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Student Guide: How Particles Affect Porosity

Downloadable and printable Student Guide PDF.
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Summary

Have you ever noticed that rocks are not completely solid? They have tiny pockets of air inside them. Just how much air is inside of a rock is called its porosity. In this activity using beans, plastic cups, and a measuring cup, you will model how the size of the particles in a rock affect its porosity.

Useful Vocabulary

  • Porosity: How much air space is inside an object compared to the total amount of space the object takes up.
  • Particles: Small pieces of matter.
  • Rocks: Hard materials that are made up of tiny particles packed together. In between the particles are spaces filled with air, other types of gases, or liquid. The porosity of a rock (how much space is between its particles) can be used to characterize the rock and identify what type it is.

Materials

To do this activity, you will need:

  • 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)
  • Water (at least 400 mL).

Directions

  1. Look at the two cups filled with peas, beans, or lentils. How do the sizes and shapes of the particles in each cup compare with each other? Which cup do you think has more air pockets based on the type of particles in it?
  2. Carefully fill the metric measuring cup with water to the 200 milliliter (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 exactly with the 200 mL mark. (Note: If you are using cups larger than 9 ounces [oz.], you may need to fill the measuring cup with more than 200 mL. Ask your teacher for guidance.)
200 mL of water in a measuring cup
Figure 1. Measure out 200 mL of water in the metric measuring cup.

Measuring cup filled with 200 mL of water showing the meniscus
Figure 2. 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. Slowly and carefully pour water in a small, steady stream from the metric measuring cup into one of the cups with the peas, beans, or lentils. Any spilled water that lands outside the cup can cause an error in your measurement. Pour water into the cup until you fill it to the cup's rim (at the same level as the peas, beans, or lentils at the top), but do not let the water overflow from the cup.

Water being pored into a cup of split peas
Figure 3. Slowly and carefully pour water into the cup.

Cup of split peas filled with water
Figure 4. Fill the cup until the water reaches the very top of the cup's rim (to where the peas, beans, or lentils are at the top), but make sure water does not overflow from the cup.
  1. How much water is left in the measuring cup? Try to be as accurate you can.
  2. Calculate the amount of empty space from all the air pockets combined that was in the cup with the peas, beans, or lentils. Do this by subtracting the amount of water left in the measuring cup from the amount that was originally in the measuring cup (200 mL). For example, if you were left with 85 mL of water after pouring water into the cup, the empty space that was in the cup would have been equal to 115 mL (because 200 mL - 85 mL equals 115 mL). How much empty space was in the cup with the peas, beans, or lentils?
  3. Repeat these steps with the other cup of peas, beans, or lentils. Make sure to refill the metric measuring cup with water exactly to the 200 mL mark, and again add the water slowly to the second cup. How much water is left in the measuring cup this time? How much empty space did the second cup with the peas, beans, or lentils have?
  4. Which cup of peas, beans, or lentils held more water? Which cup had the most amount of empty space? Does this reveal any relationship between the types of particles in the cup and the cup's porosity? Can you relate your findings to how the size and shape of particles in a rock affect its porosity?