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Porosity and Particle Size

Difficulty
Time Required Very Short (≤ 1 day)
Prerequisites None
Material Availability Readily available
Cost Very Low (under $20)
Safety No issues

Abstract

Often, when we think of something that is solid we think about rocks. But in reality, rocks have tiny holes of air inside them. This is called porosity. In this science project you can find out what it means to be "solid as a rock!"

Objective

Test if the porosity of a rock matrix is affected by particle size.

Credits

Sara Agee, Ph.D., Science Buddies

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Last edit date: 2013-01-10

Introduction

Did you know that rocks are not completely solid? Rocks have tiny pockets of air inside them. This is obvious when you look at a piece of volcanic rock, but it happens in dense rocks like granite, too. It is just that the pockets of air are very small.

You will notice that if you pick up same-sized pieces of volcanic rock and granite, that they do not weigh the same. The granite feels heavier than the volcanic rock. The holes of air in the volcanic rock make it feel lighter because it is more porous and less dense. The granite is less porous and more dense. The porosity of a rock can be used to characterize the rock and identify what type of rock it is.

Rocks are made up of tiny particles, specifically minerals that can form different crystal shapes. These particles are packed together. In between the particles are spaces that are filled with air, gas, or liquid. The shape and size of the particles affect the way that they pack together in a certain amount of space, which affects a rock's porosity. Porosity refers to the ratio of the volume the air 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 can, which means that packing larger particles together leaves more room for air and gas to fill in between the particles, making the rock more porous.

But rocks change over time. When enough pressure, or force, is put on a rock, the pressure can make the rock pack its particles closer together. Usually pressure builds up on a rock over a long period of time, as soil and other rocks end up on top of it. You may be able to imagine how this process, known as compaction, makes the rock's porosity decrease with time.

In this geology science project, you will investigate the effect of particle size on porosity by making a model to test your hypothesis. Which particles will leave the most space and make a more porous matrix, small particles or large particles?

Terms and Concepts

  • Rocks
  • Density
  • Particles
  • Minerals
  • Crystals
  • Porosity

Questions

  • How is porosity related to particle size?
  • How much space is left between particles of rock or soil?
  • How can porosity be measured?

Bibliography

Materials and Equipment

  • Clear plastic cups (4 to 6)
  • Water
  • Permanent marker
  • Metric measuring cup
  • Differently-sized rock particles (4 to 6 groups). Use the same type of rock, such as granite.

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Experimental Procedure

  1. First, you will need to find your rock particles. You will want 4-6 differently sized samples of rock. You can often buy these from a landscape or construction supply store. They sell crushed granite in different sizes. Get some large-, medium-, small-, and fine-grade samples.
  2. Fill each clear plastic cup up to the top with each of your samples. Label each cup with the size of the particles in the sample.
  3. Fill your metric measuring cup with water to the 100 milliliter (mL) mark.
  4. Pour water into one of the cups of rocks until it is full to the rim of the cup, at the same level as the top of the rocks. Pour slowly and gently so you do not spill water out of the measuring cup, as this will cause error in your measurement.
  5. Record the amount of water left in the measuring cup. It should be less than 100 mL. Write your result in your lab notebook in a data table like Table 1 below. Your result is the "Water After." The "Water Before" is 100 mL because this is the amount of water you started with.

    Grain Size Water Before (mL) Water After (mL) Empty Space (mL)
    Large      
    Medium      
    Small      
    Fine      

    Table 1. Record your results in your lab notebook in a data table like this one.

  6. Now calculate the amount of empty space in the cup of rocks. To do this, subtract the "Water After" from the "Water Before" that you wrote in your data table. Write the amount of empty space you calculated under "Empty Space" in your data table.
    1. For example, if after pouring water into your cup you were left with 60 mL of water, then you would subtract 60 mL from 100 mL to get 40 mL. This means that there is a volume of 40 mL of empty space between the particles in your cup.
  7. Repeat steps 3-6 for each cup of rocks.
  8. Make a graph of your data to compare your results.
    1. You can make a graph by hand or use a website like Create a Graph to make a graph on the computer and print it.
    2. Put the size of the rocks on the x-axis (the horizontal axis going across the bottom) and the amount of empty space on the y-axis (the vertical axis going up and down).
  9. Which particle size had the most amount of empty space? The least amount of space? Was there a pattern to your data? Does this reveal any relationship between particles size and pore space?

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Variations

  • Here you used differently sized pieces of the same material, crushed granite. What about other materials and minerals? Try comparing different types of crushed rock like volcanic rock, gravel, quartz, sandstone, or limestone.
  • Soil is a mixture of rocks, minerals and organic matter. Porosity is also a property of soil. Try the same experiment using different types of soil: clay, loam, sandy, silty, potting soil, compost, etc. The only thing to do differently is place 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?
  • You can calculate the porosity of each of the cups of differently-sized rocks you used in this experiment. To do this, divide the amount of empty space each cup of rocks had by the total volume the cup can hold (without rocks in it). (To find out the total volume the cup can hold, fill the empty cup completely full with water and then pour the water into a measuring cup and read the water level.) For example, if the empty space took up 50 mL and the cup could hold 100 mL total, the porosity would be 50%. What is the porosity of the cups of the differently-sized rocks?
  • Porosity can also be linked to the soil type, amount of compaction and dryness of the soil. Try these other Science Buddies experiments too:

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