Lesson by Max Millard and Kevin Gortney for Presidio Middle School, 8th grade science, 4-19-07Adjust Background: Darker / Lighter


Introduction/background information:

If you've ever lain on your back in a swimming pool, you have demonstrated the ability of an object to float in water. You float because the water displaced by your body weighs the same as your body. In the pool, the water is pushing on your body's mass with a stronger force than gravity is pulling it down. This is called "positive" buoyancy, or the ability of an object to stay afloat. Simply put, if an object in water displaces its own weight in the water, the object will float. If the object is denser (heavier) than the water it displaces, it will sink (also known as "negative" buoyancy). Ships float because they are less dense than the surrounding water and so the force of gravity pulling on the ship is less than the force of water pushing the ship up. The term SALINITY or SALT CONTENT describes the concentration of salt in a given amount of water. Salinity is expressed in parts per thousand (ppt). This means one gram of salt mixed with 999 milliliters (almost 1 liter) of water is one part per thousand. The average salinity of ocean water is about 35 ppt that is, 35 grams of salt per liter, although it can fluctuate between 32 and 37 ppt, depending on how much freshwater is available. Freshwater normally has a salinity of less than 0.5 ppt. Brackish water, found where a river flows into the sea, can have a salinity as high as 17 ppt. Because most ocean water contains high amounts of dissolved minerals such as salt, it is more dense than freshwater, so objects in seawater will float higher and easier than they do in freshwater. Imagine the extra particles of minerals acting as "tiny little hands" pushing a ship up while gravity is pulling it down. The more "tiny little hands" there are to push the object up, the easier it is for the object to float. A ship holding a given amount of cargo will float high in seawater. When the ship arrives in less saline waters it will ride lower because there are less dissolved minerals in the water. The "Plimsoll mark" shows the maximum amount of cargo a ship can have in freshwater and seawater to ensure that a heavily loaded ship does not sink as it reaches freshwater.

F: Freshwater
B: Brackish Water
S: Saltwater

Plimsoll marks on ship

The Problem:

Captain Jack Sparrow does not believe the old wives' tale that a pirate ship heavily loaded with treasure and cargo will float lower in the water and possibly sink as it travels from the open ocean into a harbor at the mouth of river. You, being his trusty first mate, understand that something in the ocean water allows the ship to float better than if it was in freshwater. Your job is to convince the Captain that loading up in open ocean water and traveling towards the mouth of a river could mean death to the crew and the loss of all the buried treasure you have uncovered. Based on the background information you have about the relationship between salinity and buoyancy, conduct the following experiment to determine if salt concentration in water has an effect on the ability of an object of a specific mass to float (buoyancy).


I. Purpose: To investigate buoyancy by:

A. Designing a boat out of aluminum foil that will hold the most pennies while floating.
B. Examining the relationship between the salinity of water and the buoyancy of objects.
C. Controlled, manipulated and responding variables.
D. Hypothesis statement: What shape of boat do you think will hold the most pennies square, circle, or rectangle? What differences do you expect when you put the boat in fresh, brackish and very salty water?

II. Materials

cardboard square template (25 sq. in.)
cardboard circle template (25 sq. in.)
cardboard rectangle template (25 sq. in.)
heavy-duty aluminum foil
basin of fresh water
basin of brackish water (17 ppt)
basin of heavily salted water (50 ppt)
paper towels

III. Procedure

1. The class is divided into teams of two to three people each. The teams decide what shape of foil they want. Then they cut it out and mold it into a boat that is designed to hold pennies.
2. Each team makes a drawing to show which shape of foil they chose and what the boat looked like (small and deep, wide and narrow, etc.). Then the team tests its boat in one of the three liquids, carefully adding the pennies until the boat reaches maximum capacity. A teacher must observe each boat while it is still afloat, to record the number of pennies.
3. If time allows, the teams may dry off their boat with paper towels and do the same experiment with the second and third liquid medium. Again, a teacher must record the number of pennies supported.

Questions to Consider:

What observations did you make as you were conducting the experiment?
What results did your experiment show?
Was your hypothesis correct? If not, why not?
Did changing the salinity of the water affect the buoyant properties of the "ship"? If so, explain how this occurred.
Having completed your experiment and drawn your own conclusions about the relationship between salinity and buoyancy, what would you say to Captain Jack Sparrow now? (Use the terms salinity, buoyancy or buoyant force, and gravitational pull in your discussion.)


Every PERSON is responsible for turning in one typed report attached to this page, that includes: (I) purpose statement, (II) materials, (III) procedure, (IV) data table and (V) conclusion. Parts II and III may be copied from this sheet, while Parts I, IV and V are the original work of each student.