth materials.
• Some earth materials are heavy for their size, while others are relatively light for their size.
• Earth materials that are heavy for their size take up less space than materials that are light for their size, for an equal weight.