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' Principle states that the buoyant force on a body immersed in a fluid is equal to the weight of the fluid displaced by that object. An object floats on a fluid if its density is less than that of the fluid. Buoyant force equals the weight of fluid displaced. We can therefore measure the salinity and density 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 of salt water samples. They will use Archimedes' Principle to solve buoyancy problems.

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 do not 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. Show students a 500 ml beaker with 300 ml of water and 150 ml beaker with 100 ml of salt, then ask the student to predict the volume of the two substances mixed together. Mix the salt and water and ask for and discuss explanations of what happened. Measure out equal volumes of water and salt water and ask students to predict which will weigh more and which has higher density. Introduce and explain Archimedes' Principle.
3. Let students complete the lab.