by Naomi White, Science Teacher, Jefferson High School, LAUSD

STUDENT HANDOUT

Background and Information:

In oceans and estuaries, salinity is an important factor. Most animalsand plants can only tolerate a small range in salinity. In estuaries, thesalinity may change depending on the tide. Estuarine organisms must havegreater tolerance. You can use Archimedes' Principle to measure salinitybecause things float higher in salty water than in fresh water.

ARCHIMEDES' PRINCIPLE
"The buoyant force of a body immersed in a fluid is equal to the weight ofthe fluid displaced by that object. Therefore, an object floats on a fluidif its density is less than the fluid.
BUOYANT FORCE = WEIGHT OF FLUID DISPLACED
Laboratory Procedure:

1. Use the balance, graduated cylinder, rock salt and water to make thesalinity assigned to the group. Put the solution in the beaker and mark the% salinity.
2. Using the marker and the eyedropper, draw small graduations startingwith the tip. Number the graduations beginning with the eyedropper tip.
3. Fill the test tube with the salt solution. Place the eyedropper upsidedown into the test tube. Record the water level on the dropper in thechart. Repeat procedures with pure water and each of the solutions createdby the other lab groups.
4. Repeat the procedure with the two mystery solutions. Decide theirsalinity by comparing their results to the results of the unknownsolutions.

Observations:

Mystery A = _______%

Mystery B = _______%

Discussion Questions:

1. Why is salinity an important factor in aquatic systems?

2. How is Archimedes' Principle important when making a salinometer?