Lab Activity: Pore Space in Soils
Objective: Measure and compare the porosity of sand and gravel.
Introduction: Whether soil is composed of coarse pieces of rock or very fine
particles, there is always some empty space between the pieces of solid material.
This empty space is called pore space. Porosity is calculated by dividing the
volume of the pore space by the total volume of the soil sample. Thus, if 50 cm3
of soil contains 5.0 cm3 of pore space, then the porosity of the soil sample is 5.0
cm3/50cm3 = 0.10 x 100 = 10%. The result is generally written as a percentage.
Prelab preparation
1. What is pore space?
2. How is pore space calculated?
3. Which rock particle has the greatest sediment size—sand or gravel?
Materials
Beaker (100 mL)
(100 mL)
sand
gravel
Graduated cylinder
Procedure
1. Fill a beaker to the top rim with water. Transfer this water to a graduated
cylinder to determine the volume of the beaker. Record the volume
2. Dry the beaker and fill it to the top with large gravel. Gently tap the
beaker to settle and compact the gravel. Add more gravel as needed to fill
the beaker until the gravel is level with the top.
3. Fill the graduated cylinder with 100 mL of water. Carefully pour the water
from the cylinder into the beaker with the large gravel until the water
level just reaches the top of the gravel.
4. Calculate the porosity of the gravel. Record the porosity as a decimal and
as a percentage.
5. Repeat Steps 2-4 using the sand and record your data.
6. Drain and dry both the sand and gravel. Mix together equal volumes of
sand and gravel. Repeat Steps 2-4 using the mixture.
1
Table 1: Pore Space Volume of Gravel, Sand, and a Mixture of Sand and Gravel
Gravel
Sand
Mixture
Volume of beaker
(mL)
Volume of rock
particles (mL)
Volume of pore
space (mL)
Analysis and Conclusion
1. Compare the gravel porosity with the sand porosity.
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2. Explain how porosity depends on particle size.
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3. What effect did mixing the materials have on the porosity? Explain this
effect.
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Extensions
1. Each of the three graphs in Figure 14.1 represents porosity. Particle size is
plotted on the vertical axis, increasing from bottom to top. Porosity is
plotted on the horizontal axis, increasing from left to right. Which of the
graphs below best represents what you discovered in the lab? Explain
your answer.
A
Particle size
Large
Particle size
Large
C
Particle size
Large
B
Small
Small
Small
2
C
3