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  Studying Conductivity
 

Student Name(s): ______________________________________________

Date: ________________

Conductivity is a measure of water's ability to conduct electrical current. Measurements of conductivity provide a general indication of water quality. The geology of a lake's watershed establishes the normal ranges for conductivity in a lake. Some pollution discharges and polluted runoff into lakes can cause changes in conductivity, especially if the pollutants include inorganic dissolved solids such as ions: bicarbonate, sulfate, chloride, calcium, magnesium, sodium, potassium, and phosphate.

In this lesson, you will explore the conductivity of various samples in the laboratory, and you will use WOW data to investigate conductivity in a lake. All WOW conductivity data are temperature compensated to 25°C (usually called specific EC). We do this because the ability of the water to conduct a current is very temperature dependent. We reference all EC readings to 25°C to eliminate temperature differences associated with seasons and depth. Therefore EC 25°C data reflect the dissolved ion content of the water (also routinely called the TDS or total dissolved salt concentration).

Part I - Conductivity in the Laboratory

Knowledge Base
Review your knowledge of materials that are good conductors and poor conductors of electricity. How does conductivity relate to lakes? How might material in lakes affect conductivity?

Experimental Design
You need the following equipment to complete the lab:

  • A hot water bath for samples
  • Conductivity pen (or other conductivity meter)-determine if it is temperature compensated (see RUSS parameters).
  • Five 50 ml beakers or small clear glasses
  • A squirt bottle filled with distilled water
  • A thermometer or temperature probe

Based on your knowledge of conductivity, rank predictions for the different samples of water in Table 1 (1 = best conductor [highest conductivity] to 13 = poorest conductor). Record explanations for your rankings in your lab journal.

As you complete this experiment, it is important that you rinse the sample beaker with distilled water between each test.

Data Collection
Enter your measurements for conductivity into Table 1. Then select four samples that you want to test at warmer temperatures. After testing those samples at room temperature, the samples should be placed in a hot water bath until the samples have increased in temperature by 10 degrees. (Rinse the thermometer each time it is used to check the sample temperature.)

Table 1: Conductivity in Lab Water Samples

Sample

Temperature of Sample

Predicted Conductivity Ranking

Measured Conductivity

Conductivity when sample's heated by 10 degrees (test only 4)

Distilled water

       

Tap water

       

Lake water

       

Tap water with baking soda

       

Tap water with table salt

       

Tap water with granite chips

       

Tap water with nitrate rich fertilizer

       

Tap water with clay

       

Tap water with organic material (peat)

       

Tap water with soil

       

Tap water with phosphate fertilizer

       

Tap water with oil

       

Tap water with sand

       

Data Management and Analysis
1. What variables might affect the differences in your observations for each sample?
2. How does the temperature of the sample affect its conductivity?

Interpretation of Results
3. What might be inferred about conductivity readings in lakes?

Reporting Results
Turn your answers in to your teacher when you have completed both parts of this lesson.

Part II - Conductivity in Lakes

Knowledge Base
Use Table 2 for this portion of the lesson. You will be collecting data from a lake during the summer. Reflect on what you know about conductivity, including your hands-on experiences in part 1 of this lesson. Then begin by filling in your predictions for ranking conductivity levels within the lake depths.

Experimental Design
Decide which of the WOW lakes you will investigate. Select a day from within the summer season to collect your conductivity data.

Data Collection
Access the conductivity data from the WOW Web site and record it in Table 2.

Table 2: Conductivity in a Lake

Lake Name:

Date data was collected:

Time data was collected:

Depth (in meters)

Predicted Ranking

Conductivity Measurements

Actual Ranking

1

     

2

     

3

     

4

     

5

     

6

     

7

     

8

     

9

     

10

     

11

     

12

     

13

     

14

     

15

     

Data Management and Analysis
Create a line graph of your data on conductivity (either create the graph by hand or use a spreadsheet program like Excel).

Review conductivity data for the lake on other dates.

Interpretation of Results

  1. What might cause the conductivity readings to vary within different layers of the lake?
  2. What can you infer about the relationships between time of day or time of year and conductivity values?
  3. What types of lake pollutants might be suspected based on conductivity measurements?

Data Presentation
Turn your results in to your teacher.

 


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date last updated: Wednesday October 06 2004