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Facilitator/Educator Guide: How Particles Affect Porosity

When we think of something hard and solid, we may often think of rocks. But in reality, rocks have tiny pockets of air inside them that, taken together, are called porosity. In this activity using beans, plastic cups, and a measuring cup, you will help students model how the size of the particles making up a rock affect that rock's porosity.

Activity's uses: Demonstration or small group exploration
Area(s) of science: Earth & Environmental Science, Engineering
Difficulty level:
Prep time: < 10 minutes
Activity time: < 10 minutes
Key terms: Geology, porosity, rocks
Downloads and Links: Facilitator / Educator Guide PDF.
Student Guide web page or PDF.

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Background Information

When you look closely at volcanic rocks, tiny pockets of air are obvious inside them. But these air pockets exist in dense rocks like granite, too, although the pockets are very small. If you pick up same-sized pieces of volcanic rock and granite, you will notice they do not weigh the same. The granite feels heavier than the volcanic rock. The holes of air in the volcanic rock make it lighter because it is more porous and less dense. The granite is less porous and more dense. Taken together, these air pockets in a rock define its porosity, which can be used to characterize the rock and identify what type it is.

Rocks are made up of tiny particles, specifically minerals that can form different crystal shapes. Although these particles are packed together, in between them are spaces filled with air, other types of gases, or liquid. The shape and size of the particles affect how the particles can pack together, including how tightly together, which affects a rock's porosity. Porosity refers to the ratio of the volume the spaces take up in the rock to the total volume of the rock. In general, larger particles cannot pack together as well as smaller particles, which means that packing larger particles together leaves more space for air between the particles. You can imagine this if you have one cup of marbles and another cup of sand. You will see many more large spaces between the marbles than between the grains of sand.

In this science activity, students will investigate the effect of particle size and shape on porosity by making a model using beans, plastic cups, and a measuring cup. Which particles will leave the most space and make a more porous rock model?

For Discussion

This science activity can serve as a starting point for a variety of science and geology discussions. Here are a few examples of questions that can be used to start a discussion:

  • How do you think porosity is related to particle size?
  • Do you think particle shape affects porosity?
  • Can you think of types of rocks that are more porous than other types of rocks?
  • How can porosity be measured?

Materials

Needed for preparing ahead:

  • Clear plastic cups (2 for each demo or small group). They should each be at least 9 oz. in size.
  • Dry garbanzo beans or black-eyed peas (enough to completely fill a plastic cup for each demo or small group)
  • Dry split peas, green or yellow, or lentils (enough to completely fill a plastic cup for each demo or small group)
  • Alternatively, instead of beans and/or peas, you could use two very differently sized round objects, such as large and small marbles, or differently sized rocks that are the same type of rock. The objects should not float or dissolve in water. (You will need enough of each size of object to completely fill a plastic cup.)

Needed for each demo or small group at the time of the science activity:

  • Clear plastic cup filled with dry garbanzo beans or black-eyed peas (1)
  • Clear plastic cup filled with dry split peas or lentils (1)
  • Metric measuring cup (1). The more metric gradation markings it has, the easier it will be to use.
  • Water (at least 400 mL). You may need to use more water if you use cups larger than 9 oz. in size.
Photograph of rocks materials for activity
Figure 1. You need only a few simple household materials to do this fun science activity.

What to Do

Prepare Ahead (< 10 minutes)

  1. For each demo or small group, fill one of the clear plastic cups to the top with the dry garbanzo beans or black-eyed peas. It is better to overfill it a little than to underfill it.
  2. For each demo or small group, fill a second clear plastic cup to the top with dry split peas or lentils.
Split peas in a plastic cup
Figure 2. When filling the cup with beans or peas, fill it all the way to the top. It is better to slightly overfill the cup than to underfill it.

