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Thirsty Rocks: Please "Porous" a Drink!

Summary:
This activity will introduce students qualitatively (if observation only is used) and/or quantitatively (if lab exercise is used) to the concepts of porosity and permeability, using a hand-sized specimen of dry sandstone. Materials:
  • One-pound plastic Solo tub, clear
  • 350 - 400 ml of water
  • 500 ml graduated cylinder
  • Dry sandstone rock pieces (hand-sized), approximately 250 g each (range 200 - 300 g). These are available from local landscaping firms.
  • Decigram pan balance (or digital balance)
  • Clock or watch
  • Paper toweling

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Source:
David F. Mastie, Retired Earth Science Teacher, Pioneer High School, Ann Arbor, Michigan.
Grade level:
4 - 10
Time:
15 -20 minutes plus an additional 5 minutes for discussion
Student Learning Outcomes:
  • Students will learn that the porosity of a sedimentary rock like sandstone is a measure of its empty space (pore space).
  • Students will learn that the permeability of a sedimentary rock like sandstone is a measure of the interconnections of the pore spaces.
Lesson format:
Experimental Observation, Hands-on activity

National Standards Addressed:

DIRECTIONS:

  1. [If qualitative activity alone, do steps 2-5 below; if quantitative activity is desired, do all steps as a lab exercise.]
  2. A day ahead, dry out the sandstone specimens by baking them for an hour at 300 degrees to eliminate residual water. Cool before use.
  3. Place about 350 ml of water into a one-pound plastic Solo tub. Record exact amount in milliliters.
  4. Mass a dry sandstone specimen on the balance. Record the answer in grams.
  5. Place the massed sandstone specimen into the water-filled tub for at least 7 minutes. Draw and label what you observe happening.
  6. At the end of this time, remove the rock and blot it as dry as possible. (Remove all excess surface water.)
  7. Using the graduated cylinder, measure the water loss from the tub. Record the loss in milliliters.
  8. Re-mass the rock and record your data in grams.
  9. Calculate the percent of gain due to water absorption by subtracting the dry weight of your specimen from its wet weight. (Divide this number by the dry weight and multiply by 100 for percent.)
  10. Enter your results (in percent gain) onto the board or overhead projector, along with the results from each group.
  11. Compare the results from group to group. Discuss variables and outcomes. Extend to other phenomena (sponges, etc.) Clarify the terminology of porosity and permeability.

ASSESSMENT:

Ask students to predict percent gains in a variety of specimens of other sizes and under different conditions (variables).

BACKGROUND INFORMATION:

Most sandstones are composed largely of resistant quartz fragments. The fragment size varies from 1/16 to 2 mm. Most sand grains are rounded and sorted by size in their natural environments. If they're closely packed, the pore space might be as little as 25%; if they're loosely packed, the pore space might be as much as 50%. The sorting and the cementation process will definitely affect the pore space and hence the amount of water absorption.

It is estimated that less than 1% of the water on Earth is groundwater. Although this sounds small, it is 70 times larger than the volume of all of the water in freshwater lakes and flowing in streams. It is this part of the water cycle that this lesson focuses upon. Different branches of the water cycle operate on vastly different time scales. Ground water remains in rock from one year to 10,000 years. Fluids, like water, move through a rock, like sandstone, easily. We call this body of permeable rock filled with water an aquifer. The rock sample you used functions as a model of this system.

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Last modified October 23, 2003 by Jennifer Bergman.

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