Therese Donatello - St. Edward School  

The Relationship Between Salinity and the Density of Water

Therese Donatello              St. Edward School  
                               4343 W. Sunnyside Ave.
                               CHICAGO IL 60630          
                               (773) 736-9133

Objective(s):

The Junior high student will review finding the density of a solid.
The student will be able to:
  a) find the density of a liquid
  b) understand the meaning of solubility and salinity
  c) show the relationship between salinity and density of water.

Materials Needed:

balance, testtubes with stoppers, blocks of wood, rulers, graduated cylinders,
sand, water, salt (NaCl), tall clear plastic cups, thumbtacks, pencils with
erasers, food color, and grease pencils.

Strategy:

Lab 1 Floating pencil. 
  1) Place a thumbtack in the eraser end of a pencil. 
  2) Place water into a clear plastic cup to a level that is 1 cm from the rim. 
  3) Place the pencil in the water, eraser end first. 
  4) Mark the water level on the floating pencil with a grease pencil. 
  5) Remove the pencil from the water. 
  6) Place the food colored solution into the cup as you did in step 2. 
  7) Place the pencil into the solution as you did in step 3. 
  8) Again mark the water level on the floating pencil.  Is there any change
     in the levels?  Why?  

Lab 2 Density of a solid and a liquid. 
  1) Measure the length, width,and height of the block.
  2) Using the balance find the mass of the block.
  3) Divide the mass by the volume to find the density of the block.
  4) Using the balance find the mass of an empty testtube and stopper.
  5) Measure 10 mL of water in a graduated cylinder and transfer to the testtube.
  6) Measure the mass of the testtube, stopper, and water.
  7) Subtract the mass of the testtube and stopper from the testtube, stopper,
     and water to find the mass of the water.
  8) Find the density of the water as you did in step 3.

Lab 3 Solubility

  1) Place 10 grams of salt into 10 mL of water.  What happens to the salt?
  2) Place 10 grams of sand into 10 mL of water.  What happens to the sand?
  3) What do think would happen if you found the density of this liquid?
  4) Follow steps 5 to 8 in lab 2.
  5) Was the density the same?  Why or why not?


Performance Assessment:

1. What do you think is the relationship between salinity and density?
2. How would you set up an experiment to show this relationship? 
3. What do you think would happen if you placed food color in the salt 
   solution and poured it slowly down the side of the glass?  Try it and
   see what happens.