Science Activity (< 10 minutes)

  1. Each classroom demo or small group should have one clear plastic cup filled with dry garbanzo beans or black-eyed peas, one clear plastic cup filled with dry split peas or lentils, one metric measuring cup, and water (at least 400 milliliters [mL] is needed).
  2. Explain to students that one cup is filled with smaller, flatter particles (the split peas or lentils) while the other cup is filled with larger, rounder particles (the garbanzo beans or black-eyed peas). You can ask the students to think about which cup is more porous, or has more air pockets, based on the types of particles in it.
  3. Help students fill the metric measuring cup with water to the 200 mL mark. Make sure the water level is exactly at 200 mL by looking straight-on at the water level and seeing that the bottom of the meniscus (the curved surface of the water) lines up with the 200 mL mark.
  4. Note: If you are using cups larger than 9 ounces (oz.), students may need to fill the measuring cup with more than 200 mL of water. Try to scale accordingly. For example, if you are using 16 oz. cups instead, have students fill the measuring cup with 350 mL of water.
200 mL of water in a measuring cup
Figure 3. Have students measure out 200 mL in the metric measuring cup.

Measuring cup filled with 200 mL of water showing the meniscus
Figure 4. Help students measure 200 mL of water exactly by making sure the bottom of the meniscus (where the black arrow is pointing) lines up exactly with the 200 mL mark.
  1. Have students slowly and carefully pour water from the metric measuring cup into one of the cups with the peas, beans, or lentils. Tell them to pour a small, continuous stream, and warn them that any spilled water can cause an error in their measurement. They should pour water into the cup until the water reaches the top of the cup's rim (at the same level as the peas, beans, or lentils at the top), but they should not let the water overflow from the cup.
Water being pored into a cup of split peas
Figure 5. Ask students to slowly and carefully pour water into the cup.

Cup of split peas filled with water
Figure 6. Students should fill the cup until the water reaches the very top of the cup's rim (shown here with the peas at the top), but does not overflow from the cup.
  1. Ask students to observe how much water is left in the measuring cup. Instruct them to be as accurate as they can in their measurement.
  2. Then have students calculate the amount of empty space from all the air pockets combined that were in the cup by subtracting the water left in the measuring cup from the amount that was originally in the measuring cup (200 mL). For example, if students were left with 85 mL of water in the measuring cup after pouring water into the cup, the empty space that was in the cup would have been equivalent to 115 mL (because 200 mL - 85 mL equals 115 mL). If you want, ask students to write their results down on a piece of paper.
  3. Have students repeat this process with the other cup. Make sure they fill the metric measuring cup with water exactly to the 200 mL mark, and again slowly add it to the second cup. Ask students to observe as accurately as possible how much water is left in the measuring cup this time, and again calculate the amount of empty space that was in the cup in terms of milliliters. You may ask them to write their results down.
  4. Ask students which cup of particles has more empty space (is more porous), based on their results, and why they think they got the results that they did.

Expected Results

The cup with the larger, rounder particles (the garbanzo beans or black-eyed peas) should have more air spaces than the cup with the smaller, flatter particles (the split peas or lentils). This means that a greater amount of water should have been able to fit in the cup filled with the larger, rounder particles compared to the cup with the smaller, flatter particles. For example, if you filled one 9 oz. cup with black-eyed peas and a second cup with lentils, the empty space of the former cup may have held about 115 mL of water while the empty space in the latter only held about 85 mL of water. This activity uses a scale model to represent porosity in rocks. In reality, the individual particles in rocks are much smaller than the beans, peas, or lentils you used in this experiment. However, because larger particles in general cannot pack together as tightly as smaller particles, a rock made of larger particles will usually be more porous than a rock made of smaller particles.

For Further Exploration

This science activity can be expanded or modified in a number of ways. Here are two options:

  1. Have students calculate the porosity of each of the cups of different particles in the activity. To do this, divide the volume of water the cup could hold with beans in it (which corresponds to the total volume of empty space between the particles in a cup without water) by the total volume of water the cup could hold without beans in it (which is the same as the total volume of the "rock"). For example, an empty 9 oz. cup can hold 266 mL of water. If the cup held 100 mL of water when full of beans, it has a porosity of 100/266, which equals 0.376, or 37.6%.
  2. Soil is a mixture of rocks, minerals, and organic matter. Porosity is also a property of soil. Try the same activity using different types of soil: clay, loam, sand, silt, potting soil, compost, etc., but put a screen on top of the cup to keep organic matter from floating out as you pour the water into the cup. Do different types of soils have different porosities?

Credits

Teisha Rowland, PhD, Science Buddies
Sponsored by a generous grant from Chevron