This activity can be incorporated into a discussion of Chemistry, Density, and Properties of Matter, or you can use it on its own. Student will need to be instructed in the proper use of a scale, graduated cylinders, and the calculation of density from the mass and volume of an object.
In addition to scales and graduated cylinders, you will also need some gravel for this activity. What I have found works best is to get washed fish-tank gravel from a pet store. Sand is too fine. Construction gravel is too coarse. Fish-tank gravel is also available is a more rounded form that has been tumbled to remove sharp edges. If you only have access to a hardware store, look for “pea gravel” and 3/8-inch or smaller is a reasonable size. Some gravel is “washed.” That would be better, too. You want something small and clean.
You will need roughly 1 pound of gravel per student or group. I prefer to have students work individually whenever possible, but if you do not have enough material, you may try the activity with just 100ml of gravel per student, or have them work in groups. When you have finished, you will need a place to dry the gravel before you can reuse it for the same activity. If you need to use it again right away, draining it on a screen or in a strainer should get you close enough. Wet gravel sticks to the sides of containers, though. Dry gravel moves easily.
Whenever I have students work with gravel, sand, water, or any material that would be difficult to clean up, I have them work over trays. I have large plastic trays that are for under-bed storage. I also have many large aluminum trays. One of the lab skills I emphasize is to always pour over a container.
The reason I have students add the water to the gravel using a dropper is that if you allow them to pour water into the gravel, they invariably overshoot and add too much. Le them know that they do not have to count the drops or even measure the water as it goes in. They will be weighing the gravel after the water is in and calculating the difference in weight to find the volume of water added.
If you are not familiar with this activity, the fascinating thing about it is that the volumes in the two cups are not equal. In Cup 1 you have 200ml of gravel and approximately 80ml of water. In Cup 2 you have 100ml of gravel and approximately 80mlof gravel. Both cups have 280ml of material added to them, but the volume of Cup 1 is 200ml, and the volume of Cup 2 is 280ml. The water is able to take the place of the air in the gravel, but when adding gravel to the water, there are no spaces in the water for the gravel to go. The gravel displaces its own volume in water.
Make sure that the plastic cups you are using will hold 280ml. If you have smaller cups, the activity works fine changing the initial volumes of water and gravel to 100ml.
One interesting extension of this activity is a simple discussion of measuring techniques. Ask students if when they measured a volume of gravel in the graduated cylinder, if in fact their results were accurate. If you fill a graduated cylinder with gravel up to the 200ml mark, is that 200ml of gravel? Explain why. How else could you have measured the gravel to obtain 200ml of gravel, and how much space would that take up? If you were to use water displacement to measure 200ml of gravel, how would that change this activity?
I have included a rubric and both Word and PDF versions of the activity and rubric. I have also included photos from class and of the materials.