Student Exploration: Porosity (ANSEWR KEY)

Student Exploration: Porosity
Vocabulary: aquifer, gravel, permeability, porosity, sand, saturated, sediment, silt, surface water
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
Two students are given cubic boxes, measuring 10 cm on a side. Robert puts a single glass marble with a diameter of 10 cm in the box. Susan puts 1,000 1-cm glass marbles in her box.
Whose box has more empty space? Explain. 
Whose box will be heavier? Explain.  
Gizmo Warm-up
Some rocks and sediments have a lot of empty space in them. This allows liquids such as water and oil to pass through and be stored in them. Permeability describes how easily liquid passes through a material, while porosity describes how much liquid can be stored in the material. These properties are explored in the Porosity Gizmo™.

Turn on the Macroscopic view of Gravel, Sand, and Silt. Gravel consists mostly of rock fragments greater than 2.0 mm in diameter. Sand consists of grains that are between 0.0625 and 2.0 mm in diameter. Silt consists of grains that are between 0.0039 and 0.0625 mm in diameter.
Which sediment do you think will allow water to pass through most easily?
Which sediment do you think could hold the most water?
An aquifer is a rock layer that stores and allows the flow of groundwater. Compared to other types of rock layers, how permeable and porous would an aquifer be?


Activity A:

Permeability
Get the Gizmo ready:
 If necessary, click Reset below each container of sediment.
 

Question: How does permeability relate to the grain size of sediment?
Observe: Above the gravel container, click ON. Observe water moving through the gravel.
Does the water pass easily through the gravel? 
Repeat the same procedure with the sand and silt. What do you notice?
Gather data: Surface water pools on top of the sediments. Surface water can indicate that sediments are saturated (full of water) or that the sediments are not very permeable.
Click Reset below each container. Release about 100 mL of water into the gravel beaker, and press OFF. Record the approximate amount of surface water you see just after you press OFF. Repeat the same procedure for the sand and silt.
Gravel: ______________             Sand: ______________                      Silt: ______________
Analyze: Based on your data, which is the most permeable sediment? _________________
Infer: How do you know the surface water is not there because the sediment is saturated?
Draw conclusions: How is permeability related to the size of the grains that make up the sediment? ________________________________________________________________
Apply: Suppose you were digging a well into saturated sediments. Why is the sediment’s permeability an important factor in deciding where to put your well?
      
Activity B:

Porosity
Get the Gizmo ready:
 Click Reset below each container of sediment.
 

Question: How does porosity relate to the grain size of sediment?
Predict: Which sediment do you think has can store the most water? __________________
Explain your choice: ________________________________________________________
Gather data: Release 400 mL of water into each sediment. Wait until each sediment is fully saturated with water. Record the amount of surface water in the first row of the table below.

Sediment:
Gravel
Sand
Silt
Surface water (in mL):
   
Water in sediment (in mL):
   
Sediment volume (in mL):
400 mL
400 mL
400 mL
Porosity (%):
   
Calculate: Find the amount of water that is stored within each sediment by subtracting the surface water volume from the original water volume (400 mL). Add this data to the table.
Calculate: To calculate the porosity percentage, divide the volume of water in the sediment by the sediment volume, and then multiply by 100. Add this data to the last row of the table.
Analyze: Which is the most porous sediment? _____________   The least? _____________
Draw conclusions: Compare the porosity number to the grain size of each sediment.
Does grain size determine the porosity of a sediment type? ____________________

Explain: 

Explain what you think controls a material’s porosity. 

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