Nancy J. Schmitt
There are two basic ways that the level of a contaminant may be brought under the threshold stipulated by the EPA. One is to increase the amount of water, essentially diluting the solution. The other way is to eliminate the amount of the contaminant.
The following situation problems are about solutions. In order for students to be successful in solving these problems they need to be comfortable with scientific notation, since the amounts being discussed are very small and very large. The other skills that the students need to be proficient with are using percentages. To check these skills a few solution problems are presented to the students.
Solution Problems:
1.
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If you have one liter solution that is 20% saline and you want to have a solution that is 10% saline, how much water must you add?
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2.
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If you have one liter of a solution that is 20% saline and one liter of another solution that is 10% saline and you mix them together, what is the percent of saline in the resulting solution?
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3.
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If you have one liter of 20% saline solution, how much salt must be added to make the solution 30%?
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4.
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If you have two liters of 15% saline solution and you wish to create a solution that is 25%, how much 40% solution do you need to add?
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Real -Life Solution Problem:
Just as solutions are mixed together, in the real world contaminant levels may increase or decrease as clean or contaminated water is added to a pond from rain water or a flowing brook or river. During the summer, we have many "no swimming" alerts as certain contaminant levels rise above the maximum levels. This happens when there has been a heavy rain fall and sewage treatment area become overwhelmed and some untreated sewage ends up in the rivers and brooks. In addition heavy rains allow many items on the ground to be swept along and into the rivers and brooks as runoff. Many times, within a day or so, the swimming ban is lifted. This is due to the fact that the contamination levels have decreased because clean water has been introduced. Most times the water testers are not concerned about how long it will take for the water to be safe, but the beach goers and vacationers would like to know in order to make appropriate plans. You want to help the vacationers out and plan to use some of your knowledge of solutions to estimate how much clean water must get to the beach to make it safe.
For this lesson, you may wish to choose a local swimming area and have the students research how big the water area is and how deep, to estimate how much water is in it. If the area is being fed by a brook or river, an estimate can be made of how much additional water is entering this area. For the sake of the problem, this could be considered all clean water. Remember as clean water comes in, dirty water goes out.
Below is a table of estimated data that could be used before the research project. This information has to do with Lake Zoar in Monroe, Connecticut. This lake was created by a hydroelectric dam that was built on the Housatonic River. It is one area that bald eagles come to feed on fish during the winter. Beaches are frequently shut down for bacteria counts after heavy rains. In addition, the river bed is contaminated with PCBs from a factory upriver in Massachusetts.
Lake Zoar, Monroe CT
1.
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How much water must be replaced?
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2.
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How much water is moving in replacing dirty water every second?
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3.
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How much water is replacing dirty water every hour?
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4.
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How long will it take before the level of bacteria is back to normal?
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Express your water amounts in gallons and time in hours.. Remember that one cubic foot of water contains 7.48 gallons.