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

LESSSON SHEET

[ Student Handout ]

Time Frame: one period

Concept:
Salinity increases the density of the water sample. Archimedes' Principlestates that the buoyant force on a body immersed in a fluid is equal to theweight of the fluid displaced by that object. An object floats on a fluidif its density is less than that of the fluid. Buoyant force equals theweight of fluid displaced. We can therefore measure the salinity anddensity of a water sample by measuring how high an object floats in it.

Objective:
Students will construct and calibrate a device to measure the densities ofsalt water samples. They will use Archimedes' Principle to solve buoyancyproblems.

Materials:

Per Student:
• student handout D

Per Group:
• indelible ink marker
• eye dropper
• balance
• rock salt
• beaker
• test tube

Preparation:

1. Make up two mystery salt solutions. Mark the beakers A and B, but donot reveal the salinity to the students.

Procedure:

1. Assign each group a different salinity to mix: 5, 10, 15, 20, or 25%salinity.
2. Introduce the discussion about density and salt solutions. Showstudents a 500 ml beaker with 300 ml of water and 150 ml beaker with 100 mlof salt, then ask the student to predict the volume of the two substancesmixed together. Mix the salt and water and ask for and discuss explanationsof what happened. Measure out equal volumes of water and salt water and askstudents to predict which will weigh more and which has higher density.Introduce and explain Archimedes' Principle.
3. Let students complete the lab.