|For this activity, you 'll need sand, gravel, a syringe, a small cup, water,|
and a towel. Frannie predicts that gravel is more porous than sand.
Porosity, in hydrogeology, is the capacity of rock, sand, soil, or other sediments to hold water. We can measure it through the ratio of the volume of empty space in a particular sentiment to the total volume of the sediment.
We’re going to test the porosity of gravel and sand to discover which one is more porous.
First, we’re going to find out the volume of an empty plastic cup by filling up our syringe to the 35 cubic centimeter (cc) line. Dispense the water from the syringe into the cup until it’s full to the rim. Subtract the amount of water left in the syringe from the starting amount of 35cc and record this value. Frannie found that her cup holds 30 cc.
Empty and dry the cup and then fill it with dry gravel. Fill the syringe again with 35 cc and dispense that water into the cup with gravel. Subtract the amount left in the syringe from the starting amount of 35cc and record this value. Frannie found that the cup with gravel holds 17 cc of water.
Empty and dry your cup before filling it with sand. Next fill the syringe with 35 ccs and dispense that water into the cup. Subtract the amount left in the syringe from the starting amount of 35cc and record this value. Frannie’s cup with sand holds 12 cc of water.
Fannie discovered that the gravel is able to hold more water and is more porous than sand.
To find the porosity of each material, we first have to determine the volume of material in each cup. The volume of the sand or gravel in the cup will be equal to the volume of the water the cup can hold when full. So the volume of our gravel and sand is 30 cc.
Remember that porosity is just the ratio of empty space over sediment so our porosity for gravel is p=17 cc/35 cc = .48 = 48%
Our experimentally determined porosity for sand is p=12 cc/35 cc = .34 = 34%
|Frannie was right! Gravel is more|
porous than sand!
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