salinity of ocean water

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Earth Science with Vernier 15 - 1

Salinity of Ocean Water

If you were to view the planet Earth from space, you would see that most of its surface is coveredby water. Most of this is ocean water which cannot be consumed. Why cant ocean water be

consumed? The reason is that ocean water contains large amounts of salt, which make itundrinkable.

Salinity is a measurement of the saltiness or concentration of salt in water. Ocean water containsmany different salts, but the most abundant is sodium chloride, also known as table salt. Sodiumchloride makes up 86% of all the ions present in ocean water. Other salts that can be found inocean water at significant levels are calcium chloride and magnesium chloride.

Why is the ocean salty? When the earth was being formed, volcanoes ejected large amounts oflava and chemicals (including salts) into the oceans and atmosphere. Some of these saltsdissolved in the water. Rain also washes salts from the land into the ocean. Over time the level ofdissolved salts in the oceans increased to the level it is today.

While the average salinity of ocean water is 35 ppt. there are several factors that can increase ordecrease its salinity. At the polar regions, freezing of ocean water increases the salinity of thesurrounding water. Evaporation in hot arid regions also increases the salinity. The Dead Sea,located in Israel, has a salinity nearly seven times that of most ocean water. At the oceanssurface, rain, snow, and melting ice are all responsible for decreasing the salinity. As rivers enterthe ocean, they carry large volumes of fresh water into the ocean, causing the salinity to decrease.

In this experiment, you will use a Conductivity Probe to measure the salinity of salt water. In PartI, you will measure the change in ocean salinity due to evaporation. In Part II, you will measurethe change in salinity near the mouth of a river as it flows into an ocean.

OBJECTIVES

In this experiment, you will

Measure salinity of water sample using Conductivity Probe. Determine the effect of evaporation on the salinity of ocean water. Determine the salinity change when a river flows into an ocean. Calculate salinity changes.

MATERIALS

computer text book (5 to 10 cm thick)Vernier computer interface masking tape

LoggerPro plastic tubingConductivity Probe plastic syringe2 aluminum pans wash bottle with distilled waterlarge plastic tub lamp with a 60 W (or greater) bulbmedium plastic tub 400 mL beakerring stand 1.5 L saltwatertest tube clamp 1 L distilled water


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PRE-LAB QUESTIONS

1. In Part I of this experiment, you will shine a light on an aluminum pan filled with saltwater.Another pan filled with saltwater will be kept out of the light. What do you predict willhappen to the salinity in each pan?

Figure 1

2. In Part II of this experiment, you will setup two tubs of water. The freshwater from thesmaller tub will slowly flow into the larger tub with saltwater. What do you predict willhappen to the salinity in the larger tub?

Figure 2

PROCEDURE

1. Connect the Conductivity Probe to the Vernier computer interface. The switch on theConductivity Probe should be on the 020000 S/cm setting.

2. Prepare the computer to collect data by opening the file 15 Salinity of Ocean from the

Earth Science with Vernierfolder.

3. Obtain 500 mL of the saltwater provided by your teacher.


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4. Measure the initial salinity of your saltwater.

a. Rinse the probe with distilled water and gently blot it dry with atissue.

b. Lower the probe into the saltwater so that the hole in the probeend is completely submerged as shown in Figure 3. Swirl the

solution briefly.c. Once the reading has stabilized, record the salinity in your datatable. The units are parts per thousand, ppt.

d. Rinse the probe with distilled water and gently blot it dry with atissue.

Part I Sunlight

Day 1

5. Obtain two aluminum pans and a desk lamp.

6. Set up the lamp and place one of the aluminum pans in front as shown in Figure 1.

7. Pour 250 mL of saltwater into the aluminum pan.

8. Turn on the lamp and position the bulb so that it is pointing down towards the pan. The lampbulb should be positioned so that it is 10 cm from the water in the pan.

9. Place the second aluminum pan a few feet away from the light. Pour 250 mL of saltwater intothis second pan and let both sit overnight.

Day 2

10. Turn off the desk lamp and slide the pan out from under the light.

11. Carefully pour the saltwater from the pan into a clean beaker. Label the beaker Light.

12. Carefully pour the saltwater from the second pan into a second beaker. Label this beaker NoLight.

13. Repeat Steps 14 to measure the salinity of water in both beakers.

Part II Freshwater Rivers

14. Repeat Steps 14, only this time, you will need 1 liter ofsaltwater instead of 500 mL.

15. Obtain two plastic tubs. Set Tub 1 (the smaller of the two if they

are different sizes) on top of a textbook. Place Tub 2 on thetable next to Tub 1 as shown in Figure 2.

16. Pour 1 liter of tap water into Tub 1. Pour 1 liter of saltwater intoTub 2.

17. Secure the Conductivity Probe to a ring stand using a test tubeclamp as shown in Figure 4.

Figure 3

Figure 4


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18. Gather a piece of plastic tubing and a plastic syringe. Connect the syringe to one end of thetubing.

19. Place the free end of the tubing under the water in Tub 1. Pull back the plunger of the syringeuntil you have drawn 5 mL of water into the syringe. Place the syringe under the water inTub 2.

20. Tape the tubing in place on the edges of the tubs with masking tape.

21. Position the Conductivity Probe in Tub 2 so that it is 5 cm from the end of the tubing. Makesure the hole on the probe is fully submerged.

22. Click to begin the 10 minute data collection.

23. Carefully, under water, disconnect the syringe from the tubing. At this point water should beflowing out of Tub 1 and into Tub 2. The freshwater should be flowing into the tub filledwith saltwater in much the same way that freshwater from a river flows into the ocean. Note:Do not move the probe or tubing during data collection.

Figure 5

24. When data collection is done after 10 minutes, click the Statistics button, , to display aStatistics box. Record the initial and final salinity values in your data table.

25. Print copies of the graph as directed by your teacher.


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DATA

Part I Part II

Pan 1

Light

Pan 2

No Light

Freshwater

River

Initial salinity (ppt)

Final salinity (ppt)

Salinity change (ppt)

PROCESSING THE DATA

1. In the space provided in your data table, subtract to find the salinity changes.

2. Discuss how the salinity changed in Part I. Did you answer the Pre-Lab Question correctly?

3. Discuss how the salinity changed in Part II. Did you answer the Pre-Lab Question correctly?

4. Aquatic animals cannot survive in an environment where the salinity levels fluctuate greatly.Explain why the salinity levels in the ocean stay relatively constant.

5. Provide a geographical example of the process that took place in Part I?

6. Provide a geographical example of the process that took place in Part II?


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EXTENSIONS

1. Find a map of world-wide ocean salinities. Select two regions and explain why they are moreor less saline than the 35 ppt average.

2. Obtain some actual ocean water and test its salinity. Hint: It will have to be diluted first.


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Vernier Lab Safety Instructions Disclaimer

THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB.

This copy does not include:

z Safety information

z Essential instructor background information

z Directions for preparing solutions

z Important tips for successfully doing these labs

The complete Earth Science with Vernierlab manual includes 33 labs, 6 projects, andessential teacher information. The full lab book is available for purchase at:http://www.vernier.com/cmat/esv.html

Vernier Software & Technology13979 S.W. Millikan Way Beaverton, OR 97005-2886

Toll Free (888) 837-6437 (503) 277-2299 FAX (503) [email protected] www.vernier.com

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