Abstract

Objective

In this experiment you will test several freshwater and saltwater samples to see how much salt they contain.

Introduction

People often wonder how salty the ocean is, but what does this mean? What you are really asking is how much salt is dissolved in the water, called salinity. When something is dissolved in water it often seems to disappear. But even though the water may look clear the dissolved substance is still there. When something can dissolve in water we say that it is soluble. Salt happens to be very soluble in water compared to other substances, like sugar for example.

Different bodies of water contain different amounts of dissolved salts and have different salinities. The two main types of aquatic environments are freshwater habitats (lake, river, pond, stream) and salt water habitats (ocean, gulf, sea, bay). Another type of aquatic environment is called an estuary, which forms where a fresh body of water mixes together with a salty body of water. Estuaries are commonly found at the mouths of rivers and deltas, and act as a kind of nursery for the ocean.

Sea salt can be harvested from seawater and used for cooking and eating. This sea salt is being harvested in the south of France at a saltern. (Wikipedia, 2006c)

How is salinity measured? The amount of salt dissolved in the water is often measured in parts per thousand (ppt) which is very similar to the grams of salt per liter of solution (g/L). That means if you have a 1 ppt salt solution, then if you filled up a 1 liter water bottle, it would have about 1 gram of salt inside. Seawater is usually somewhere between 30 and 35 ppt, which means that a 1 liter bottle of seawater would have 30–35 grams of salt inside!

In this experiment you will learn a simple way to measure the amount of salt in a sample of water by using evaporation. After the water of a solution evaporates, the dissolved salts are left behind. By collecting water samples from different bodies of water and evaporating them, you can find out how much salt there is in an ocean, lake, bay, or stream near your city.

Terms, Concepts, and Questions to Start Background Research

To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!

• salt water habitats
  • oceans
  • seas
  • gulfs
  • bays
  • estuaries
• fresh water habitats
  • lakes
  • streams
  • ponds
  • rivers
• salinity
• parts per thousand (ppt)

• What is the difference in salinity between salt water and fresh water aquatic environments?
• How salty are estuaries?
• How can salinity be measured?

Bibliography

• Here are three Wikipedia sites with useful information on salinity, seawater and sea salt:
  
  • Wikipedia contributors, 2006a. "Salinity," Wikipedia, The Free Encyclopedia, [Accessed October 15, 2006] http://en.wikipedia.org/wiki/Salinity
  • Wikipedia contributors, 2006b. "Seawater," Wikipedia, The Free Encyclopedia, [Accessed October 15, 2006] http://en.wikipedia.org/wiki/Sea_water
  • Wikipedia contributors, 2006c. "Sea Salt," Wikipedia, The Free Encyclopedia, [Accessed October 15, 2006] http://en.wikipedia.org/wiki/Sea_salt
• Here you can read all about salt, what it is made of, where we get it and what we use it for:
  SI, 2006. "The Salt Institute Home Page," Washington, D.C.: Salt Institute. [Accessed October 15, 2006] http://www.saltinstitute.org/
• UCAR, 2001. "Salinity" The University Corporation for Atmospheric Research (UCAR). [Accessed October 15, 2006] http://www.windows.ucar.edu/tour/link=/earth/Water/salinity.html&edu=elem

Materials and Equipment

• 12 oz. glass canning jars with lids
• cookie sheet
• oven
• metric measuring cup
• metric scale, such as the Fast Weigh MS-500-BLK Digital Pocket Scale, 500 by 0.1 G, available from Amazon.com

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Experimental Procedure

1. You will be collecting several samples from different locations and making measurements during this experiment. You will want to make a data table to write down and organize your data:

   Location	Freshwater or Saltwater?	Weight of Jar Before (g)	Weight of Jar After (g)	Weight of Jar (g)	Volume of Water (L)	Amount of Salt (g)	Salinity (g/L)
   negative control	distilled
   Berkeley Marina	salty
   Walden Pond	fresh
   Cedar Creek	fresh
   Ocean Beach	salty
   etc...

2. Collect samples of water from several sites that you think will have different salinities. Choose some freshwater sites (lakes, rivers, streams, springs, etc.) and some marine sites (beaches, marinas, bays, estuaries, etc.) for your study. Write the locations in your data table.
3. Collect each sample by filling a canning jar with the water at the site and securing the lid. Then write the date, time, and place that you collected your sample on the jar with a permanent marker.
4. After you collect your samples, each jar should be full to the brim with water and sealed tightly. If they are not full, you will need to remove some water from a few of the jars until the water level for all of the jars is the same.
5. Take a new, empty jar and weigh the jar with your metric scale in grams. Write the weight in grams in your data table.
6. In the new empty jar, fill the jar with distilled water to the same level as the other jars. Then measure the amount of water in liters (L) by pouring it into a large, metric liquid measuring bowl. Write this measurement down in your data table. It will be the total volume of liquid you collected from each site. Then re-fill the jar and use it as your negative control.
7. Arrange all of your jars on a cookie sheet and carefully place them in a sunny location. Carefully remove the lids and leave them there for several weeks to evaporate. If you have your parents help, you can also bake the jars in the oven at a very low temperature (200^oF) until the water has evaporated. This will take a few hours.
8. After the jars have dried completely, weigh each jar in grams, and write the weight in grams in the data table.
9. Subtract the weight of the empty jar (step 5) from the final weight of each jar you filled with a water sample. This is the amount of salt that was present in that water sample.
10. Now divide the amount of salt in grams by the amount of water in liters, and this will give you the salinity, or concentration of salt, of the sample measured in grams per liter (g/L).
11. Make a bar graph of your data to compare the salinity of each water sample. Make a scale on the left side of the graph (y-axis) representing the different ranges of salinity from your study. Then draw a bar for each sample up to the corresponding salinity level. Which samples had the most salt? Which had the least?

Variations

• If you have access to one, you can also use a refractometer to measure salinity. As the amount of salt in the water increases, so does the index of refraction. Try repeating the experiment taking measurements with a refractometer. Do you get similar results? Which method is more accurate? For a more advanced project idea on the index of refraction, see the Science Buddies experiment Measuring Sugar Content of a Liquid with a Laser Pointer.
• Many organisms that live in saltwater environments cannot live in a freshwater environment, and vice versa. Why do you think that is? Do an experiment using a very simple salt water organism, the Brine Shrimp, to see what effect changes in salinity can have. You can buy a Brine Shrimp hatchery at your local pet store to hatch and rear the brine shrimp. Then you can expose the brine shrimp to water samples of different salinities. What will happen?

• For more science project ideas in this area of science, see Ocean Sciences Project Ideas.

Credits

Sara Agee, Ph.D., Science Buddies

Last edit date: 2011-11-18 12:00:00