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? ? ? ? ? ? ? ? I am going conduct an experiment to find the porosity of different rocks. The porosity of a rock is how much water it can hold, or the volume of air gaps that it has. To do this experiment I will soak different rocks in water and measure the differences.
The equation for the porosity, expressed as a fraction is X 100 VOLUME OF PORE SPACE TOTAL VOLUME OF ROCK? ? ? ? ? ? ? ? There are quite a lot of factors affecting the porosity of rocks. They include the following: Grain size If the grains are large then there is more likely to be bigger gaps in which water can get into. Sorting If the grains are not well sorted then the smaller grains fill in the gaps that the big ones have left. If the rock is well sorted then there is more likely to be larger gaps. Grain shape There are lots of different shapes that the grains can be.
If the are irregular, then they will tend to interlock and there will be less gaps, but if they are rounded then there will be more gaps. Degree of compaction If the rock was under a lot of pressure when it was formed then it is more likely to have all the gaps squashed out. If it was formed under high pressure then it may be impermeable, in which case this experiment will have no effect on it. Amount and type of cement. Usually the cement is impermeable so if there is a lot of cement then the will be much less space for the water to go. Method? ? ? ? ? ? ? ?
The method that I am going to use is to soak the selected rock in water for at least 24 hours to make absolutely sure that all of the water had gone into the rock. Then I must take it out and wipe off the surface water to make sure that that does not give me a false weight. Then I will weigh it to an appropriate accuracy of 0. 1 g. Then to work out how much water had been gained I will need to dry it in an oven making sure that there is a hole to let the moisture out of. A practical time for leaving this in the oven is for 24 hours.
Then I must weigh the dry rock. After this I will fill a displacement can with water just up to the funnel hole and put the rock into it, and then measure the water that has been displaced. ? ? ? ? ? ? ? ? There are some problems that I will probably encounter. When I soak the rock, if the rock only has bubbles in an impermeable rock then if these bubbles do not link up I will not be able to get water into the spaces.
Also on rock like that it will be hard to just wipe off the surface water without taking water that has actually been soaked up. Also when I soak the rocks it will be very hard to get out air bubbles, stopping some of the water getting in, out. When I dry the rock it will be very hard to get rid of all of the water and to make sure that none of it falls back onto the rock as condensation. ? ? ? ? ? ? ? ? The equipment provided is below: Beaker of water Electronic balance Measuring cylinder Displacement can Oven Gloves Safety goggles The calculation that I have to do is below W 1 = Weight of saturated rock W 2 = Weight of dry rock This means that W 1? W 2 gives the weight of water taken in by the rock. This gives the volume of water as 1 g of water is 1 cm 3.
Let V = the total volume of the rock. W 1 -W 2? ? ? V POROSITY = ? ? ? ? ? ? ? ? ? ? ? ? x 100 The rocks that I will be using are Red Sandstone, Chalk and Vesicular Basalt. The vesicular basalt is a rock that is impermeable but has large holes from bubbles left in it. The sandstone and chalk are both rocks that should easily soak up the water. ? ? ? ? ? ? ? ?
After I have done the experiment I could look at the individual grains of rock under a microscope to see the different shapes and sorting and other variables that I mentioned earlier. ? ? ? ? ? ? ? ? For professional geologists, it is very important to know what the porosity of rocks is. The information can be used to find where gas or oil supplies may be as they will rise through the pores in the rock. Also it is important to know the porosity of the rock when building a reservoir, or when dumping nuclear or toxic waste to make sue it doesn?
t leak into any water supplies.
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