Science students are typically amazed to find out that rocks absorb water. In reality, the amount of water rocks absorb is based on the porosity of the rock. Porosity of rocks is based on the number of holes, pores, or cracks within and on rocks. Some pores are on the surface of rocks and are microscopic; however, many pores are small and sometimes large enough to be visible without the aid of a magnifying glass. All rocks are porous to some extent including granite, marble, sandstone, and limestone.
The most porous rock is pumice, which is a resultant of erupting volcanoes. Pumice is formed from pyroclastic igneous rock, called lava. The foaming action of igneous rock flows or lava creates gas bubbles within the rock. As the lava cools the gas bubbles leave holes, pores, and cracks. An interesting factor about pumice rocks is that they can float and are often found floating on the ocean’s surface after an island volcano erupts.
A rock absorbs water or other liquids that seep in through pores and cracks in the rock to fill accessible holes. When all the accessible holes or pores fill with water the rock, the rock has become saturated. The amount of holes, pores, and cracks determine the percent of porosity of rocks. Another factor of the porosity of rocks is called permeability. This factor is the ability of rocks to absorb, hold, and allow a liquid to pass in and through.
Porosity of Rocks
Percents are based on the U.S. Department of Energy’s Information Library (Science, Society, and America’s Nuclear Waste).
- Granite: 1 percent
- Marble: 2 percent
- Shale: 10 percent
- Limestone: 15 percent
- Sandstone: 25 percent
- Pumice: 50+ percent
Rocks Experiment
This is investigation is to determine which rocks absorb water and how much they absorb. Students use basic and integrated science process skills in the completion of this investigation.
Materials
- Large clear plastic container
- 500 mL beaker
- Brick
- Sandstone rock
- Limestone rock
- Pumice rock
- Water
- Marker
- Scale
Procedures
Part 1
Students:
- Pour a measured amount of water into the plastic container, making sure there is enough water to cover the brick.
- Record the amount of water placed in the plastic container.
- Record observations of the brick prior to placing it in the water.
- Measure the mass of the brick.
- Place the brick in the center of the water-filled plastic container.
- Mark on the side of the container the height of the water after placing brick in the water.
- Leave the brick in the container for 45 minutes.
- Remove the brick from the container.
- Ensure that all excess water is drained from the brick back into the container.
- Mark the side of the container after removing the brick.
- Measure the mass of the brick.
- Record your observations of the brick and the water in the container.
- Pour the excess water back into the measuring cup to determine how much water the brick absorbed.
- Subtract the remaining amount of water from the amount of water poured into the container at the beginning.
- The answer provides the amount of water absorbed by the brick.
Part 2
Repeat the steps in Part 1 using a sandstone rock, a pumice rock, and a limestone rock
Part 3
- Compare the decrease in volume of water for each rock and brick to determine which absorbed the most water. This is determined by calculating the percentage difference of water volume change in the plastic container.
- Write a report about your observations, data, findings, and conclusions; writing in science provides guidance.
Student Questions
- Which rock gained the greatest percentage increase in water absorbed?
- Which rock gained the greatest percentage increase in mass?
- Is there a correlation between percentage increase if water absorbed and mass?
- Which rock is the most porous?
- Which rock is more permeable?
Questions to ask students in science, along with additional questions developed based on students’ responses will guide the discussion of the investigation.
Making Connections with Properties of Rocks
As students discover the porosity properties of rocks, they also learn to consider porosity when describing the physical properties of rocks and other objects. They also make another connection with the concept of permeability, because with rocks it defines absorption ability. Typically permeability is used to define a filtering membrane’s ability to allow liquids to pass through and not solids.
David R. Wetzel - Dr. David Wetzel's experience includes more than 25 years in continuing, adult, and teacher education.
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