Abstract

Objective

In this experiment you will test if the porosity of a rock matrix is affected by particle size.

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's 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.

Imagine that a rock is made up of tiny particles of minerals and crystals that are packed together. In between the particles there will be extra space which is filled with air and gas. The size of the particles will affect the way that they pack together in a certain amount of space. Larger particles will not pack together as well as small particles, leaving more room for air and gas between the particles and making the rock more porous.

In this experiment, 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, Concepts, and Questions to Start Background Research

To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!

• rocks
• minerals
• crystals
• porosity
• particles
• density

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

Bibliography

• SEED, 2006. "Porosity," Schlumberger Excellence in Educational Development (SEED) program, Schlumberger Ltd. [accessed: 4/26/06]
  http://www.seed.slb.com/en/scictr/lab/porosity/index.htm
• Wikipedia contributors, "Porosity," Wikipedia, The Free Encyclopedia. [accessed: 4/26/06]
  http://en.wikipedia.org/w/index.php?title=Porosity&oldid=49469765
• Brynie, F.H., 2005. Parent's Crash Course: Elementary School Science Fair Projects, Hoboken, NJ: Wiley Publishing Inc. pp 165-168.

Materials and Equipment

• clear plastic cups
• water
• permanent marker
• measuring cup
• differently-sized rock particles

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 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 mL mark.
4. Pour water into the first sample until it is full to the rim of the cup. 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 a data table:

   Grain Size	Water Before (mL)	Water After (mL)	Volume Displaced (mL)
   Large
   Medium
   Small
   Fine

6. Now calculate the amount of empty space in the sample by subtracting the amount of water you measured from 100 mL. 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 space between the particles in your cup.
7. Repeat steps 2-6 for each sample.
8. Make a graph of your data to compare your results.
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?

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?
• Porosity can also be linked to the soil type, amount of compaction and dryness of the soil. Try these other Science Buddies experiments too:
  • Soil Color and Moisture
  • Measuring Soil Compaction

• For more science project ideas in this area of science, see Geology Project Ideas.

Credits

Sara Agee, Ph.D., Science Buddies

Last edit date: 2006-08-15 17:10:45